diff --git a/port/api/u_port_uart.h b/port/api/u_port_uart.h
index 7dbcf513..891ba28a 100644
--- a/port/api/u_port_uart.h
+++ b/port/api/u_port_uart.h
@@ -38,6 +38,19 @@ extern "C" {
 # define U_PORT_UART_EVENT_QUEUE_SIZE 20
 #endif
 
+#ifndef U_PORT_UART_WRITE_TIMEOUT_MS
+/** uPortUartWrite() should always succeed in sending all characters;
+ * however, when flow control is enabled, it is possible that the
+ * receiving UART at the far end blocks transmission, potentially
+ * indefinitely, causing uPortUartWrite() to hang.  It is not
+ * desirable to cause the whole application to fail because of an
+ * IO function; this [deliberately very large] defensive time-out
+ * may be employed by an implementation of uPortUartWrite()
+ * as a guard against that.
+ */
+# define U_PORT_UART_WRITE_TIMEOUT_MS 30000
+#endif
+
 /** The event which means that received data is available; this
  * will be sent if the receive buffer goes from empty to containing
  * one or more bytes of received data. It is used as a bit-mask.
diff --git a/port/platform/stm32cube/src/u_port_uart.c b/port/platform/stm32cube/src/u_port_uart.c
index f2e55ba0..78459ff0 100644
--- a/port/platform/stm32cube/src/u_port_uart.c
+++ b/port/platform/stm32cube/src/u_port_uart.c
@@ -1307,23 +1307,25 @@ int32_t uPortUartWrite(int32_t handle,
                        const void *pBuffer,
                        size_t sizeBytes)
 {
+    int32_t sizeOrErrorCode = (int32_t) U_ERROR_COMMON_NOT_INITIALISED;
     const uint8_t *pDataPtr = pBuffer;
-    uErrorCode_t sizeOrErrorCode = U_ERROR_COMMON_NOT_INITIALISED;
     uPortUartData_t *pUartData;
     USART_TypeDef *pReg;
+    bool txOk = true;
+    int64_t startTimeMs;
 
     if (gMutex != NULL) {
 
         U_PORT_MUTEX_LOCK(gMutex);
 
-        sizeOrErrorCode = U_ERROR_COMMON_INVALID_PARAMETER;
+        sizeOrErrorCode = (int32_t) U_ERROR_COMMON_INVALID_PARAMETER;
         pUartData = pGetUartDataByHandle(handle);
         if (pUartData != NULL) {
             pReg = gUartCfg[pUartData->uart].pReg;
-            sizeOrErrorCode = sizeBytes;
-
             // Do the blocking send
-            while (sizeBytes > 0) {
+            sizeOrErrorCode = (int32_t) sizeBytes;
+            startTimeMs = uPortGetTickTimeMs();
+            while ((sizeBytes > 0) && (txOk)) {
                 LL_USART_TransmitData8(pReg, *pDataPtr);
                 // Hint when debugging: if your code stops dead here
                 // it is because the CTS line of this MCU's UART HW
@@ -1333,17 +1335,24 @@ int32_t uPortUartWrite(int32_t handle,
                 // it or the CTS pin when configuring this UART
                 // was wrong and it's not connected to the right
                 // thing.
-                while (!LL_USART_IsActiveFlag_TXE(pReg)) {}
-                pDataPtr++;
-                sizeBytes--;
+                while (!(txOk = LL_USART_IsActiveFlag_TXE(pReg)) &&
+                       (uPortGetTickTimeMs() - startTimeMs < U_PORT_UART_WRITE_TIMEOUT_MS)) {}
+                if (txOk) {
+                    pDataPtr++;
+                    sizeBytes--;
+                }
             }
-            while (!LL_USART_IsActiveFlag_TC(pReg)) {}
+            // Wait for transmission to complete so that we don't
+            // write over stuff the next time
+            while (!LL_USART_IsActiveFlag_TC(pReg) &&
+                   (uPortGetTickTimeMs() - startTimeMs < U_PORT_UART_WRITE_TIMEOUT_MS)) {}
+            sizeOrErrorCode -= (int32_t) sizeBytes;
         }
 
         U_PORT_MUTEX_UNLOCK(gMutex);
     }
 
-    return (int32_t) sizeOrErrorCode;
+    return sizeOrErrorCode;
 }
 
 // Set an event callback.