US2473202A - Telegraph system with code conversion and error detection - Google Patents

Description

June 14,A 1949. H. v-. HIGG'l-r'r 2,473,202

TELEGRAPH SYYSTEH-WITH CODE CONVERSION ANU ERROR DETECTION y 4 Sheets-Sheet 1 Filed Sept.4 21, 1948 ttamey H.' V HIGGITT TELEGRAPH SYSTEM WITH CODE CONVERSION June' 14, 1949.

AND ERROR DETECTION 4 Sheets-Sheet 2 Filed sept. l21, 1948 dNml.

wm Nk June 14, 1949. H. v. HIGGITT 2,473,202

TELEGRPH SYSTEM WITH CODE CONVERSION AND ERROR DETECTION Filed Sept. 21, 1948 4 Sheets-Sheet 3 gar@ WIL/493i# .V @MM im Attorney H. v; HIGG'l-r'r TELEGRAPH SYSTEM WITH CODE CONVERSION` -June V14, 1949.

AND ERROR DETECTION Filed seln. 21, 1948 4 Sheets-Sheet 4' 03. M Attorney Inver'zlar Harry V. H6331,

marking units andiour, spacing units.

clear, however, thata departure has to. bfe made:

Patented June 14, 1949 UNITED STATESv F FICE Tlil'IEGrRAPH#v SYSTEM WITH CODE CONVER- SION, ANDER/ROR DETECTION Harry VernonHggitt,],London, England, a'ssignor'` tok Cable & WirelessJLimited, London, England,

a: British company Application September 21, 1948, Serial No. 50,311 Iii Great Britain September-22, 1947 9 Claims.

1 This invention relates to telegraph apparatus for workingy with so-called` equal length*` codes, and' in'- particular; isf concerned with transnxittingk and" receiving apparatus fr'error-detecting or errorhindi'cating"equal" length codes.

Suchiequal length telegraph codes, that-is to" say; codesl infwhicl'i` all `the characters and? other' SignalsemplOyed consistofi the same number of" units,y have beenwell known for many years and the most: commonfoff these issthe five-unit code acoordingf to'which: each signal isrepresented by;

when signals-are mutilated as frequently occurs" on wirelessicircuits. There is approximately an 80-percent chance that the mutilation of a letter will result inl another letter being received. This is most objectionable when messages are sentin.

cipher for then, there is' nothing to guide a receiving operator when such a transposition occurs.

Thisfeature of the five-unitcode of giving rise to undetectable errors has been appreciated in` the past and various methods have been proposed to overcome it.` These-methods, in general, necessitate the useof an increased number of units per character. One method 'suggested consists in building upa seven-unit code having each signal consistingvof aeconstant number of marking units and a constant number of'spacing units, for exampleevery'signal consisting of 3 marking units and 4 spacing units. In that particular case, the number of possible signals is35 as compared with 32 in the primary five-unit code. Such a seven-A unit code, however, isy open, toltheobjection that the signals have `to be entirely different from and= to bear. no relation .tothoseof the standard vef unit code so that conversion of signals in. one code to. the other has `to be eiected. on achar-r acter basisandcan. only be,Y donev with compli-v4 cated apparatus. Another; proposal-to. deal with,

the undetectable errorY draw-backv of the ii-ve.- uni-t. code is-to employ for each signal eight units.,

o-iwhich the rst ve are the units of the corresponding signal in. the. primary standard vef unit code to Which three-.unitsare added in such a. way thateach signal. isconstituted by four from the general system for-theftwo signals which in thefive-unit code consist respectively of vemarking and five spacing units since it is not possible to arrive at a, four-marking four-spacing signal by the addition of'threeunits to either'of,

those two signals; thus this scheme has the drawback that these two ve-unit signals have to be converted into special'eight-unit signals cliier-A ing in constitution from all the others which leads to complications at both ends of the telegraph circuit.

The present invention is concerned in particular with' telegraph apparatus for working withV error-Lv detecting co'des in which all 'tliefsignals consist of the same num-ber offmarki'ng and the same numy ber ofl spacing elements arranged"v in different; sequences; in one particular example of suchv codes;` each signal` consists' of three marking.V ele-1- mentsz' and fourfspacingelements 2g According to. theinvention bothA codes em'- ployed--the non-error-detecting primary code: converted at the sending` end of a circuit into 4the;1 other codewhich isvan error-detecting code and is:- converted back at the receivingfendl of vthe cire cuit-employ signalsf, theI elements of whichitarel-l` divided into; two groups, of which*y one group'inil each: coole-is,Y inV general, the same in. marking and? spacing permutation for each signal while-the; other group in theerrori-detectinfg code is deter mined .by the. permutations of; both groups offthefr noneerror-detecting.code; appear'. later, in practice?.signalsrmayalso'vbe ein-fi ployed inn.which,.,in. changing from one code tol thefother, the elementsiof the -rst group haveltov bezconverted.'

It. maybepointed out that the-substantially: perfect securitywhi-ch resultsfromzthe use of: the-iy seven-unit vand the" eight-unitv codes already' re-L ferred.to ydoes 'not arise-- directly from the signals being balanced-las regardsfthenumbers of mark ing-and spacing units-but from the fact that muti-- lation which occu'rsfin any one signal isl almost-1 certain to consist of a change: of onek or more/off thefunits of the signal. from marking to spacing. or from spacing,l to markingl and not both at thery Sametime. Thus,theierror-detecting codes whichl arel employed intelegraph systems comprisingf apparatus according. to the presentinvention are: equal. length codes. consisting of signalsno'one:v of which, canbe converted into another, bythe., changeoi one or'r more unitsffromv marking. to spacing lorifice-versa. y

The procedure of'y building up they codes ein--A ployed. in practising. the present` invention may"- perhaps bebest .made` cleary by considering' two.;

Inf some` cases, .as-r will;y

examples, the first of which consists in conversion of the standard five-unit telegraph code into an error-detecting code in which each signal consists of three marking elements and four spacing elements and vice-versa. In the following table, the formation in both codes of the 32 signals employed is set out, in which M represents a marking element and S, a spacing element.

