From 26d783ab6503c396d997cbecf52eefc379843223 Mon Sep 17 00:00:00 2001
From: Chris Tacke <ctacke@opennetcf.com>
Date: Thu, 12 Oct 2023 09:52:51 -0500
Subject: [PATCH] improving error handling - chasing a customer issue

---
 src/Meadow.Modbus/Clients/ModbusRtuClient.cs | 29 +++++++++++++++-----
 src/Meadow.Modbus/Exceptions.cs              | 20 ++++++++++++--
 2 files changed, 40 insertions(+), 9 deletions(-)

diff --git a/src/Meadow.Modbus/Clients/ModbusRtuClient.cs b/src/Meadow.Modbus/Clients/ModbusRtuClient.cs
index ad53cd8..f993ff5 100644
--- a/src/Meadow.Modbus/Clients/ModbusRtuClient.cs
+++ b/src/Meadow.Modbus/Clients/ModbusRtuClient.cs
@@ -101,7 +101,11 @@ protected override async Task<byte[]> ReadResult(ModbusFunction function)
         var header = new byte[headerLen];
 
         // first read 3 bytes so we can look for an error
-        _port.Read(header, 0, 3);
+        var read = 0;
+        while (read < 3)
+        {
+            read += _port.Read(header, read, 3 - read);
+        }
 
         // check for an error bit (MSB in byte 2)
         if ((header[1] & 0x80) != 0)
@@ -109,14 +113,22 @@ protected override async Task<byte[]> ReadResult(ModbusFunction function)
             // an error response has come back - read the remaining 2 bytes (CRC of the error)
             var errpacket = new byte[5];
             Array.Copy(header, 0, errpacket, 0, 3);
-            _port.Read(errpacket, 3, 2);
 
-            expectedCrc = RtuHelpers.Crc(errpacket, 0, errpacket.Length - 2);
-            actualCrc = (ushort)(errpacket[errpacket.Length - 2] | errpacket[errpacket.Length - 1] << 8);
-            if (expectedCrc != actualCrc) { throw new CrcException(); }
+            read = 0;
+            while (read < 2)
+            {
+                _port.Read(errpacket, 3 + read, 2 - read);
+            }
 
             var errorCode = (ModbusErrorCode)errpacket[2];
 
+            expectedCrc = RtuHelpers.Crc(errpacket, 0, errpacket.Length - 2);
+            actualCrc = (ushort)(errpacket[errpacket.Length - 2] | errpacket[errpacket.Length - 1] << 8);
+            if (expectedCrc != actualCrc)
+            {
+                throw new CrcException($"CRC error in {errorCode} message", expectedCrc, actualCrc, errpacket);
+            }
+
             throw new ModbusException(errorCode, function);
         }
 
@@ -156,7 +168,7 @@ protected override async Task<byte[]> ReadResult(ModbusFunction function)
         // the CRC includes the header, so we need those in the buffer
         Array.Copy(header, buffer, headerLen);
 
-        var read = headerLen;
+        read = headerLen;
         while (read < buffer.Length)
         {
             read += _port.Read(buffer, read, buffer.Length - read);
@@ -165,7 +177,10 @@ protected override async Task<byte[]> ReadResult(ModbusFunction function)
         // do a CRC on all but the last 2 bytes, then see if that matches the last 2
         expectedCrc = RtuHelpers.Crc(buffer, 0, buffer.Length - 2);
         actualCrc = (ushort)(buffer[buffer.Length - 2] | buffer[buffer.Length - 1] << 8);
-        if (expectedCrc != actualCrc) { throw new CrcException(); }
+        if (expectedCrc != actualCrc)
+        {
+            throw new CrcException("CRC error in response message", expectedCrc, actualCrc, buffer);
+        }
 
         if (resultLen == 0)
         {   //happens on write multiples
diff --git a/src/Meadow.Modbus/Exceptions.cs b/src/Meadow.Modbus/Exceptions.cs
index 6d93251..1b25918 100644
--- a/src/Meadow.Modbus/Exceptions.cs
+++ b/src/Meadow.Modbus/Exceptions.cs
@@ -9,12 +9,28 @@ namespace Meadow.Modbus;
 /// </summary>
 public class CrcException : Exception
 {
+    /// <summary>
+    /// The expected CRC value
+    /// </summary>
+    public ushort ExpectedCrc { get; } = 0;
+    /// <summary>
+    /// The calculated CRC
+    /// </summary>
+    public ushort ActualCrc { get; } = 0;
+    /// <summary>
+    /// The message failing the CRC check
+    /// </summary>
+    public byte[] MessageBytes { get; } = default!;
+
     /// <summary>
     /// Initializes a new instance of the <see cref="CrcException"/> class with a default error message.
     /// </summary>
-    internal CrcException()
-        : base("CRC Failure")
+    internal CrcException(string message, ushort expectedCrc, ushort actualCrc, byte[] messageBytes)
+        : base(message)
     {
+        ExpectedCrc = expectedCrc;
+        ActualCrc = actualCrc;
+        MessageBytes = messageBytes;
     }
 }