SerialPort communication in C# splitting frames

问题

I need to write a program, that will listen to communication in ModBus network through RS485. I am connected to the network with RS485 <> USB dongle.

I can read some data using SerialPort.DataReceived event, but it gives strange results. Data is often split, when it should come in one piece. (Modbus Master transmits every 100ms).

class Serial
{
    private SerialPort port;
    Queue<byte[]> buffer;

    public Serial()
    {
        buffer = new Queue<byte[]>();
        port = new SerialPort("COM3", 19200, Parity.Even, 8, StopBits.One);
        port.DataReceived += port_DataReceived;
    }

    public void Open()
    {
        if (port.IsOpen)
        {
            port.Close();
        }

        port.Open();
    }

    void port_DataReceived(object sender, SerialDataReceivedEventArgs e)
    {
        byte[] buff = new byte[port.BytesToRead];
        port.Read(buff, 0, port.BytesToRead);
        buffer.Enqueue(buff);
    }
}

I don't have any start sign in transmission. Delay between frames is min. 3.5 chars, and max delay between chars is 1.5 chars.

回答1:

This is entirely normal, serial ports are very slow devices. The DataReceived event is fired as soon as one byte was received. You'll need to call Read() and pay attention to the value it returns, the number of bytes it was able to retrieve from the input buffer. Which might be more than one but is only very rarely equal to the number of bytes in a "packet", that could only happen if the machine got very slow for some reason.

Beware that the debugger is one way to make it that slow, a breakpoint or single-stepping the event handler code gives the driver enough time to receive all the bytes in a packet. So that the Read() call returns them all. But that stops working as soon as you stop debugging that code.

You could use the ReceivedBytesThreshold property to delay the event but that can only work when a packet has a fixed size. Simply append the bytes you get into byte[], using the 2nd argument of the Read() call. And don't process the packet until you have them all.

来源：https://stackoverflow.com/questions/13417442/serialport-communication-in-c-sharp-splitting-frames

标签

serial-port

modbus