Table 1 Five'unit primary Seven Unit errorco e detecting codo (Standard five unit code) Signals First Second First Second Group Group Group Group 1st Series MMS MM MMS MSSS MSM SM MSM SMSS i2 SMM MS SMM SSMS SS SSSM 2nd Series MSS MM MSS MMSS SBIS Slv SMS SMMS l2 SSM M S SSM SSMM SS MSSM 3rd Series SSS MM SSS MMMS SlVl SMMIVI 4 MS lVISl/IM SS MlvlSNI 4th Series MMM MM MSS MSMS SM SMSM 4 MS SSM MSMS SS SMSM It will be noted that, in both codes, the rst group consists of three signal elements and in the rst three series of signals, amounting to 28 signals in all, the signal elements of the first groups are the same in both codes. In the fourth series of signals which consist of four signals, the elements of the first group have to be changed in converting from one code to the other as will be explained more fully below.

In the primary ve-unit code, the second group of each signal consists of two signal elements and there are four ways in which this group can be formed viz. MM, SM, MS and SS. In converting to the seven-unit code, the second group is converted into four elements in accordance with the following scheme of conversion. In the first series of signals, it will be noted that in both codes, the first group in each signal contains two marking elements so that in the second group of the signals in the seven-unit code, only one marking element is required and to represent MM, SM, MS or SS of the second group of the five-unit signals, the first, second, third and fourth element respectively of the second group in the seven-unit code is made marking.

In the second series of signals, however, in both codes, the rst group of each signal contains only one marking element so that in the second group of the signals in the seven-unit code, two marking elements are required and this second group is formed in a manner similar to that in the first series except that a second marking element is made to follow immediately after the marking element which has been provided in the conversion as described above for the rst series, for which purpose, the rst element is regarded as following immediately the last element.

In the third series of signals, it is seen that there is no marking element in the first group in either code so that in the seven-unit code, three marking elements are required in the second group and are obtained in each instance by adding a further marking element after the other two marking elements. In the fourth series of signals, there are three marking elements in the first group in both codes so that in the seven-unit code no marking element is required in the second group. Since this only yields one sevenunit signal (i. e. MMMSSSS) and since four are needed to complete the 32 signals corresponding to those of the ivenunit code, it is arranged that in the seven-unit code, the rst group is changed to MSS to form two of the signals and to SSM for the other two signals while the second group of the seven-unit signal is either MSMS or SMSM. Thus as shown in the above table, the four sevenunit signals are:

MSS MSMS MSS SMSM SSM MSMS SSM SMSM In the second example, the primary code is a six-unit equal length code which is converted for transmission purposes into an eight-unit error detecting code, in which each signal consists of four marking elements and four spacing elements. Signals in this error-detecting code may be reconverted into the primary six-unit code at the receiver. The following table shows how 64 different signals may be obtained in five series, the letter M in the signals representing a marking element and S a spacing element.

Table 2 Six-unit Primary Eight-unit error- Code detecting code Signals First Second First Second Group Group Group Group 1st series MMMS MM MMMS MSSS MMSM SM MMSM SMSS lVISMM MS MSMM SSMS i6 SMMM SS SMMM SSSNI 2nd series. MMSS MM MMSS MMSS MSMS SM IVISMS SMMS SMlwS MS SNVMS SSMM SMSM SS SlVISh/l MSSM SSMM SSMM 24 MSSM MSSM 3rd series MSSS MM MSSS MMMS SMSS SD/I SMSS SMMM SSMS MS SSMS MSMM 16 SSSM SS SSSM MMSM 4th series i, SSSS MM SMSM MSMS SM SMSM MS SMMS MSMS 4 SS SMSM 5th series MMMM MM MSSM MSMS SM SMSM MS MSMS MSMS 4 SS SMSM It will be noted again that the signals in the two `codes are each divided into two groups and that in both codes, the rst group consists of four signal elements; the first three series of signals amounting to 56 signals in all. The signal elements of the first group are the same in both codes as before. In the 4th series of signals, which includes four signals, and the 5th series which contains a further four signals, the elements of the rst group in the primary code have to be changed in converting one code to the other, as will be explained later.

In the first four series, the second group of each signal in changing from the primary code to the errordetecting code, is converted respee tively into the same four "elements as ln YTable 1 above.

. As regards the rst group in the 4th series, there are no marking units in vthe primary code so that the rst group of lthe eight-unit code is made to contain two marking elements in each signal and the second group can then be the same as in Table 1. In the 5th series of signals, there vare four marking units .in the iirst group in the ,primary code which would only provide one signal in the eight-unit code, therefore the iirst group is changed in the eight-unit code as shown in Table 2 and the second group in that code is the same as in the 4th series.

.The converting apparatus suitable for use in vconnection vwith the above two examples at the transmitting and receivingendsof a line will now be described by Way of 4example with reference to the accompanying drawings, in which- Figure 1 is a diagram of connections of appaf ratus at a transmittingl station for converting standard five-unit telegraph code into a sevenunit error-detecting code;

,Figure 2 is la diagram ofv connections for yreceivingsuch signals and for re-converting them into five-unit signals;

Figure 3 is a diagram of connections of a transmitting system for converting a six-unit primary code into'an eight-unit error-detecting code; and Figure `i isl a .diagram of connections of a receiving system suitable for converting signals in the eight-unit code back into the six-unit primary code. y

Referring now to Figure 1, it is assumed that standard five-'unit telegraph signals on a perforated tape operate any number up to live of the peckersPa, Pb, Pc, Pd, Pe. The peckers thus operated pass through `perforations in the tape simultaneously for a suitable time in each signal period, e. g., for the time of two signal units as indicated by Xin Figure 1. Each pecker when thus operated, closes contacts I, 2, thus connecting a circuit through a winding of the corresponding relay A, B, C, D or E from the positive side of the battery at 3 to earth at 4. The relays which are thus energised, are vlocked up through contact springs al eI and front contacts 5 thus connecting locking windings of the respective relaysl 'A E from vthe battery at l'6, through a brush 'B72 and continuous segment 1 to earth at 8, so that the relays in question are held locked until the end of transmission of a seven-unit signal.

