C# - polling a device by serial port

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Faced a problem. I need to poll the device by serial port using the Modbus protocol.

But polling the device takes a long time - about 2 seconds.

True, my program still additionally constantly polls devices in a separate thread, but I wanted it to be faster.

Maybe someone can help me how to optimize my code. I would be very grateful.

What I have tried:

Baud rate = 19200.This is what my program displays: Request: 07.04.2020 12:51.48.038 01 03 04 00 00 03 04 FB Answer: 07.04.2020 12:51.49.949 (1,911 seconds) 01 03 06 51 47 07 12 20 84 20 29 00

C#

public override RcResult ExecuteModBus(RcModBusDevice device, byte[] request, out byte[] answer)
    {
        answer = null;
        var result = new RcResult();
        OnLogMessage?.Invoke(this, "ExecuteModBus LOCK?");
        lock (communication)
        {
            OnLogMessage?.Invoke(this, "ExecuteModBus LOCK!");
            var dt = DateTime.Now;
            if (device != null)
            {
                serialPort.WriteTimeout = device.SendTimeout;
                serialPort.ReadTimeout = device.ReceiveTimeout;
            }
            serialPort.DiscardOutBuffer();
            serialPort.DiscardInBuffer();

            OnLogMessage?.Invoke(this, "REQUEST->:" + Utils.BytesToHex(request));
            try
            {
                serialPort.Write(request, 0, request.Length);
            }
            catch(Exception ex)
            {
                result.ErrorCode = 1;
                result.ErrorText = ex.Message;
                return result;
            }
            var tmp = new byte[0x2000];
            int dataLength = 0;
            try
            {
                for (int i = 0; i < tmp.Length; i++)
                    tmp[dataLength++] = (byte)serialPort.ReadByte();
            }
            catch (Exception ex)
            {
            }

            var crc = Utils.GetCRC(tmp, 0, dataLength - 2);
            if (crc[0] != tmp[dataLength - 2] || crc[1] != tmp[dataLength - 1])
            {
                result.ErrorCode = 1;
                result.ErrorText = "Bad CRC";
            }
            answer = new byte[dataLength];
            Array.Copy(tmp, 0, answer, 0, dataLength);
            OnLogMessage?.Invoke(this, "ANSWER<-:" + Utils.BytesToHex(answer));
            if (device != null)
                SaveToLog(DbID, device.DbID, dt, Utils.BytesToHex(request), DateTime.Now, Utils.BytesToHex(answer));
            if (dataLength == 0)
                result.ErrorCode = 1;
        }
        OnLogMessage?.Invoke(this, "ExecuteModBus UNLOCK");
        return result;
    }

Posted 20-Apr-20 23:59pm

Demuter

Updated 21-Apr-20 6:33am

v2

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2 solutions

Solution 1

Start by finding out where it's spending it#s time: pepper it with Stopwatch Class (System.Diagnostics) | Microsoft Docs[^] instances and log the various parts of the operation to a file at the end. Then focus on the longest part, and add more Stopwatches to try and narrow down where it's spending it's time.

When you have that, you can start looking at speeding it up, but until you have actual numbers it's all guesswork. When you know, you can focus on the "why is this slow?" - but not until you have data and a reference to check your improvements against.

Posted 21-Apr-20 0:13am

OriginalGriff

Comments

Demuter 21-Apr-20 6:47am

This code runs the longest. I tried to reduce the size of the buffer, but the time did not change ...

var tmp = new byte[0x2000];
                //var tmp = new byte[0x11];
                int dataLength = 0;
                try
                {
                    for (int i = 0; i < tmp.Length; i++)
                        tmp[dataLength++] = (byte)serialPort.ReadByte();
                }
                catch (Exception ex)
                {
                }

[no name] 21-Apr-20 6:51am

What is the value for ReceiveTimeout resp, serialPort.ReadTimeout?

Also I would suggest to log the time for the first byte received. This will show you how long the other part needs to answer your request.

OriginalGriff 21-Apr-20 7:12am

Sensible advice. :thumbsup:

[no name] 21-Apr-20 13:13pm

Thank you :happy:

Demuter 21-Apr-20 7:55am

Value for serialPort.ReadTimeout = 1000.
I made a conclusion as you suggested - the first answer in 0.033 seconds.

