DE19955330A1 - Data transmission between field bus and communications network - Google Patents

Abstract

The method involves transmitting data from a field bus (1), especially a PROFIBUS, to a network, on which cells with a fixed length are transferred, especially an ATM network (3), whereby the data transmission speed on the field bus is lower than on the network. A field bus message is divided at a segmentation in several partial packets, which respectively fill a cell only partially. The size of the partial packets is preferably predetermined in such way, that the time which is required by the field bus for a reception of a partial packet, is longer than the time necessary for transmitting the packet to the network.

Description

The invention relates to a method for the transmission of Data from a fieldbus, in particular a fieldbus after the PROFIBUS specification, on a network on which cells with fixed length are transmitted, especially to an ATM Network, according to the preamble of claim 1 and an ent speaking coupling device according to the preamble of claim 5.

For example from the article "High-speed networks in automation: ATM "by Christian Cseh and Jürgen Jasperneite, published in etz, issue 6/1999, Pages 6 to 9, it is known that high speed networks such as ATM networks, Fast and Gigabit Ethernet in automation can be used. Such high speed networks allow in addition to the classic requirements of Automation systems multimedia applications, for example as the transmission of audio and video data. In addition to the Performance parameters, services and functions are important which is a communication system in automation technology at the interface to the application or a service user he brings. These services include the transfer of data in Real time or with guaranteed time limits for tax and Control tasks and for example a data transfer without certain quality of service parameters, such as those used when sending Be drive or quality data for statistical evaluations or archiving occurs.

Between different communication systems, for example between a fieldbus and an ATM network, must be coupling devices exist. On the one hand, these are necessary in order to be Management level to be able to access data from the lower levels second, to establish a connection between different ones to create fieldbuses via an ATM network.  

Asynchronous transfer mode (ATM) is a mostly connection-oriented technology for the transmission of Fixed-size packets, so-called ATM cells. The ATM technology enables data of an automation technology application and multimedia data with strict real-time requirements or a high bandwidth requirement almost simultaneously to send over a network. The advantages of this procedure lie in the integration of data, voice, image and video transmission, in the assurance of certain quality features for the requested transmission route and in the Ska allocability of the allocated bandwidth. ATM is based on asynchronous time multiplexing of ATM cells of a fixed Cell size of 53 bytes.

When transmitting fieldbus telegrams, e.g. B. PROFIBUS Telegrams over an ATM network result despite the high Transmission rate of the ATM network, e.g. B. 155 Mbps or 622 Mbit / s, large delay times. Doing so of segmentation delays because of the amount of data that required to fill an ATM cell, completely ver The cell must be able to be added before segmentation ends provided in a buffer and for transmission on the ATM network can be registered. There is a long data packet whose volume exceeds the filling quantity of a cell, several cells have to be formed and buffered. Next there are waiting times at the switches of the ATM network, because the cells according to the ordered quality of service, i.e. H. according to agreed priority and bandwidth, different quickly into the cell grid of the time-multiplexed transmission be incorporated. Outgoing side, d. H. in the paddock device in which received from the ATM network Cells a fieldbus telegram must be restored all cells of a message are available to the telegram with a constant bitrate on the connected to be able to deliver its fieldbus. With an acknowledged data transfer results for the ant running in the opposite direction  word telegram also such a delay in the Conversion.

Monitoring a complete message cycle, the consists of a request and a response telegram, can be done by examining a so-called slot time that specifies the time within which the response telegram follows the sending of a request telegram must arrive. If the slot time is not observed, the night applies straightening cycle as disturbed. When configuring the slot Time must be the delay times involved in the conversion arise, are sufficiently taken into account. For quick Applications that require a short re action time, but would have to be a very short slot time be set due to the delay times that for data transmission via fieldbus with intermediate tetem ATM network arise, despite the large transport capacity of the ATM network could no longer be met.

The invention has for its object a method for Transfer of data from a fieldbus to an ATM network find that an improved transmission speed enables. A corresponding coupling device is also required create.