, The first three relays A, 1B and C deal with the three signal elements forming the firstgroup of a signal as shown in Table 1 above and each has seven contactsprings a'l a1., bI b1 and c'I.,. c1.

'When the respective relays are energised, their seventh springs a1, b1 or c'l connect battery at `9 to the first three transmitting segments SI, S2, 4S3 for the seven-unit code to send marking elements for the rst, second and third series of signals but the connections to battery pass through back contacts I of vthree modifying relays 'ML M2 and AM3 which alter the application -ofbattery to the rst three distributor segments SI, S2, S3 when sending one of the fourth series ofl signals as shown in 'Table l. Then the relays M2 and M3 or the relays 'MI and M2 are energised and change the appropriate two mark- Ing elements to spacing elements as required for the rst group "in the seven-unit code `fory the fourth series of signals'as fsee'n in Table I. This result is "clearly only obtained when Vthe first three'sg'nal elements :are 'marks in the five-unit primar-y code whereupon the relays A, B and C are energis'ed and which of the modifying relays MI, M2 and M3 are enetgised depends on the lstate of thefth pecker relay E. In other words, this depends upon whether the last signal element in the live-unit code of the signals of the fourthseries is 'a mark or. a space, as can be followed from Table 1. If the fth signal element is a mark, the table shows that the second and third elements in 'the seven-unit code should be spaces; then the relay E is energised and battery at Il is connected through the springs a6, bt, c6 and their front contacts in series and through the spring e3 of the relay E and its front contact by way of -conductor 12, through the windings yof the relays M3 and M2 in series and thence to earth. The relays M2 and M3 are thus energised, breaking connection at AIl) and giving spaces for the rsecond and third signal elements on the segf ments S2 and S3. However, if the fifth signal element is a space, as can be seen from Table l, the first two elements of the seven-unit signal have to be spaces and vthen the vconnection from battery at Il goes through the back contact L3 oi the relay spring e3 and proceeds by the vcom d'ucto'r I4 to energise the modifying relays MII 'and M2 which break circuit at I0 and 4alter the rst and second sig-nal elements to spaces.

The relaysD, E control four supplementary r'elays F, G, H, K which, at their contact springs f3, g3, h3 and k3, connect battery at I5 respectively to the distributor segments S4, S5, S6 and S1. The results obtained vare as follows: I

If the fourth and fth elements of the fiveunit signals are both marks, the relays D and E are energised and the iirs't supplementary relay F is energised 'from battery at IS, relay spring e2, iront contact l1, relay spring d2, front contact I`8 through the winding of relay F to earth at I9; thus relay F at its spring f3 connects bat tery to the distributor :segment S4, consequently the fourth element of the seven-unit signals is a mark as is required, as can be seen from the first 'signal in each of the series in Table 1.

If, however, the fth element of the live-unit signal is a mark but the fourth is a space, as occurs in the vsecond signal in the live-unit 'code ineach sries in Table l, the relay E is energised but the relay D is not energised so that the second supplementary relay (Er is now energised from battery at 16, springs e2, d2, back ycontact 20 and winding lof relay lG to earth at 2l, and now the fth element in the seven-unit vcode Vbecomes a mark.

If now the fourth element in the 'live-unit code is a mark but the lifth is a space, corresponding to the third srignalvin each series vin Table 1, Iit will be Aseen that the third supplementary relay His energised from battery at I6, spring e2, back contact 22, vspring d3, front contact 23 and winding of relay H to earth at v2li so 'that now, jas is required, the sixth element in the seven- 'unitsignal is a mark.

Yet again, if both the `fourth and fth elements of the live-unit 'signal are spaces, the fourth supplementary relay K is energised from battery at lt, spring e2, back contact 22, spring d3, back contact 25, winding of relay K to earth at 26 so 'that now 'the relay K at its spring 7c3 applies marking current to the seventh segment S1 Aso that vthe seventh signal element in the seven-unit code is now a mark.

The above four conditions complete the reduirements for forming the second group of signal elements of the rst series of signals in the seven-unit code, as shown in the fourth column of Table l. If, however, the rst groups of the signal elements in both codes each contains only one mark, as in the second series of signals, or contains no mark as in the third series of signals, a circuit is made from battery 2l through a back contact of at least one of the third springs a3, b3, c3 and through at least one of the second springs a2, b2 and c2 of the relays A, B and C and through the frontcontact of one of the springs fl, gl, hl and lcl of the relays F, G, H, K to apply battery to the transmitting segment following that transmitting segment which is connected to battery by one of the springs f3, g3, h3 or k3, as described above, which is in accordance with what is required for the second and third series of signals in the seven-unit code shown in Table 1. This is arranged by connecting battery at 21 to the springs a3, b3 and c3 of the relays A, B and C and connecting the back contacts 28 of those springs respectively to the springs b2, c2 and a2 of the relays B, C and A next in order, for which purpose the relay A is treated as following the relay C and, further, by connecting the back contacts 29 of the springs a2, b2 and c2 together and to the springs fl, gl, hl and kl of the supplementary relays F, G, H and K by the conductor 30. When, as in the second series of signals, only one of the relays A, B and C is energised, there is a connection from battery 2l through one of the springs a3, b3 and c3 and its back contact and one of the springs a2, b2, c2 and its back contact 29 to the conductor 30 and thence through one of the springs fl, gl, hl and kl to the segment S5, S6, S1 or S4, following that which has already received a mark as described above.

In addition, if the rst group of signal elements contains no marks, a third segment of the segments S4, S5, S6 and S1, receives a mark from battery in accordance with the third series of signals in Table l. This is eiected by connecting battery at 3l through relay spring a5 and back Contact 32, spring b5, its back contact 33, spring c5 and its back contact 34 which it will be seen, is connected by a conductor 35 to the springs f2, g2, h2, k2 of all the relays F, G, H, K.

