diff --git a/ports/nrf/boards/PCA10028/mpconfigboard.h b/ports/nrf/boards/PCA10028/mpconfigboard.h
index 3dc8ed3459b53..df2e4e85d931f 100644
--- a/ports/nrf/boards/PCA10028/mpconfigboard.h
+++ b/ports/nrf/boards/PCA10028/mpconfigboard.h
@@ -60,3 +60,8 @@
 #define MICROPY_HW_SPI0_MISO        (28)
 
 #define HELP_TEXT_BOARD_LED         "1,2,3,4"
+
+// The JLink CDC on the PCA10028 cannot accept more than 64 incoming bytes at a time.
+// That makes the UART REPL unreliable in general.  But it can be improved to some
+// extent by setting the raw-paste buffer size to that limit of 64.
+#define MICROPY_REPL_STDIN_BUFFER_MAX (64)
diff --git a/ports/nrf/boards/PCA10040/mpconfigboard.h b/ports/nrf/boards/PCA10040/mpconfigboard.h
index b965bf319d082..2b1c4c7ef5cfb 100644
--- a/ports/nrf/boards/PCA10040/mpconfigboard.h
+++ b/ports/nrf/boards/PCA10040/mpconfigboard.h
@@ -64,3 +64,8 @@
 #define MICROPY_HW_PWM2_NAME        "PWM2"
 
 #define HELP_TEXT_BOARD_LED         "1,2,3,4"
+
+// The JLink CDC on the PCA10040 cannot accept more than 64 incoming bytes at a time.
+// That makes the UART REPL unreliable in general.  But it can be improved to some
+// extent by setting the raw-paste buffer size to that limit of 64.
+#define MICROPY_REPL_STDIN_BUFFER_MAX (64)
diff --git a/ports/nrf/main.c b/ports/nrf/main.c
index 784a031ce29bb..21a71c7c9e48c 100644
--- a/ports/nrf/main.c
+++ b/ports/nrf/main.c
@@ -40,6 +40,7 @@
 #include "py/gc.h"
 #include "py/compile.h"
 #include "py/persistentcode.h"
+#include "extmod/misc.h"
 #include "extmod/modmachine.h"
 #include "shared/runtime/pyexec.h"
 #include "readline.h"
@@ -172,7 +173,8 @@ void MP_NORETURN _start(void) {
             MP_OBJ_NEW_SMALL_INT(MICROPY_HW_UART_REPL),
             MP_OBJ_NEW_SMALL_INT(MICROPY_HW_UART_REPL_BAUD),
         };
-        MP_STATE_VM(dupterm_objs[0]) = MP_OBJ_TYPE_GET_SLOT(&machine_uart_type, make_new)((mp_obj_t)&machine_uart_type, MP_ARRAY_SIZE(args), 0, args);
+        mp_obj_t uart = MP_OBJ_TYPE_GET_SLOT(&machine_uart_type, make_new)((mp_obj_t)&machine_uart_type, MP_ARRAY_SIZE(args), 0, args);
+        mp_os_dupterm_obj.fun.var(1, &uart);
     }
     #endif
 
diff --git a/ports/nrf/modules/machine/uart.c b/ports/nrf/modules/machine/uart.c
index 8d5a73e095f5e..4c75bf8209830 100644
--- a/ports/nrf/modules/machine/uart.c
+++ b/ports/nrf/modules/machine/uart.c
@@ -46,6 +46,11 @@
 #include "nrfx_uarte.h"
 #endif
 
+#if defined(NRF52832)
+// The nRF52832 cannot write more than 255 bytes at a time.
+#define UART_MAX_TX_CHUNK (255)
+#endif
+
 typedef struct _machine_uart_buf_t {
     uint8_t tx_buf[1];
     uint8_t rx_buf[1];
@@ -104,6 +109,7 @@ typedef struct _machine_uart_obj_t {
     uint16_t timeout_char;  // timeout waiting between chars (in ms)
     uint8_t uart_id;
     bool initialized;       // static flag. Initialized to False
+    bool attached_to_repl;
     #if MICROPY_PY_MACHINE_UART_IRQ
     uint16_t mp_irq_trigger;   // user IRQ trigger mask
     uint16_t mp_irq_flags;     // user IRQ active IRQ flags
@@ -118,6 +124,13 @@ static machine_uart_obj_t machine_uart_obj[] = {
 };
 
 void uart_init0(void) {
+    for (int i = 0; i < MP_ARRAY_SIZE(machine_uart_obj); i++) {
+        machine_uart_obj[i].attached_to_repl = false;
+    }
+}
+
+void uart_attach_to_repl(machine_uart_obj_t *self, bool attached) {
+    self->attached_to_repl = attached;
 }
 