To solve this problem, the new method of gangs mentioned in the characterizing part of the An pronounced 1 features and a corresponding coupling gets the features of the characterizing part of claim 5 on. Advantageous further developments are in the subclaims described the invention.

The invention has the advantage that a faster segment of the fieldbus telegram reached can be, if the incoming serial data in the Segmentation layer broken down into smaller subpackages and the cells are therefore not completely filled. The  ATM cells are filled with smaller amounts in the ATM network sent out. This will significantly reduce the Transmission delay in segmentation reached. A Transmission of fieldbus telegrams across an ATM network is possible with a higher transmission speed overall Lich and a shorter slot time can be set, so that short response times, as with fast real time applications are required, are adjustable. Through the Invention will expand the scope of ATM Networks also achieved on the process-related area. It does not apply the waiting time for the complete arrival of one under Um long fieldbus telegram before segmentation started and the subsequent transmission of the ATM cells can be initiated.

If the size of the subpackages is specified such that the Time it takes to receive a partial packet from the fieldbus is longer than that for sending the subpacket to the ATM network time required, will be advantageous prevents a jam when sending the ATM cells on the ATM network is created. The deployed after segmentation Cell contents can be accessed directly from the ATM network are given.

In addition to the characters that are supposed to be transmitted present information, can also Ele elements with accompanying fieldbus-specific information, such as B. Start, parity or stop bit. So that will a transparent transmission of a fieldbus telegram the ATM network achieved. The telegram can thus be re assembly can be restored unchanged.

In contrast to the AAL5 service, no additional In formation, such as B. the number of data filled Octets in the cell or a CRC check in a trailer of the Cell to be added. The fieldbus telegram must therefore  not fully available before sending out the Cells can be started.

For larger fieldbus telegrams, the amount of data of which is an opti filling capacity of a cell exceeds several ATM Cells at equidistant time intervals with a small part packages filled and prepared for shipping. That has the Advantage that a cell has already been segmented and the partially filled cell to a first-in-first-out Send buffers of the ATM access are already passed on can, as soon as the respective opti times the amount of data was received. Will a cell with low Data volume filled as the optimal - that is z. B. at the last cell for a fieldbus telegram or for a ATM cell for a short message - the ver remaining rest in a simple manner with a logic level the level of the silence on the ATM network corresponds. The last cell for a field bus grams and also the ATM cell for a short message can in a predetermined octet. Another octet can contain a count number to emp on the upstream side, the availability of all ATM cells used for Transmission of a fieldbus telegram were formed via to be able to check.

Especially if the fieldbus complies with the PROFIBUS specification suffices and the maximum size of the subpackages three characters one PROFIBUS telegram is a fast token circulation guaranteed.

Using the drawings, in which an embodiment of the Invention is shown, the following are the invention as well as configurations and advantages explained in more detail.

Show it:

Fig. 1 is a block diagram of a communication system,

Fig. 2 is a block diagram of a coupling device for Verbin dung a field bus with an ATM network,

Fig. 3 is a timing diagram illustrating the delays in a telegram transmission,

Fig. 4 shows the basic structure of a fieldbus telegram and the associated ATM cells to explain segmentation and

Fig. 5 is a table for comparing the terms.

A communication system for industrial communication has according to FIG. 1 a field bus 1 and a field bus 2, which are interconnected via an ATM network 3. The fieldbus 1 is coupled to the ATM network 3 via a coupling device 4 and a switch 5 , the fieldbus 2 via a coupling device 6 and a switch 7 . At a switch 8 , a host computer 9 of an automation system, not shown, is connected. There are further switches in the ATM network 3 , which are also not shown in FIG. 1 for reasons of clarity. A sensor 10 , a sensor 11 and an actuator 12 are connected to the fieldbus 1 . In a closed control loop, this could be, for example, a flow measuring device, a pressure measuring device or a slide with a positioner. The fieldbus 2 connects a sensor 13 , an actuator 14 and an actuator 15 to one another. In the communication system shown, fieldbus telegrams can be exchanged within the fieldbus 1 between the participants 10 , 11 and 12 and within the fieldbus 2 between the participants 13 , 14 and 15 . In addition, it is possible to transmit fieldbus telegrams, for example, from the subscriber 10 to the subscriber 14 via the fieldbus 1 , the ATM network 3 and the fieldbus 2 . For this purpose, a fieldbus telegram is mapped by the coupling device 4 on ATM cells, from which the fieldbus telegram is in turn restored by the coupling device 6 . In this exemplary embodiment, field buses 1 and 2 meet the PROFIBUS specification. The coupling devices 4 and 6 are each designed as a user network interface (UNI) with a physical PROFIBUS interface on the one hand and a standardized UTOPIA interface on the other. The coupling devices 4 and 6 are designed bidirectionally and each occupy an entry and exit at the nodes of the switches 5 and 7 .

