Asynchronous Client Example

The asynchronous client functions in the same way as the synchronous client, however, the asynchronous client uses twisted to return deferreds for the response result. Just like the synchronous version, it works against TCP, UDP, serial ASCII, and serial RTU devices.

Below an asynchronous tcp client is demonstrated running against a reference server. If you do not have a device to test with, feel free to run a pymodbus server instance or start the reference tester in the tools directory.

#!/usr/bin/env python
'''
Pymodbus Asynchrnonous Client Examples
--------------------------------------------------------------------------

The following is an example of how to use the asynchronous modbus
client implementation from pymodbus.
'''
#---------------------------------------------------------------------------# 
# import the various server implementations
#---------------------------------------------------------------------------# 
from pymodbus.client.async import ModbusTcpClient as ModbusClient
#from pymodbus.client.async import ModbusUdpClient as ModbusClient
#from pymodbus.client.async import ModbusSerialClient as ModbusClient

#---------------------------------------------------------------------------# 
# configure the client logging
#---------------------------------------------------------------------------# 
import logging
logging.basicConfig()
log = logging.getLogger()
log.setLevel(logging.DEBUG)

#---------------------------------------------------------------------------# 
# choose the client you want
#---------------------------------------------------------------------------# 
# make sure to start an implementation to hit against. For this
# you can use an existing device, the reference implementation in the tools
# directory, or start a pymodbus server.
#---------------------------------------------------------------------------# 
client = ModbusClient('127.0.0.1')

#---------------------------------------------------------------------------# 
# helper method to test deferred callbacks
#---------------------------------------------------------------------------# 
def dassert(deferred, callback):
    def _tester():
        assert(callback())
    deferred.callback(_tester)
    deferred.errback(lambda _: assert(False))

#---------------------------------------------------------------------------# 
# example requests
#---------------------------------------------------------------------------# 
# simply call the methods that you would like to use. An example session
# is displayed below along with some assert checks.
#---------------------------------------------------------------------------# 
rq = client.write_coil(1, True)
rr = client.read_coils(1,1)
dassert(rq, lambda r: r.function_code < 0x80)     # test that we are not an error
dassert(rr, lambda r: r.bits[0] == True)          # test the expected value

rq = client.write_coils(1, [True]*8)
rr = client.read_coils(1,8)
dassert(rq, lambda r: r.function_code < 0x80)     # test that we are not an error
dassert(rr, lambda r: r.bits == [True]*8)         # test the expected value

rq = client.write_coils(1, [False]*8)
rr = client.read_discrete_inputs(1,8)
dassert(rq, lambda r: r.function_code < 0x80)     # test that we are not an error
dassert(rr, lambda r: r.bits == [False]*8)        # test the expected value

rq = client.write_register(1, 10)
rr = client.read_holding_registers(1,1)
dassert(rq, lambda r: r.function_code < 0x80)     # test that we are not an error
dassert(rr, lambda r: r.registers[0] == 10)       # test the expected value

rq = client.write_registers(1, [10]*8)
rr = client.read_input_registers(1,8)
dassert(rq, lambda r: r.function_code < 0x80)     # test that we are not an error
dassert(rr, lambda r: r.registers == [10]*8)      # test the expected value

rq = client.readwrite_registers(1, [20]*8)
rr = client.read_input_registers(1,8)
dassert(rq, lambda r: r.function_code < 0x80)     # test that we are not an error
dassert(rr, lambda r: r.registers == [20]*8)      # test the expected value

#---------------------------------------------------------------------------# 
# close the client
#---------------------------------------------------------------------------# 
client.close()