CODEWITHSUNDEEP
pythonserial-portpyserialmodbus
Chris Goder
Chris Goder

Reputation: 118

Trouble send hex (Modbus Packet) via pyserial

I am having some trouble with sending the following hex codes 0x01 0x03 0x00 0x00 0x00 0x01 0x0a 0x84 to a serial device (Voltage Sensor) which will return an int for current voltage using Python and pyserial. My code is as follows:

import serial
import time

ser = serial.Serial(
port=1,
baudrate=38400,
parity=serial.PARITY_NONE,
stopbits=serial.STOPBITS_TWO,
bytesize=serial.EIGHTBITS,
timeout=0,
xonxoff=0,
rtscts=0)    

ByteStringToSend = "\x01\x03\x00\x00\x00\x01\x0a\x84"
ser.write(ByteStringToSend)
time.sleep(1)
RecievedData = ""
while ser.inWaiting() > 0:
    RecievedData = ser.read(1)
return RecievedData

The problem is it appears Python is sending each byte as an individual packet see below image from a serial monitor

see image at http://www.centralinfo.com.au/images/SerialOutput.png

The first 8 bytes (00 - 07) is from the python app (note the altinate colors for the different packets) The next 8 bytes (08 - 0f) is a VB.net app sending the same data which works.

My question is how do I send the 8 hex bytes above so it is in one packet (Modbus Protocol) as per the vb.net app?

VB Code for comparison:

     ' open the serial port if it is closed
            If Me.SerialPort1.IsOpen = False Then
                Me.SerialPort1.PortName = ComPort
                Me.SerialPort1.BaudRate = "38400" 'Set Baud rate
                Me.SerialPort1.RtsEnable = False ' Set RTS
                Me.SerialPort1.DtrEnable = False ' Set DTR
                Me.SerialPort1.Parity = IO.Ports.Parity.None
                Me.SerialPort1.StopBits = IO.Ports.StopBits.Two
                Me.SerialPort1.DataBits = 8 ' Set data length
                Me.SerialPort1.Handshake = Handshake.XOnXOff
                Me.SerialPort1.ReadTimeout = 10000
                Me.SerialPort1.WriteTimeout = 10000
                Me.SerialPort1.Open()
            End If

    Try
            Dim CommandBlock(7) As Byte
            CommandBlock(0) = &H1
            CommandBlock(1) = &H3
            CommandBlock(2) = &H0
            CommandBlock(3) = &H0
            CommandBlock(4) = &H0
            CommandBlock(5) = &H1
            CommandBlock(6) = &HA
            CommandBlock(7) = &H84
            Me.SerialPort1.Write(CommandBlock, 0, CommandBlock.Length)
            Thread.Sleep(100)
            Return True
        Catch ex As Exception
            Return False
        End Try

Thanks in advance for your time Chris

Upvotes: 1

Views: 5602

Answers (1)

Alexandre Vinçon
Alexandre Vinçon

Reputation: 954

Your VB app enables XON/XOFF protocol, while your python app does not. Without XON/XOFF turned on, I suspect that your python app is waiting for a certain amount of time between each bytes before sending it, thus the receiving device sees each byte as an individual "packet".

Upvotes: 1

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