In FIG. 2, the construction of the coupling device is shown in detail 4 and 6. The coupling to a fieldbus 20 takes place via a bus transceiver 21 according to the RS485 specification. This is followed by a circuit 22 for electrical isolation. A retimer 23 takes over the direction-dependent management of the fieldbus access and the data handling. The telegrams received by the fieldbus 20 are stored in a buffer 24 in a byte-oriented manner. After adding a header and a trailer in a conversion layer, they are forwarded to a segmentation layer. The service types AAL3 / 4 and AAL5 can be used in a non-connection-oriented form for the PROFIBUS data structure. The cells now available are stored in a transmit buffer 25 and registered via a UTOPIA interface 26 to the physical layer of the ATM network (not shown in more detail) for further transport. In the opposite direction, the UTOPIA interface 26 stores the incoming cells in a buffer 27 , from where they are processed in the assembly layer. Cell-accompanying information is separated and the user data is extracted in a conversion layer and collected in the buffer 24 . After all cells belonging to a PROFIBUS telegram have arrived, recognizable by the end byte of the PROFIBUS telegram, the data can be sent to the fieldbus 20 in PROFIBUS format.

Two variants are possible for segmentation. With full transparency of the data transmission, the serial bit stream of a fieldbus telegram including start, stop and parity bits is broken up into octets. As a result, an overhead of 27% is also transmitted. In contrast, in a time-effective variant, only the eight user data bits of a character are transmitted after the start, stop and parity bit have been recognized, checked and separated in a UART 70 . In the opposite direction, in the first variant the octets of the cells are sent immediately one after the other, while in the second variant the useful information in the UART 70 is supplemented by start, stop and parity bits. Extensive potential carryover is prevented by the galvanic isolation in the circuit 22 . For example, it can be implemented with optocouplers. The segmentation and reassembly is controlled by a controller 28 , the UTOPIA interface 26 by a controller 29 .

Based on the timing diagram in Fig. 3, which serves only the qualitative explanation and is not to scale, who explains the different delay times that arise in the transmission of a fieldbus telegram over an ATM network to another fieldbus. In the example shown in FIG. 3 above, a subscriber, for example sensor 10 in FIG. 1, starts sending a 100-character PROFIBUS telegram at time t = 0, as indicated by a horizontal bar 30 . The transmission process takes a little more than 700 µs at a transmission rate of 1.5 Mbit / s. A bar 31 shows the time range in which the PROFIBUS telegram is received by, for example, the coupling device 4 , which is connected to the sensor 10 via a 200 m long copper line of the fieldbus 1 . The bar 31 is consequently shifted to the right by approximately 1 μs with respect to the bar 30 . Compared to the other delays, the runtime delay is almost negligible.