In the fourth series of signals, the flrst group in the live-unit code is three marks which, as seen from Table 1, has to be converted to one mark followed by two spaces or two spaces followed by one mark, as determined by the fth signal element in the primary five-unit code; that is to say, if the fifth signal element is a mark and the relay E consequently energised, the rst group in the seven-unit code has to be MSS, while if the fifth signal element is a space, that group has to be SSM. This conversion is effected oy energising the three modifying relays Ml, M2, MS' in the way described above as determined by the conditions of the relay E. Also in this event, the first three relays A, B, C are arranged to connect battery at 36 through springs a4, b4 and c4 and their front contacts 31 in series to all the springs f2, g2, h2 and k2, with the result that the one of the relays F, G, H, or K which is energised, applies battery at 36 to the segment S4, S5, S6 or S1 which is two segments removed from that which receives a mark from the spring f3, g3, h3 or k3 on the relay which is energised. To make this clear, if relay F is energised, its spring f3 applies battery at l5 to the segment S4 but also through the circuit just described, battery' at 36 is con- 8 nected through the spring f2 and its front contact 38 to the conductor 39 and thence to segment S6 which is two segments removed from the segment S4. This is the necessary condition as can be seen from the second group in the seven-unit code in the fourth series as set out in Table 1.

Thus, all the conditions are fullled for providing conversions as set out in Table 1 for setting up the seven-unit error-detecting code signals at the transmitting segments Sl S1, with a positive battery connection for each battery marking element and a free segment for each spacing element. These elements may conveniently be assumed to constitute the signals of one of a number of time-division channels in a multiplex telegraph system of the ordinary kind. The brush Brl which sweeps over the segments Sl S'l sends the signals to the line L or through the conductor L, energises a line relay, as is necessary in sending double current signals.

The equipment for receiving the signals in the seven-unit error-detecting code and re-converting them into five-unit signals of the primary code is shown in Figure 2. The seven-unit signals are received by the line relay LR whose spring Zr has marking and spacing contacts M, S, the former of which is connected to a brush Brl which sweeps over seven short segments RSl RSI, making contact in the middle of the respective periods of reception of the seven signal elements. Since each seven-unit signal contains three marks unless it has become mutilated, current i-s passed through the windings of three of seven receiving relays N, O, P, Q, R, S and T, each of which windings is connected to one of the segments RSI RST. The three relays are energised in accordance with the particular signal being received by the line relay LR. The brush Brl is driven in synchronism with the received signals by any of the usual means. The relays N T which are thus energised, are locked up through contact springs nl tl, each of which is conected to battery and its front contact 4l) and a brush Br2 working over a long segment 4l in the circuits of the locking windings of the relays N T which are energised so that these three relays are held locked until the operation of re-transmitting the ve-unit signals formed in the receiver is completed, as will be described later.

When three of the relays N T, neither more nor` less, are thus energised, a circuit is completed through an error indicator or testing relay TR. The winding of that relay is con- 'l nected to battery at 42 by a bank of contacts controlled by springs n2; o2, o3; p2, p3, p4; q2, q3, 14,115; r2,1^3, T4, T5; s2, s3, s4, and t2 t3. It will be seen that battery is connected to the spring n2 whose back contact 43 and front contact 44 are connected respectively to the springs o2 and o3. The front contact 45 of the spring o2 and the back contact 46 of the spring o3 are both connected to the spring p3; while the back contact 4l of the spring o2 is connected to the spring p2 and the front contact 48 of the spring o3 is connected to the spring p4. The rest `of the bank of contacts are connected forwards in a similar manner until at the end the spring t2 has its front contact 49 and the spring t3 has its back contact 50 both connected to the Winding of the test relay TR, the other end of which is earthed. If the circuit through the bank of contacts is traced it will be found that there is a connection to the winding of the relay TR only when precisely three of the relays N T are energised. When as a result, the rest relay TR is energised, its' spring trl` at its back contact 5I disconnects a connection 52 which leads to an error-indicator, while the spring trl at its contact 53 applies battery' to a number of relay springs and contacts which will be described' in detail later. However, the battery connection to the spring t'rl passes from battery atv 54, through a brush Br3 and a segment 55, which only come into 'contact when the three of the relays N T in question have been locked.

When the test relay TR is thus energised, it connects battery at 54 through the contact k53, inter alia to springs n4, o5, p5 respectively of the relays N, O and P, through a conductor 56. The springs n4, o5, and p5 through their front contacts .57, when energised continue the connection from battery at 54 and through back contacts 58 and springs ma, mb, mc .of three modifying relays Ma, Mb, Mc and thence to the first three segments 59, 60 and 6l of the ve segments 59 G3; of the retransmitting distributor. It will be recollected that in the case of' a.v signal in the fourth series, as shown in Table l, the second group of the signal in the seven-unit code is eitherl MSMS r SMSM, so that in the former event, the fourth and sixth relays Q and S are energised and in the latter event the relays R and T. Certain springsof the relaysviz. Q6, q'l'; r6, r1; S5, s6 and t4, t are so interconnected that when. the. second group lof a signal in the seven-unit code takes either of the above two forms and consequently the relays Q and S or R and T are energised, the three modifying relays Ma, Mbs, Mc are operated and Athen the test relay' TR., through f its spring trl and front contact l53 connects bat'- tery at 54 through the conductors 5.6 and. 64 to the front contacts 65 of all three. of the modifying relays Ma, Mb, Mc and also applies battery through the conductorlit` to the said bank. of springs and contacts of therelays Q T, in fact, at the back contact 61. of theL spring t4 and the front contact 68of the spring t5.

