From 2926a66fe0112900aab2b2d5e371581a3892920f Mon Sep 17 00:00:00 2001
From: JeroenSt <nospam@nospam.org>
Date: Sat, 27 Aug 2022 09:11:36 +0200
Subject: [PATCH] Added returning of modbus error codes to TCP client
 https://github.com/arendst/Tasmota/issues/9586#issuecomment-1225437630

---
 .../xdrv_63_modbus_bridge.ino                 | 24 ++++++++++++-------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/tasmota/tasmota_xdrv_driver/xdrv_63_modbus_bridge.ino b/tasmota/tasmota_xdrv_driver/xdrv_63_modbus_bridge.ino
index 208c1f1ac..5d5ac8f5d 100644
--- a/tasmota/tasmota_xdrv_driver/xdrv_63_modbus_bridge.ino
+++ b/tasmota/tasmota_xdrv_driver/xdrv_63_modbus_bridge.ino
@@ -240,13 +240,6 @@ void ModbusBridgeHandle(void)
     buffer = (uint8_t *)malloc(9 + (modbusBridge.dataCount * 2)); // Addres(1), Function(1), Length(1), Data(1..n), CRC(2)
     uint32_t error = tasmotaModbus->ReceiveBuffer(buffer, modbusBridge.dataCount);
 
-    if (error)
-    {
-      AddLog(LOG_LEVEL_DEBUG, PSTR("MBS: MBR Driver receive error %d"), error);
-      free(buffer);
-      return;
-    }
-
 #ifdef USE_MODBUS_BRIDGE_TCP
     for (uint32_t i = 0; i < nitems(modbusBridgeTCP.client_tcp); i++)
     {
@@ -261,7 +254,15 @@ void ModbusBridgeHandle(void)
         header[3] = 0;
         header[6] = buffer[0]; // Send slave address
         header[7] = buffer[1]; // Send function code
-        if (buffer[1] <= 4) 
+        if (error)
+        {
+          header[4] = 0; // Message Length Hi-Byte
+          header[5] = 3; // Message Length Low-Byte
+          header[8] = error;
+          nrOfBytes += 1;
+          client.write(header, 9);
+        }
+        else if (buffer[1] <= 4) 
         {
           header[4] = ((modbusBridge.dataCount * 2) + 3) >> 8;
           header[5] = (modbusBridge.dataCount * 2) + 3;
@@ -285,6 +286,13 @@ void ModbusBridgeHandle(void)
     }
 #endif
 
+    if (error)
+    {
+      AddLog(LOG_LEVEL_DEBUG, PSTR("MBS: MBR Driver receive error %d"), error);
+      free(buffer);
+      return;
+    }
+
     ModbusBridgeError errorcode = ModbusBridgeError::noerror;
     if (modbusBridge.deviceAddress == 0)
     {