In ATM processing, in which the user data are segmented and generated cells are stored in a transmission buffer, a time which corresponds to a bar 32 elapses in the case of segmentation according to AAL5, as is already known. The time for sending three generated cells is represented by three bars 33 , 34 and 35 , respectively. The transmission time over an ATM network cannot be clearly predicted. It depends on the number of nodes to be passed and the dwell time of the individual cells in the cell buffers of the switches to be passed. The waiting time is determined on the one hand by the utilization of the channels and paths, and on the other hand by the agreed services. The runtime on a medium is about 5 µs / km, the switching time through a switch is about 50 µs. Delayed by a transmission time A, the three ATM cells are received by another coupling device connected to the ATM network, for example the coupling device 6 in FIG. 1. The respective reception times are indicated by bars 36 , 37 and 38 in Fig. 3. Upon receipt of the three ATM cells, a reassembly to a PROFIBUS telegram takes place during a time period corresponding to a beam 39 which during a marked by a bar 40 time to the connected to the coupling device fieldbus example as the field bus 2 in Fig. 1, is sent out. The total delay in the transmission of a fieldbus telegram via an ATM network with segmentation according to the known type corresponds to the time interval between the beginning of the bar 30 and the beginning of the bar 40 . In the example shown, it is more than 1000 µs. A response time of 1 ms, which is usually required for real-time automation technology applications, can therefore hardly be met.

For comparison, the delay times for the new type of segmentation are also shown in FIG. 3. It starts again with the transmission of a 100-character fieldbus telegram, the duration of which is represented by the bar 30 '. According to a bar 41 , segmentation can already be started in a coupling device if three characters, which correspond to the optimal filling quantity of an ATM cell, have been received via the fieldbus interface. From the fieldbus telegram with 100 characters, 34 ATM cells, the transmission times of which are marked as bars 42 to 43 , are generated. After a time A, which in turn is required for transmission over the ATM network, the ATM cells according to bars 44 to 45 are received by another coupling device connected to the ATM network. Immediately after receipt of the first ATM cell 44 , a continuous reassembly of the fieldbus telegram is started, which thus takes place over a period of time corresponding to a bar 46 . So that fluctuations in the cell transport time on the ATM network can be compensated, a delay W should be started with the transmission of the fieldbus telegram, which preferably corresponds to the reception time of three cells. The duration of the transmission time is marked with a bar 47 . The delay time, which is caused by the transmission of the fieldbus telegram via the ATM network, can be read from the diagram as the time difference between the beginning of the bar 47 and the beginning of the bar 30 . It is approximately 250 µs. The reduction in the delay time which results from the new type of segmentation compared to a segmentation of a known type corresponds to a time period G of almost 800 microseconds. This illustration clearly shows that the delay time of a fieldbus telegram is significantly influenced by the processing time for segmentation and reassembly. These are the times that have been greatly reduced with the new type of segmentation.

The new type of segmentation for trans parente transmission over the ATM network at a PROFIBUS message is program 50 with reference to Fig. 4, five numeral 51. , , 55 exists. Each character, for example the character 51 , is composed of a start bit 56 , eight data bits 57 , a parity bit 58 and a stop bit 59 . A first ATM cell 60 is formed from the first three characters 51 , 52 and 53 , which are represented with 33 bits and correspond to the optimal fill quantity. Significant octets of the ATM cell 60 as well as a further ATM cell 61 are marked with a consecutive numbering 1 on the lower edge. .53 provided. In the octets 1.. .6 there is a header of the respective ATM cell 60 or 61 , a sequence number being entered in octet 6, which is entered for the consecutive numbering of the ATM cells belonging to a fieldbus telegram. The 33 bits for the representation of the three characters 51 , 52 and 53 completely fill the octets 7, 8, 9 and 10 and the octet 11 partially. The unused rest of octet 11 is supplemented by a replenishment amount 62 , which corresponds to the silence level. The octets 12th .53 of the ATM cell 60 remain empty. In a corresponding manner, 22 bits are written into the octets 7, 8 and 9 of the ATM cell 61 to represent the characters 54 and 55 of the fieldbus telegram 50 . A remaining remainder of octet 9 and octets 10 and 11 are in turn supplemented by a replenishment quantity 63 , octets 12 . , , 53 remain empty. For easier identification of the last transmitted ATM cell, an end identifier 64 can be stored in octet 12 of cell 61 . In this form, ATM cells 60 and 61 are transmitted over the ATM network. In the reverse order, the receiving coupling device from the ATM cells 60 and 61 restores the fieldbus telegram 50 and sends it out to the connected fieldbus. During the reassembly, the cell useful content is separated from the ATM cell, transferred to a transmission shift register and sent to the outgoing fieldbus segment at the transmission side before the given data rate. In the case of long telegrams which are divided into several ATM cells, one should wait until the next ATM cell has arrived before the partial packet of an ATM cell has been completely sent and is ready for transfer to the transmission register. This only causes a delay in the runtime of an ATM cell. For this purpose, a stack register for one or two partial packets should be provided on the output side of the coupling device in order to compensate for a jitter in the transmission of the ATM cells, which can be caused, for example, by different waiting times at the switches. If a cell is filled with only one or two characters, the fill quantity can be transferred to the stack register. Monitoring the sliding mechanism can e.g. B. due to the lack of start bits in a replenishment quantity, the end of the fieldbus program.