The test relay TR also 4applies battery to spring-s Q78, 18, s1, and t6 oi the relays. Q T. These springs q8, f8, sl, t@ and the springs q9, r9, S8 and t1 of the same four relays and their contacts are so interconnected that whether onlyl one of the receiving relays Q; T is operated, o r whether two of those relays which are adjacent are operated, or whether three of those re.- lays are operated, the result is that only one of four additional relays W, X, Y and Ziis ener;-

gised. For this purpose, the' relay Q is regardedv as following the relay T. The relayv W, X, Y or Z which is operated ,is that corresponding to the. rst vrelay of the operated sequence of the relays Q T when more than onevof the latterjare operated. The above result can. be conrmed by checking the connections of ther springs 18,

qs, T8, rs, s'l, e8 and t6, tl. Thus, for example, if relay Q only of the four in question 4is ener gised the coresponding additional relayI W- ,willbeenergised from conductor 56.- th-rough conductor 5.9, spring t6, its back contact 1.0, conductor 1l, spring Q9, its front contact: 12, ywinding of, relay W'tor earth. Obviously the windings of the relays X, Yand Z have their circuits interrupted at the springs ,79, si), tl' respectively. .IL again, ofthe relays Q T, the relays QA andR onlyA are vc energised,,the relay W is energised as before, the windings of relays Y and Z are 'interrupted at they springs s8V and .t1 and although. the spring r9 makes c.on,tact the circuit. 4of. relay" X isnow interrupted at ,thespring-ql As annal example,

if relaysy Q, R and T are energised and relay S is not energised, relay Z isenergised, which is that corresponding to the relay T which is the rst of the'operatedrelays T, Q and R in the sequence as explained above. Actually, relay Z is `energised from the conductor 59, spring s1 and its` back contact 13, spring t1 and its front contact 14, through' the Winding of relay Z to earth; the circuitof relay W is interrupted at the spring t6, the circuit of relay X is interrupted at the spring q andthe circuit of the relay Y is, of course, interrupted at the spring S8 of the relay S which is not excited. These are typical examples and allA other possibilities can be checked in a similar way.

Thus, in the. first, second' and third series of signals, the relays W Z are operated to correspond to the second group of signal elements of the first' series in the seven-unit errord'etecting code.V At the ltime referred to, the test relay TR.' hasv connected battery through conductors 5.5' and 'l5 to the spring wl of the relay'W, and through conductors 56 and'G to the spring y of relayfY, while the springs and 2: of the relays X andl Z are connected* together; further the back contact 7T of the spring wl is connected to the back contact 18 of the spring e; the front contact 'I9 of' thespri'ngr wl and the front contact of the spring y are both directly connected to the fourth segment62 of the trans'- m'itting commutator; the spring w2 of the relay W isi connected to theback contact 8| of the spring y, while the front 'contact 82 of the spring w2 and' the'frontk contact 83 of the spring x are both 'connected tothe fifth retransmitting segment 63, through `the back contact 84 and spring mrof an additional modifying relay Mm. As a result of these connections, the second group ofv the ve-unit signals appears on the retransmitting segments 6.2 and 63 as MM, SM, MS or SS respectively, depending upon whether the addi` tional relay W, X, Y or Z is'operated. It can be seen from Table 1 above that this is the necessary condition Vfor reproducing the second group ofthe signals in the=veunit code.

In1 the case of the fourth series of signals, from what has been stated above; and fromexamination of Table 1, it will be realised` that the relays QV through the bank'of springs t4, t5, s5, s6, T5, r'l,

q`6, q1, by way of'conductor 8 5 to the windings ofthe three modifying relays Ma, Mb, Mc all of which relays are, therefore, energised and apply marking elements to the segments 59, 60 and 6l as already described. Furthermore, if only the relay N is energis'ed, as happens in the fourth series when the first signal elementof the sevenu'nit' signali'sv a mark, a circuitl is continued from the conductor 85`through the front'contact 86 and spring n3 of the relay N and conductor 8l' to energisethev additional modifying relay Mx.

As. a result', battery is' applied to the last retrans mitting segment 631 from conductor 55 through conductor 8,8, front contact'8`9 andspring msc of the relay Mr and,` this makes the fth and last signal element of: the. retransmitted signal in the vemunt code a mark, as is required and shown in the fourth series in Table 1. With this arrangement, although the operation ofthe relays W andLY applies a marking elementto the fourth retransmitting segment B2 only, the fifth retransmitting segment 63 also receives a marking element, in the way just mentioned, when the first signal element of the seven-unit signal is a mark. This accounts for the rst and third signals of the fourth series. When the relays X and Z are operated, no marking element is applied by their contacts to either of the retransmitting segments 62 and 63. However, in the way just explained, when the first signal element of a seven-unit signal is a mark, a marking element is applied to the retransmitting segment 63 by the additional modifying relay Mr and thus the second and fourth signals of the fourth series are formed.

The ve retransmitting segments 59 B3 may be used in conjunction with a preceding segment 90 and a following segment 9| to send on a standard five-unit signal together with a start and stop signal to the line L through the brush Br4 working over the segments 59 63, When the test relay TR is not energised, battery is applied through the back contact 5I and conductor 52 to an error-indicator in any convenient manner to print an error-indicating signal on the printer normally operated by the signals from the retransmitting segments 59 63.

One way of doing this is to make the printer employed selective to the signal consisting of ve spaces which is not used in the standard five-unit code. Then the printer is arranged t print the error-indicating signal when this selection occurs. When the test relay TR is not operated, the ve space signal is sent out by the brush Br4 because until the relay TR is energised, the battery at 54 is not connected to the relays. In such a case, the connection 52 and a separate error-indicating circuit is not required.

In some cases, it may be suillcient to indicate an error by inserting a letter space, that is to say, that whenever a mutilated character is received it appears as a space in which the correct character can be manually written in later; such a Space signal can be retransmitted by providing a second spring tr2, shown in dotted lines, on the test relay TR, which spring has a back contact 92 connected by a conductor 92a to the third retransmitting segment 6 I.

There are three possible sevenunit signals in addition to the 32 shown in Table l; they are:

SMSMSMS signal No. 33 SMSSMSM signal No. 34 MMMSSSS signal No. 35

The three signals, however, cannot readily be sent under control of iive-unit perforated tape because only 32 different five unit signals are possible but these eXtra signals could be sent by providing switches for connecting the segments Si S1 in Figure l to alternative contact levers which may be controlled manually or otherwise according to requirements. The above signal No. 33 or No. 34 may, for example, be used to give instructions to the sending operator of the return circuit, being sent once, twice or another number of times to convey different meanings such as stop, go ahead or repeat last message. The signal may cause a bell to ring.