[no name] 21-Apr-20 7:57am

Ok, so I would say you spend/loose 1000mS until you stop your loop ;)

What I remember from Modbus is, the end of the transfer is when there is no character received for "at least 3½ character times of silence"

Demuter 21-Apr-20 8:30am

Maybe this will help, in setting up the modem by which I ping the device, I have: Inactivity Timeout=30 seconds...

[no name] 21-Apr-20 10:55am

But I speak from serialPort.ReadTimeout. Set it to e.g. 10, this means 10mS timeout (about 20 chars silent at 19'200) after which your loop stops.
[Edit] Forgot to mention this 10mS should be set after the first character is received.

Btw. I would catch TimeoutException explicitely because it is an expected Exception. All other exception should be handled special because they are an error condition.

Demuter 21-Apr-20 15:14pm

It works! Thank you very much.
But can you tell me how to get around a new error ... =))

Now that I’m reading several registers in a loop, probably the program doesn’t have time to get an answer and the data is not coming.
Is it possible to solve only an increase in time? Or are there more options?
Thank you again!

I have not seen your code. Now I'll try it and then I will write how things are going.

[no name] 21-Apr-20 15:59pm

:thumbsup:
You are very welcome. Looking forward for the results of your test.

Demuter 22-Apr-20 4:37am

Your code works, but the first catch(TimeoutException ex) sometimes gets an error: The operation timed out.
And then there is a problem as in my comment above ...

[no name] 22-Apr-20 5:51am

Make sure that at the beginning ReadTimeout is big, something like 1000mS like you did it before.

OriginalGriff 21-Apr-20 6:53am

So ... it's the other end.
Your code is sitting there waiting for data to arrive - you can't speed that up, it's pretty much totally up to the device at the other end of the link.
Think about it: assuming that you are communicating via a 9600 baud link, that's ~1000 characters per second at best, so your 17 characters will take less than two hundreds of a second to be physically received, and fed to your app. If that code is taking the best part of 100 times that then the delay is with the data provider, not the receiver.

You can't code around that!

Garth J Lancaster 21-Apr-20 9:27am

I second OriginalGriff's conclusion - I've worked a lot with Modbus, and the last particular device (for example) was a small microprocessor-board, running the Modbus protocol but monitoring a lot of sensors - it could have moments where it just couldnt respond to a request for register readings

Demuter 21-Apr-20 10:24am

I wouldn’t be worried if there wasn’t a normal speed on tсp. Response rate by tсp is about 0.15 sec. So I'm thinking that this is somewhere a problem in my code...
Maybe somewhere else in the code ...

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User 11060979 · Accepted Answer · 2020-04-21T06:34:00

After all discussions I would try something like below in your receiver code section to speed up communication.
I hope it helps and that I did not make to much mistakes.

// The following expects that serialPort.ReadTimeout is setup
// for "message timeout" which is usually much bigger than character 
// timeout after the first byte has been  received
//
var tmp = new byte[0x2000];
int dataLength = 0;
try
{
   while(true)
   {
      byte receiveByte= (byte)serialPort.ReadByte();

      // We received one byte. Check buffer size
      if (dataLength >= tmp.Length) 
      {
         // Buffer to small
         result.ErrorCode = 1; // Appropriate ErrorCode
         result.ErrorText = "Buffer Overrun";
         return result;
      }
      
      // Add it to receive buffer
      tmp[dataLength++] = receiveByte;

      // Set character Timeout after we received the first byte of the answer
      if (dataLength == 1)
      {
         // Set new character timeout according to Modbus spec.
         // At least 3.5 characters silence but here we take about 
         // 5 times more chars to be on the safe side
         //
         //                   10Bit/Byte     mS      ca. 5x3.5
         serialPort.ReadTimeout= (10    *   1000   *   18)      /   serialPort.Baudrate; 
      }

      // Don't eat CPU too aggressive (has to be proofed whether it really helps)
      Thread.Sleep(0);
   }
}
catch (TimeoutException ex)
{
   // Expected exception. No more character received in the specified
   // character timeout period
}
catch (Exception ex)
{
   // Other than (TimeoutException is an error condition
   result.ErrorCode = 1; // Appropriate ErrorCode
   result.ErrorText = ex.Message;
   return result;
}