The functions of segmentation and reassembly can as a pure hardware solution, for example using an FPGA or EPLD module, by means of a sufficiently fast Microcontrollers with appropriate programming or can be realized as a combination of both.

For the determination of the optimal filling quantity of an ATM cell applies that the transmission time of an ATM cell with that on the ATM Network predetermined data transmission speed lower must be as the send time of the fieldbus data set, the one finally the replenishment amount is written into an ATM cell becomes. You may be able to add additional criteria to the feast definition of the optimal filling quantity from the protocol specifics the different fieldbus systems.

For a fieldbus according to the PROFIBUS specification, this means:
The smallest amount of data to be transported with transparent transmission is a character with 11 bits for the short acknowledgment. The next largest telegram is the token with three characters, the fast passage of which is seen as particularly important. This amount of data is advantageously assumed to be the optimal fill amount of an ATM cell. Larger fieldbus telegrams are divided into several ATM cells and the last ATM cell that belongs to the transmission of a fieldbus telegram is specially marked, for example by the end identifier 64 in FIG. 4.

The table in FIG. 5 shows a comparison of the runtimes of fieldbus telegrams and ATM cells. The respective network is specified in the first line, the data transmission rate in the unit MBit / s in the second line, the number of bits per field bus telegram or ATM cell in the third line and the required runtime in the fourth line.

Only when combining an ATM network with the Transfer rate 51.84 Mbit / s and a PROFIBUS bus with The cell runtime is 12 Mbit / s longer than the runtime for three Character on the PROFIBUS. For all other combinations the running time of the cell is shorter, so that no information jam is to be expected.

Claims (5)

1. Method for the transmission of data from a fieldbus ( 1 ), in particular from PROFIBUS, to a network on which cells with a fixed length are transmitted, in particular to an ATM network ( 3 ), the data transmission speed on the fieldbus ( 1 ) is significantly lower than on the network ( 3 ), characterized in that a fieldbus telegram is divided into several partial packets during segmentation, each of which only partially fills up a cell. 2. The method according to claim 1, characterized in that that the size of the subpackages is specified such that the Time it takes to receive a partial packet from the fieldbus is longer than that for sending the subpacket to the Network time required. 3. The method according to claim 1 or 2, characterized in that in addition to the characters ( 57 ), which actually represent the information to be transmitted, elements ( 56 , 58 , 59 ) with fieldbus-specific accompanying information, such as. B. start, parity or stop bit are transmitted. 4. The method according to any one of the preceding claims, characterized in that the fieldbus ( 1 ) meets the PROFIBUS specification and the maximum size of the partial packets is three characters ( 51 , 52 , 53 ) of a PROFIBUS telegram ( 50 ). 5. Coupling device for the transmission of data from a fieldbus ( 1 ), in particular from PROFIBUS, to a network on which cells with a fixed length are transmitted, in particular to an ATM network ( 3 ), the data transmission speed on the fieldbus ( 1 ) is significantly lower than on the network ( 3 ), characterized in that a segmentation device is designed in such a way that a fieldbus telegram ( 50 ) is divided into several partial packets, each of which only partially fills up a cell ( 60 ).