Thus, as each of the above signals Nos. 33 and 34 has a mark as its second element, the spring o4 of the relay O may connect the conductor 93 through a front contact M to a cut-off relay COR, the circuit being from battery at &3, brush BTB, segment 55, spring trl, contact 53, conductor 66 and through springs t4 or t5, S5 or s6,

r6 or r1, Q6 or q1, conductor 85, conductor 93 and contacts 96, o4 and the Winding of relay COR. The relay COR at its spring cOr2 and back contact 95 cuts off the tive-unit output from the brush Br4 to the line L and at its front contact 96, connects battery to line to give the stop signal. The relay also at spring corl and front contact S1 connects battery to a bell 9S or other convenient indicator. By connecting battery at contact 96 to the line L, the stop signal is prolonged and continuous running of the printer avoided. Clearly, however, it would be possible to provide the relay CORl with a single spring which could cut oil the output from the brush Br4 to the line L and to connect that out put to a connection to the bell or other indicator. For that purpose a conductor 99 from the brush Br4 would be connected to the spring of the relay COR, the back Contact of the spring being connected to line L and its front contact to the bell or other indicator circuit.

If it is desired to use the above two signals No. 33 and No. 34 individually, the springs q6, ql, T5, rl, S5, s6, t4, t5 and their contacts would have to be separated and would be arranged to control separate cut-off relays each providing one of these two signals in a way which is thought to be obvious. If it is desired to utilise the above signal No. 35, it may be received by connecting another cut-off relay, as shown in Figure 2, at COR2, to the spring q5 of the relay Q. Since the first three elements of signal No. 35 are marks, there is a circuit from battery at 42 through spring n2, its front Contact 44, spring o3 and its front contact 48, spring p4 and its front contact IDB and conductor It! to the Winding of the relay COR2.y

The latter, when energised, maintains the battery connection to the line L through its spring 02 and front contact I02a connected to battery. An additional contact spring (not shown) on the relay CCR2 would be used to operate a bell or perform any other function desired.

In the transmitting system shown in Figure 3 in which signals are transmitted in an eight-unit error-detecting code derived from perforated tape perforated with the signals of six-unit primary code, the circuits are simplified with respect to Figure 1 by the use of a number of contact rectiiiers. However, as before, the Derforated tape controls the operation of six peckers Pa., Pb, Pc, Pd, Pe and PZ; each mark in a signal causes the corresponding pecker Pa PZ to close its contacts l, 2 and, as before, to connect battery at 3 to the actuating winding of the corresponding relay A, B, C, D, E and L to earth at 4. Also each of these relays on being actuated closes the circuit of a locking winding battery at 6 through spring al ZI and its front contact 5 to a continuous segment 1 and through a brush BTZ to earth at 8. The peckers pass through the perforations in the tape for a suitable period for example, that represented by X in Figure 3.

The conversion of the signals is set out in Table 2 above from which it will be noted that the rst group of each signal in both codes is the same and contains four signal elements. In the rst, second and third series, this rst group contains respectively three, two and one mark so that for these three series of signals, three, two and one of the relays A D are energised. For any of these signals, battery at |04 is connected to a point |05 through one of three rectiers RI, R2 and R3. Details of the connections are as follows: In the first series where there are three amants marks in the rst group, it Willibe seen that-owing tothe connections of the spring a?V of relay: A, springs b2 and b3 of relay B, spring c2, c3, and'. c4 of relay C and springs d2, d3, d4 and dsof relay D, thevbattery at |04 isconnected to apoint |06 which can be checked byv tracing the circuit when any three of the relays A, B, C and. D are` energised; it will also be seen that battery at |04 is connected to a point |01 When any tvvo of the'l relays AB, C and D are energisedv and,.nally, when only one-of those relays is energised, battery is connected to the point |08. The point I 05 is connected by a conductor |09 to further springs a3, b4, e5 and d6 ofthe relays A, B, C and D so that under any oi these conditions, these springs are connected to battery at |04; thus if any one o r more of the relays A, B, C and D are energised when the signal contains mark elements in thev respective positions according to the first three series of signals in Table 2, those marks are ap'-` plied to the distributor segments SI, S2, S3 or S4.

For the fourth series of signals according to Table 2 above, ity Will be noted that there are no marking units in the first group and then b attery at |04 is connected through the springs of relays A, B, C and D already mentioned, to the point I I and thence through a rectifier R4 to the. distributing segment S2, thus giving a mark as the second. signal element in the rst group. of the eight-unit signal. It will, be observed from Table 2, however, that it is desired to. have a mark as the third` or fourth .element depending on Whetherv the last signal element in the six-.unit primary code is a mark or a space. To produce this result, the relay L has a spring Z3 with a back` contact I connected, directly to the transmitting segment S3 and al front contact I I2 connected directly to the transmitting segment S4'. In the fourth series, the iirst four units of a signal in the primary code are spaces so thaty the relays A, B, C and D are all degenergised. andy as already explained, under those conditions` there is a connection from battery at. |04 to the point IIO and this connection continues through another rectifier R5 to the spring Z3. Consequently, when the last unit of a signal in the fourth series is a mark and the relay L therefore energised this battery lconnection is continued through the spring Z3 and contact II2y to the segment S4 which thus receives `a mark; as4 is: required'. Also, if the last unit in the signal in the primary 6 unit code is a space, so that theV relay L is not energised, the battery connection passes on through the back contact III to the distributor segment S3, resulting in the third unit being a mark in the eight-unit signal which, of course, agrees with 'Table 2. For this series of signals, as there are no marking units in the rst four units, the relays A, B, C, D are all de energised so that none o f the springs a3, b4, c5 and d6 closes its contact.

