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This PR improves Modbus RTU master extensibility and broadcast support. #209

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This PR improves Modbus RTU master extensibility and broadcast support. #209

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2 changes: 1 addition & 1 deletion NModbus/IO/EmptyTransport.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -15,7 +15,7 @@ public override byte[] ReadRequest()
throw new NotImplementedException();
}

public override IModbusMessage ReadResponse<T>()
public override IModbusMessage ReadResponse<T>(IModbusMessage request)
{
throw new NotImplementedException();
}
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2 changes: 1 addition & 1 deletion NModbus/IO/ModbusAsciiTransport.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -45,7 +45,7 @@ public override byte[] ReadRequest()
return ReadRequestResponse();
}

public override IModbusMessage ReadResponse<T>()
public override IModbusMessage ReadResponse<T>(IModbusMessage request)
{
return CreateResponse<T>(ReadRequestResponse());
}
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2 changes: 1 addition & 1 deletion NModbus/IO/ModbusIpTransport.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -137,7 +137,7 @@ public override byte[] ReadRequest()
return ReadRequestResponse(StreamResource, Logger);
}

public override IModbusMessage ReadResponse<T>()
public override IModbusMessage ReadResponse<T>(IModbusMessage request)
{
return CreateMessageAndInitializeTransactionId<T>(ReadRequestResponse(StreamResource, Logger));
}
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79 changes: 74 additions & 5 deletions NModbus/IO/ModbusRtuTransport.cs
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Original file line number Diff line number Diff line change
@@ -1,4 +1,5 @@
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
Expand Down Expand Up @@ -87,18 +88,20 @@ public override bool ChecksumsMatch(IModbusMessage message, byte[] messageFrame)
return messageCrc == calculatedCrc;
}

public override IModbusMessage ReadResponse<T>()
public override IModbusMessage ReadResponse<T>(IModbusMessage request)
{
byte[] frame = ReadResponse();
byte[] frame = ReadResponse(request);

Logger.LogFrameRx(frame);

return CreateResponse<T>(frame);
}

private byte[] ReadResponse()
private byte[] ReadResponse(IModbusMessage request)
{
byte[] frameStart = Read(ResponseFrameStartLength);
byte[] frameStart = request != null ?
ReadStartFrameResponse(request) :
Read(ResponseFrameStartLength);
byte[] frameEnd = Read(ResponseBytesToRead(frameStart));
byte[] frame = frameStart.Concat(frameEnd).ToArray();

Expand All @@ -107,7 +110,7 @@ private byte[] ReadResponse()

public override void IgnoreResponse()
{
byte[] frame = ReadResponse();
byte[] frame = ReadResponse(null);

Logger.LogFrameIgnoreRx(frame);
}
Expand All @@ -122,5 +125,71 @@ public override byte[] ReadRequest()

return frame;
}

private byte[] ReadStartFrameResponse(IModbusMessage request)
{
const int HeaderLength = 2;
int frameHeadLength = ResponseFrameStartLength;
byte functionCode = request.FunctionCode;
byte slaveId = request.SlaveAddress;
int maxGarbageBytes = 1024;

byte[] window = new byte[frameHeadLength];
int count = 0;
int garbageCount = 0;

while (true)
{
// Fill the sliding window buffer
while (count < frameHeadLength)
{
byte[] b = Read(1);
window[count++] = b[0];
}

int headIdx = -1;
if (slaveId == 0)
{
// Broadcast request: look for [1-247, functionCode] as valid header
for (int i = 0; i <= frameHeadLength - HeaderLength; i++)
{
if (window[i] >= 1 && window[i] <= 247 && window[i + 1] == functionCode)
{
headIdx = i;
break;
}
}
}
else
{
// Exact match: look for [slaveId, functionCode]
if (window[0] == slaveId && window[1] == functionCode)
headIdx = 0;
}

if (headIdx >= 0)
{
// Extract the frame header starting from headIdx
byte[] frameHead = new byte[frameHeadLength];
Array.Copy(window, headIdx, frameHead, 0, frameHeadLength - headIdx);

// If there was garbage before the header, read extra bytes to complete the header
int bytesNeeded = headIdx;
if (bytesNeeded > 0)
{
byte[] rest = Read(bytesNeeded);
Array.Copy(rest, 0, frameHead, frameHeadLength - bytesNeeded, bytesNeeded);
}
return frameHead;
}

