Convert RS232 Ascii to Modbus TCP using Pymodbus

Question

I am trying to convert RS252 Ascii string data from a sensor to Modbus TCP Input/Holding registers using pymodbus Callback Sever, the server is the master reporting data when requested to a client logger, and I am not sure what I need to do to get this to work. I am currently able to read the data and log it to a csv file using this

#!/usr/bin/env python
# Log data from serial port


import argparse
import serial
import datetime
import time
import os


parser = argparse.ArgumentParser(formatter_class=argparse.ArgumentDefaultsHelpFormatter)
parser.add_argument("-d", "--device", help="device to read from", default="/dev/ttyUSB0")
parser.add_argument("-s", "--speed", help="speed in bps", default=9600, type=int)
args = parser.parse_args()

outputFilePath = os.path.join(os.path.dirname(__file__),
                 datetime.datetime.now().strftime("%Y-%m-%d") + ".csv")

with serial.Serial(args.device, args.speed) as ser, open(outputFilePath,'w') as outputFile:
    print("Logging started. Ctrl-C to stop.") 
    try:
        while True:
            time.sleep(0.2) 
            x = (ser.read(ser.inWaiting())) 
            data = x.decode('UTF-8')
            if data !="":
                outputFile.write(time.strftime("%Y/%m/%d %H:%M ") + " " + data  )
                outputFile.flush()

    except KeyboardInterrupt:
        print("Logging stopped")

The string from the sensor comes out of the device as:

 0.00 0.0 0.0 346.70 25.14

I need to have each piece as its own Modbus register and I am trying to use pymodbus on a Raspberry Pi Zero. The sensor updates 4 times a second and I am able to break the data into parts, I just haven;t gotten to that yet because I am not sure what I need to do in the Callback script, I am not that versed in Python yet I am I am still learning. I do have an understanding of Modbus TCP and have used it before on Arduino systems. Any help would be appreciated.

Answer 1

What you need is updating server, which you could use to populate the registers . You will have to focus on function def updating_writer and do the serial reads, process them and write to registers of your choice. The example is hard to read and understand in first go. I have modified the example to meet your needs. But here are some key concepts which will be handy to understand the code.

ModbusSlaveContext

BinaryPayloadBuilder

Also note, the example uses asynchronous server based on twisted, If you are new to twisted or have some constraints which will not allow you to use twisted on your target, you can achieve the same with simple threads as well. The design would be roughly like this.

1. Start your updating function in a separate thread
2. Start your TCP server at the end (blocking)

# Complete gist here --> https://gist.github.com/dhoomakethu/540b15781c62de6d1f7c318c3fc8ae22
def updating_writer(context, device, baudrate):
    """ A worker process that runs every so often and
    updates live values of the context. It should be noted
    that there is a race condition for the update.
    :param arguments: The input arguments to the call
    """
    log.debug("updating the context")
    log.debug("device - {}, baudrate-{}".format(device, baudrate))
    data = serial_reader(device, baudrate)  # Implement your serial reads, return list of floats.
    if data:
        # We can not directly write float values to registers, Use BinaryPayloadBuilder to convert float to IEEE-754 hex integer
        for d in data:
            builder.add_32bit_float(d)
        registers = builder.to_registers()
        context = context
        register = 3  # Modbus function code (3) read holding registers. Just to uniquely identify what we are reading from /writing in to.
        slave_id = 0x01 # Device Unit address , refer ModbusSlaveContext below
        address = 0x00 # starting offset of register to write (0 --> 40001)
        log.debug("new values: " + str(registers))
        context[slave_id].setValues(register, address, registers)

Once the server is running and the values are being updated, you can use a client to read values and parse it back to float.

from pymodbus.client.sync import ModbusTcpClient as Client
from pymodbus.payload import BinaryPayloadDecoder, Endian
client = Client(<IP-ADDRESS>, port=5020)

# Each 32 bit float is stored in 2 words, so we will read 10 registers 
raw_values = client.read_holding_registers(0, 10, unit=1)
if not registers.isError():
    registers = raw_values.registers
    decoder = BinaryPayloadDecoder.fromRegisters(registers, 
    wordorder=Endian.Big, byteorder=Endian.Big)

    for _ in range(5):
        print(decoder.decode_32bit_float())