In the ith series of signals, the rst groupy always consists of four marking units which means that the four relays A, B, C, D are all energised with the vresult that battery at |04 is connected through the spring a2, its iront conftact, the spring b3 and its front contact, the spring c4 and its front contact and the spring d'5 and its front contact toa point IIS from which it is carried on through another rectifier R6 to the rst distributor segment SI. It can be seen from Table 2 thatV in the fth series in the eightunit code, either the thirdor fourth unit has t9 be a mark, depending upon whether the last unit of the, sixfunitsisnalis an space or a mark; thusy battery is` also connected; to the segment S3A or. S4 tol produce this result, as described above for the fourth-series, except that now, battery is connectedto -thespring'lSfromthe point I3. throughl a. further rectiiier R'I.

The secondi group of units in the six-unit primary code is converted. into the second group intheeightfunit error-detectingv code in a manner similar to that described. with reference to Table 1' and Figure 1 for by comparison of Tables 1 andi 2 it will be seen that theconversion is preciselythe same in the'y tWocases. In the present in'- stance, however, the second mark unit to be addeds for'signal's in the second and third series, as compared with signals in theifirst series, is introducedy through thepoint |08 or the point I0'I Which are respectively connected to a point II4 through further rectifiers R8 and R9. For this purposethe-four additional relays F, G, H and K are conn ected exactly as in Figure 1 so as to be controlled by the springs e2 and e3 of the relay E and by the spring Z2 oi the-relay L. Thespring's fI, f2, andf 3 ofthe relay F, the springs gl, g2 and g3 of the relay G, the springs hI', h2 and h3- of the relay and the springs lcI, 1c2and k3 of the relay K are connected to one another exactlyas in Flgure 1 and4 to the distributor segments S5', S6, S1 and Sa. instead of segments S4, S5, S6 and S1, of course. The point I I4 is connected to the springs fl, gl, hl and la! corresponding precisely to the similar connection inFigure 1.

I-n the third series of signals, the third mark unit` has to be added as seen from Table 2, by Way of the point IIIl` through another rectier Ri, tothe point I.I5. A mark. corresponding tor thisy third4 mark is added in the same way for sig,- nals in the. fourth and fifth series but Without the second mark; itis added by Way of connections from points |03, and |I`3 through rectiers RI. I and R12 to `I I5.

A suitable receiving system for the signals produced. as. ldescribed with reference to Figure 3 is illustrated in Figure` 4., The eight-unit signals are received :by the line relay LR as in Figure 2 and, indeed,r the system is similar to that shown lin AFigure- 2 except that there is a certain; amount of simplification obtained `by the use of' rectifiers. The spring Z1: of the line relay LR has a. iront marking contact M and aback spacing contact S, and, as in Figure 2, the marking contact M is connected to a brush BTI which sweeps over .eight short segmentsy SRI SRS, making .contact with them in the middle of the respective periods of reception of the eight signal elements.

`Since .each veight-unit. signal contains four marks unless itv has ,been mutilated, current is passed. through `the windings of four of eight receiving-l relays-N, lO, P, Q, R, S, T and U, the winding of each relay being. connected to one of the, segments RSI RSS and to earth. Thus the f our relays are yenergised by ,the line relay LR in accordance with ,the number of marks in the receivedA signals with which `the brush BTI is driven Ain syriclfnfonsrn by any of the usual means. The four relays thus energised are locked Vup through AContact` springs nl ul ,ea-ch of which is connected' to ltigattery* and through its iront Contact 40' and' a brush Br2 which is'earthed andA werks over a long segment y4I so that the four relays which have :beenenergised are held locked until' the operation of rfa-transmitting the sixcriminels in the primary Code is 'completed- -When `there are thusl four marks in the re ceived eight-unit signal, neither more nor less, a circuit is completed through the test relay TR. The winding of that relay is connected to battery at 62 by a bank of contacts controlled by springs 11,2; o2, o3; p2, p3, p4; q2, Q3, q, Q5; r2, rt, ril, rfi; s2, s3, sil, S5; 2,125, t4 and uit and a3. `This `bant: of springs is similar to that Dro-- vided in Figure 2 but is modified to suit the eightunit signals and, consequently, it is not necessary to describe these contacts in greater detail although it can be seen by tracing the circuits that battery is connected to the winding of relay TR only when exactly four yof the relays N U are energised. Again, when the test relay TR is energis-ed in this way, its spring tr and its back contact 5i disconnect a connection 52 which leads to the circuit of an error-indicator. Connection is made to battery at 5d through brush BTS and a segment 55 but cont-act is only made when the four of the relays N U in question have been locked.

When the test relay TR is thus energised, it connects battery at 54 through its front contact 53 by way of a conductor 51B inter alia to springs n, o5, p5 and q1. These s-prings, through their front contacts 5l, when energised continue the connection from battery to the first four segments Ii, Hl', ||8 and ||9 Iof the six segments |2| of the retransmitting distributor.

The conversion of the last four units of the eight-unit signal into the last two units of the six-unit signal in the primary code is effected in a manner similar to that described with reference to Figure 2 except that in the case of signals in the fourth and fifth series, marking is applied to the sixth retransmitting segment |2| through the Contact spring qi and a rectier R|4 for which purpose the springs r1, r3; s6, sl; t5, t6 and ud and a5 are connected and operated in the same way as the respective four pairs of springs in Figure 2.

In the case of signals in the rst, second and third series, the units in the rst group of the received eight-unit signals are repeated or passed on without change to the retransmitting segments H16, ||8 and H9 by the contact springs 11B, 05, p5 and ql. The battery connection to the springs o5, p5 and q1 from the conductor 55 passes through the spring y and back contact |22 of an additional relay Y which is de-energised by the spring 115 of the relay N when the iirst unit of the received signal is a mark.

However, in the case of signals of the fourth series, the relay Y is operated because, as can be seen from Table 2, in that series, the iirst unit of a signal in the eight-unit code is a space and therefore the spring 115 rests on its back contact |23 and as the second group of the eight-unit signals in that series is either MSMS or SMSM, battery is connected through conductor 65 and the springs ud, a5; t5, t6; s6, s1; and r1, T8, to the conductor 85 and winding of relay Y. As a result, battery from conductor 55 is cut oit from the springs o5, p5 and ql at the back contact H22 of relay Y and, since the connection to the first segment |6 is interrupted between the spring 116 and its front contact 51, the four retransmitting segments H6, ||'i, ||8 and ||9 all receive spaces which, of course, agrees with the rst column in Table 2 above.