// Slide the window: shift left by one byte, decrease count accordingly
Array.Copy(window, 1, window, 0, frameHeadLength - 1);
count = frameHeadLength - 1;
garbageCount++;
if (garbageCount > maxGarbageBytes)
throw new IOException("Too many garbage bytes, failed to find Modbus RTU frame header.");
}
}
}
}
65 changes: 65 additions & 0 deletions NModbus/IO/ModbusSerialTransport.cs
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Original file line number Diff line number Diff line change
@@ -1,6 +1,7 @@
using System.Diagnostics;
using System.IO;
using NModbus.Logging;
using NModbus.Message;

namespace NModbus.IO
{
Expand Down Expand Up @@ -66,5 +67,69 @@ internal override void OnValidateResponse(IModbusMessage request, IModbusMessage
{
// no-op
}

public override bool ShouldRetryResponse(IModbusMessage request, IModbusMessage response)
{
if (request.FunctionCode != response.FunctionCode)
{
return true;
}

if (request.SlaveAddress != 0)
{
if (response.SlaveAddress != request.SlaveAddress)
{
return true;
}
}
else
{
if (response.SlaveAddress < 1 || response.SlaveAddress > 247)
{
return true;
}
}
return false;
}

public override void ValidateResponse(IModbusMessage request, IModbusMessage response)
{
// always check the function code and slave address, regardless of transport protocol
if (request.FunctionCode != response.FunctionCode)
{
string msg = $"Received response with unexpected Function Code. Expected {request.FunctionCode}, received {response.FunctionCode}.";
throw new IOException(msg);
}

// Validate slave address
if (request.SlaveAddress != 0)
{
// Normal (non-broadcast) request: response address must match and be valid
if (response.SlaveAddress != request.SlaveAddress)
{
string msg = $"Response slave address does not match request. Expected {request.SlaveAddress}, received {response.SlaveAddress}.";
throw new IOException(msg);
}
}
else
{
// Broadcast request: only check that the response slave address is in valid range
if (response.SlaveAddress < 1 || response.SlaveAddress > 247)
{
string msg = $"Response slave address {response.SlaveAddress} is out of valid range (1~247) for a broadcast request.";
throw new IOException(msg);
}
}

// message specific validation
var req = request as IModbusRequest;

if (req != null)
{
req.ValidateResponse(response);
}

OnValidateResponse(request, response);
}
}
}
8 changes: 4 additions & 4 deletions NModbus/IO/ModbusTransport.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -134,7 +134,7 @@ public virtual T UnicastMessage<T>(IModbusMessage message)
do
{
readAgain = false;
response = ReadResponse<T>();
response = ReadResponse<T>(message);
var exceptionResponse = response as SlaveExceptionResponse;

if (exceptionResponse != null)
Expand Down Expand Up @@ -231,7 +231,7 @@ public virtual IModbusMessage CreateResponse<T>(byte[] frame)
return response;
}

public void ValidateResponse(IModbusMessage request, IModbusMessage response)
public virtual void ValidateResponse(IModbusMessage request, IModbusMessage response)
{
// always check the function code and slave address, regardless of transport protocol
if (request.FunctionCode != response.FunctionCode)
Expand Down Expand Up @@ -260,7 +260,7 @@ public void ValidateResponse(IModbusMessage request, IModbusMessage response)
/// <summary>
/// Check whether we need to attempt to read another response before processing it (e.g. response was from previous request)
/// </summary>
public bool ShouldRetryResponse(IModbusMessage request, IModbusMessage response)
public virtual bool ShouldRetryResponse(IModbusMessage request, IModbusMessage response)
{
// These checks are enforced in ValidateRequest, we don't want to retry for these
if (request.FunctionCode != response.FunctionCode)
Expand Down Expand Up @@ -291,7 +291,7 @@ public virtual bool OnShouldRetryResponse(IModbusMessage request, IModbusMessage

public abstract byte[] ReadRequest();

public abstract IModbusMessage ReadResponse<T>()
public abstract IModbusMessage ReadResponse<T>(IModbusMessage request)
where T : IModbusMessage, new();

public abstract byte[] BuildMessageFrame(IModbusMessage message);
Expand Down
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