How do you modify the Read Holding Registers inside the Modbus RTU Server of the pyModbus RTU Server (Slave) application

Question

I have the example pyModbus Server application working, but I am not sure how to adjust/modify the Input registers after the server has been started. Address range #30000-#39999

What I would like to do is to dynamically change the Input Registers inside my Server so that as my test Client reading system reads these registers, I can modify the behavior of the reading system.

Thanks

This server code is working:

#!/usr/bin/env python
"""
Pymodbus Asynchronous Server Example
--------------------------------------------------------------------------

The asynchronous server is a high performance implementation using the
twisted library as its backend.  This allows it to scale to many thousands
of nodes which can be helpful for testing monitoring software.
"""
# --------------------------------------------------------------------------- # 
# import the various server implementations
# --------------------------------------------------------------------------- #
from pymodbus.version import version
from pymodbus.server.asynchronous import StartTcpServer
from pymodbus.server.asynchronous import StartUdpServer
from pymodbus.server.asynchronous import StartSerialServer

from pymodbus.device import ModbusDeviceIdentification
from pymodbus.datastore import ModbusSequentialDataBlock
from pymodbus.datastore import ModbusSlaveContext, ModbusServerContext
from pymodbus.transaction import (ModbusRtuFramer,
                                  ModbusAsciiFramer,
                                  ModbusBinaryFramer)
#from custom_message import CustomModbusRequest

# --------------------------------------------------------------------------- # 
# configure the service logging
# --------------------------------------------------------------------------- # 
import logging
FORMAT = ('%(asctime)-15s %(threadName)-15s'
          ' %(levelname)-8s %(module)-15s:%(lineno)-8s %(message)s')
logging.basicConfig(format=FORMAT)
log = logging.getLogger()
log.setLevel(logging.DEBUG)


def run_async_server():
    # ----------------------------------------------------------------------- # 
    # initialize your data store
    # ----------------------------------------------------------------------- # 
    # The datastores only respond to the addresses that they are initialized to
    # Therefore, if you initialize a DataBlock to addresses from 0x00 to 0xFF,
    # a request to 0x100 will respond with an invalid address exception.
    # This is because many devices exhibit this kind of behavior (but not all)
    #
    #     block = ModbusSequentialDataBlock(0x00, [0]*0xff)
    #
    # Continuing, you can choose to use a sequential or a sparse DataBlock in
    # your data context.  The difference is that the sequential has no gaps in
    # the data while the sparse can. Once again, there are devices that exhibit
    # both forms of behavior::
    #
    #     block = ModbusSparseDataBlock({0x00: 0, 0x05: 1})
    #     block = ModbusSequentialDataBlock(0x00, [0]*5)
    #
    # Alternately, you can use the factory methods to initialize the DataBlocks
    # or simply do not pass them to have them initialized to 0x00 on the full
    # address range::
    #
    #     store = ModbusSlaveContext(di = ModbusSequentialDataBlock.create())
    #     store = ModbusSlaveContext()
    #
    # Finally, you are allowed to use the same DataBlock reference for every
    # table or you you may use a seperate DataBlock for each table.
    # This depends if you would like functions to be able to access and modify
    # the same data or not::
    #
    #     block = ModbusSequentialDataBlock(0x00, [0]*0xff)
    #     store = ModbusSlaveContext(di=block, co=block, hr=block, ir=block)
    #
    # The server then makes use of a server context that allows the server to
    # respond with different slave contexts for different unit ids. By default
    # it will return the same context for every unit id supplied (broadcast
    # mode).
    # However, this can be overloaded by setting the single flag to False
    # and then supplying a dictionary of unit id to context mapping::
    #
    #     slaves  = {
    #         0x01: ModbusSlaveContext(...),
    #         0x02: ModbusSlaveContext(...),
    #         0x03: ModbusSlaveContext(...),
    #     }
    #     context = ModbusServerContext(slaves=slaves, single=False)
    #
    # The slave context can also be initialized in zero_mode which means that a
    # request to address(0-7) will map to the address (0-7). The default is
    # False which is based on section 4.4 of the specification, so address(0-7)
    # will map to (1-8)::
    #
    #     store = ModbusSlaveContext(..., zero_mode=True)
    # ----------------------------------------------------------------------- # 
    store = ModbusSlaveContext(
        di=ModbusSequentialDataBlock(0, [17]*100),
        co=ModbusSequentialDataBlock(0, [17]*100),
        hr=ModbusSequentialDataBlock(0, [17]*100),
        ir=ModbusSequentialDataBlock(0, [17]*100))
    #store.register(CustomModbusRequest.function_code, 'cm',
    #               ModbusSequentialDataBlock(0, [17] * 100))
    context = ModbusServerContext(slaves=store, single=True)
    
    # ----------------------------------------------------------------------- # 
    # initialize the server information
    # ----------------------------------------------------------------------- # 
    # If you don't set this or any fields, they are defaulted to empty strings.
    # ----------------------------------------------------------------------- # 
    identity = ModbusDeviceIdentification()
    identity.VendorName = 'Pymodbus'
    identity.ProductCode = 'PM'
    identity.VendorUrl = 'http://github.com/riptideio/pymodbus/'
    identity.ProductName = 'Pymodbus Server'
    identity.ModelName = 'Pymodbus Server'
    identity.MajorMinorRevision = version.short()
    

    # ----------------------------------------------------------------------- # 
    # run the server you want
    # ----------------------------------------------------------------------- # 

    # TCP Server

    #StartTcpServer(context, identity=identity, address=("localhost", 5020),
    #               custom_functions=[CustomModbusRequest])

    # TCP Server with deferred reactor run

    # from twisted.internet import reactor
    # StartTcpServer(context, identity=identity, address=("localhost", 5020),
    #                defer_reactor_run=True)
    # reactor.run()

    # Server with RTU framer
    # StartTcpServer(context, identity=identity, address=("localhost", 5020),
    #                framer=ModbusRtuFramer)

    # UDP Server
    # StartUdpServer(context, identity=identity, address=("127.0.0.1", 5020))

    # RTU Server
    StartSerialServer(context, identity=identity, port='com9', framer=ModbusRtuFramer)#,unit=5)

    # ASCII Server
    # StartSerialServer(context, identity=identity,
    #                   port='/dev/ttyp0', framer=ModbusAsciiFramer)

    # Binary Server
    # StartSerialServer(context, identity=identity,
    #                   port='/dev/ttyp0', framer=ModbusBinaryFramer)


if __name__ == "__main__":
    run_async_server()
 

================================ and when I run the Client code shown below, I know I'm reading the FC04

from pymodbus.client.sync import ModbusSerialClient as ModbusClient
#fro pymodbus.register_read_message import ReadInputRegisterResponse

client = ModbusClient(method='rtu', port='com19',stopbits=1,bytesize=8,parity='N',baudrate=19200,timeout=.03)

client.connect()
read=client.read_input_registers(address = 0x00, count =10, unit=1)

#data=read.registers[int(2)]
#print(read)
print(read.getRegister(1))

Answer 1

Should the port be uppercase 'COM'? and in your examples the COM ports do not match.

Server shows 'com9' and Client shows 'com19'

are you able read the registers?

cant you just write to them in the usual way?

client.write_registers(address=0, count=10, unit=1, values=(1,2,3,4,5,6,7,8,9,10))