In the 5th series of signals, the rst unit of the received eight-unit signal is always a mark so that the relay N is energised and as the conductor 85 is connected to battery for al1 these signals for the reason mentioned above, the spring 115 at its front contact |24 applies marking to the segments ||8 and H9 through rectiers RIS, Riti, RH respectively and the iirst segment Ii' receives a mark through the spring ne and its front contact 51 so that the first four units are all marks in agreement with Table 2.

There are siX possible eight-unit signals in addition to the 61|. shown in Table 2; they are:

MMMM SSSS signal No. 65 SSSS MMMM signal No. 66 MMSS MSMS signal No. 67 MMSS SMSM signal No. 68 SSMM MSMS signal No. 69 SSMM SMSM signal No. 70

As explained in connection with Figure 2, these signals cannot readily be sent under control of six-unit perforated tape because only 64 different six-unit signals are possible, However, in Figure 4, two cut-oir relays COA and COB are provided which are arranged to be operated only by signals Nos. 67 and 68 and Nos. 69 and '70 respectively. Thus signals 6l! and 68 both have their rst two units as marks so that relays N and O are both energised. It will be recollected that because of the form of the second group of these two signals, there will be a connection from battery to the conductor B5 and this connection is continued through the spring o4 of relay O and its front contact |25 and the iront contact |25 of the spring w1 of the relay N to the winding of relay COA and to earth, so that as already stated, the relay COA is then energised. In a similar way, for signals B9 and Til, the first two units are spaces so that the relays N and O are not energised. Thus, under these conditions, the connection to battery at ccnductor is continued through the spring o4, its back contact |21 and through the back contact |28 and its spring 'n3 to the winding of relay COB and to earth so that now the relay COB is energised. As illustrated, the signals 61 and 68 are sent by connecting battery at |29 by means of the spring c0112 to the conductor |35 and the signals BQ and 1|) are sent by connecting battery at |3| to the spring cob2 and conductor |32. When either of these two relays is operated, the normal output in the six-unit primary code to the line L is cut oi, either at the back contact of spring coal or the back contact of spring cobl and either of those springs at its front contact |33 or |34, connects battery to the line L providing the stop signal for the printer. Any one of these four signals may be used to signal any special function, such as has already been described with reference to Figure 2.

In the two transmitting and receiving systems described as examples, generally speaking, the necessary connections have been made and interrupted by relays and their contact springs but these may be replaced by any recognised equivalent devices for producing the same eiects such as thermionic valves which are rendered conductive and non-conductive under desired conditions.

I claim:

l. A telegraph system arranged to transmit signals in an error-detecting code all the signals of which consist of the same number of marking and the same number of spacing units, comprising transmitting apparatus arranged to convert signals in a primary equal-length code into signals in said equal-length error-detecting code, said second-mentioned signals consisting of a greater number of units than the signals in said primary code, Said transmitting apparatus comprising means for passing forward and transmitting one group of units in each of at least the majority of the signals in said primary code, as a group of units in said error-detecting code which is the same in marking and spacing permutation as the group in said primary code and means for modifying a second group of units in each signal in said primary code and converting same into a group in said error-detecting code determined by the permutations of both groups of the respective signal in said primary code.

2. A telegraph system according to claim 1, in which said modifying means convert a second group in each signal in said primary code containing only two units thus providing four permutations, into groups of units in said error-detecting code containing the necessary marking units to make up the predetermined number of marking units in each signal in said error-detecting code in consideration of the number of marking units in the first group of each signal in said error-detecting code.

3. A telegraph system according to claim 2, in which said transmitting apparatus comprises means for -producing signals in said error-detecting code in a number of series, according to which said second groups of units in the signals contain a single marking unit in successive positions, contain a second marking unit in the position follow. ing the first marking unit and also contain a third marking unit in the position following the second marking unit respectively.

4. A telegraph system according to claim 1, comprising relays controlled in accordance with the signal units in said primary code and a transmitting commutator connected to said relays so as to receive the first group of units of signals in said primary code unchanged at least for the majority of the signals.

5. A telegraph system according to claim 4, comprising means for transmitting an additional series of signals in said primary code and means for converting the first group of each of said additional signals into a form depending upon Whether the last unit of each said signals in the primary code is a marking or a spacing unit and comprising modifying relays controlled by contacts of said relay which is controlled in accordance with the last unit of the signal in said primary code.

6. A telegraph system according to claim 5, comprising transmitting apparatus arranged to convert a second group of signal units in said primary code each consisting of two units, into groups each consisting of four units in said errordetecting code, said two relays controlled in accordance with the units of the second groups of units in said primary code being arranged to control four additional relays connected to apply the second group of units in said error-detecting code to the segments of said transmitting commutator.

7. A telegraph system according to claim 1, comprising receiving apparatus for handling the received signals in said error-detecting code and comprisin-g means for passing on, at least for the majority of signals in said error-detecting code, the first group of signal units unchanged but for reconverting the second group of units in each of said signals into a second group of signal units in the original form in said primary code.

8. A telegraph system according to claim 7, comprising receiving relays with means for applying the signal units in said error-detecting code to said receiving relays in succession, said relays having a bank of interconnected contacts, a testing relay so connected to said bank of contacts that when a received signal contains the predetermined number of marking units, no more and no less, said testing relay is energised through said bank of contacts and only when energised, allows the incoming signal to be set up by connecting battery to the appropriate relay contacts after said receiving relays have all been influenced by said incoming signal units.

9. A telgeraph system according to claim 7, comprising receiving relays connected so as to be responsive to the respective units of the incoming signals in said error-detecting code, and a set of additional relays connected to set up the second group of signal units as reconverted into said primary code and also connected to said receiving relays corresponding to the signal units in the second group so as to be controlled thereby.

HARRY VERNON HIGGITT.

No references cited.

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