 static int uart_find(mp_obj_t id) {
@@ -137,14 +150,16 @@ static void uart_event_handler(nrfx_uart_event_t const *p_event, void *p_context
     if (p_event->type == NRFX_UART_EVT_RX_DONE) {
         nrfx_uart_rx(self->p_uart, &self->buf.rx_buf[0], 1);
         int chr = self->buf.rx_buf[0];
-        #if !MICROPY_PY_BLE_NUS && MICROPY_KBD_EXCEPTION
-        if (chr == mp_interrupt_char) {
-            self->buf.rx_ringbuf.iget = 0;
-            self->buf.rx_ringbuf.iput = 0;
-            mp_sched_keyboard_interrupt();
-        } else
-        #endif
-        {
+        if (self->attached_to_repl) {
+            #if MICROPY_KBD_EXCEPTION
+            if (chr == mp_interrupt_char) {
+                mp_sched_keyboard_interrupt();
+            } else
+            #endif
+            {
+                ringbuf_put((ringbuf_t *)&stdin_ringbuf, chr);
+            }
+        } else {
             ringbuf_put((ringbuf_t *)&self->buf.rx_ringbuf, chr);
         }
         #if MICROPY_PY_MACHINE_UART_IRQ
@@ -446,17 +461,29 @@ static mp_uint_t mp_machine_uart_write(mp_obj_t self_in, const void *buf, mp_uin
     #endif
 
     machine_uart_obj_t *self = self_in;
-    nrfx_err_t err = nrfx_uart_tx(self->p_uart, buf, size);
-    if (err == NRFX_SUCCESS) {
+
+    // Send data out, in chunks if needed.
+    mp_uint_t remaining = size;
+    while (remaining) {
+        #ifdef UART_MAX_TX_CHUNK
+        mp_uint_t chunk = MIN(UART_MAX_TX_CHUNK, remaining);
+        #else
+        mp_uint_t chunk = remaining;
+        #endif
+        nrfx_err_t err = nrfx_uart_tx(self->p_uart, buf, chunk);
+        if (err != NRFX_SUCCESS) {
+            *errcode = mp_hal_status_to_errno_table[err];
+            return MP_STREAM_ERROR;
+        }
         while (nrfx_uart_tx_in_progress(self->p_uart)) {
             MICROPY_EVENT_POLL_HOOK;
         }
-        // return number of bytes written
-        return size;
-    } else {
-        *errcode = mp_hal_status_to_errno_table[err];
-        return MP_STREAM_ERROR;
+        buf += chunk;
+        remaining -= chunk;
     }
+
+    // return number of bytes written
+    return size;
 }
 
 static mp_uint_t mp_machine_uart_ioctl(mp_obj_t self_in, mp_uint_t request, uintptr_t arg, int *errcode) {
diff --git a/ports/nrf/modules/machine/uart.h b/ports/nrf/modules/machine/uart.h
index 741473ab7a4e3..85c2092458541 100644
--- a/ports/nrf/modules/machine/uart.h
+++ b/ports/nrf/modules/machine/uart.h
@@ -34,8 +34,7 @@
 typedef struct _machine_uart_obj_t machine_uart_obj_t;
 
 void uart_init0(void);
-void uart_deinit(void);
-void uart_irq_handler(mp_uint_t uart_id);
+void uart_attach_to_repl(machine_uart_obj_t *self, bool attached);
 
 bool uart_rx_any(machine_uart_obj_t *uart_obj);
 int uart_rx_char(machine_uart_obj_t *uart_obj);
diff --git a/ports/nrf/modules/os/modos.c b/ports/nrf/modules/os/modos.c
index f000e1eeb6457..97ed1e1badbbc 100644
--- a/ports/nrf/modules/os/modos.c
+++ b/ports/nrf/modules/os/modos.c
@@ -30,6 +30,7 @@
 #include "py/runtime.h"
 #include "extmod/modmachine.h"
 #include "drivers/rng.h"
+#include "modules/machine/uart.h"
 
 #if MICROPY_PY_OS_URANDOM
 // Return a bytes object with n random bytes, generated by the hardware random number generator.
@@ -46,10 +47,17 @@ static MP_DEFINE_CONST_FUN_OBJ_1(mp_os_urandom_obj, mp_os_urandom);
 #endif
 
 #if MICROPY_PY_OS_DUPTERM
-// TODO should accept any object with read/write methods.
 void mp_os_dupterm_stream_detached_attached(mp_obj_t stream_detached, mp_obj_t stream_attached) {
-    if (!(stream_attached == mp_const_none || mp_obj_get_type(stream_attached) == &machine_uart_type)) {
-        mp_raise_ValueError(MP_ERROR_TEXT("need a UART object"));
+    #if MICROPY_PY_MACHINE_UART
+    if (mp_obj_get_type(stream_detached) == &machine_uart_type) {
+        uart_attach_to_repl(MP_OBJ_TO_PTR(stream_detached), false);
     }
+    #endif
+
+    #if MICROPY_PY_MACHINE_UART
+    if (mp_obj_get_type(stream_attached) == &machine_uart_type) {
+        uart_attach_to_repl(MP_OBJ_TO_PTR(stream_attached), true);
+    }
+    #endif
 }
 #endif // MICROPY_PY_OS_DUPTERM