modbus_master

Description:

modbus master

'modbus_master' is an easy to use package using which a dart program can work as a Modbus/TCP master device.

Currently users can use only these features of Modbus/TCP:

Discrete input

Read single discrete input


Coil

Read single coil
Write single coil


Input Register

Read single input register


Holding Register

Read single holding register
Write single holding register




Currently only TCP ipv4 is implemented.
A single stream object is used for response received from all slave devices.
This package handles socket networking part in a separate isolate so that main isolate is free to handle other tasks like UI part of flutter.
This package can be used on all platforms which supports dart:io & dart:isolate i.e. WINDOWS, LINUX, MACOS, ANDROID, IOS.

Important information to know about this package #
For each request, a response is always generated #

If response is not received from slave device within timeout, an error response is generated by library and is put to "responses()" stream
When first request is sent to a slave device, connection is established and connection is kept alive.
Whenever a request is sent to a slave device, first it checks whether connection is already established.

If connection is already established, then sends request.
If connection is found to be broken, then tries to establish connection.


If there are already 247 active connections, and request to a new address is sent. Then, earliest connection is broken, and this new connection is established.
If close method is executed, then first responses for all pending requests are generated. After that, all active connections are closed.

Timeout #
An error response is produced by package, if slave response is not received within timeout. Duration of timeout differs in two different scenarios:

Scenario 1: When connection is already established

if response is not produced within timeout(default 1000ms),

master produces error response




Timeout = request timeout (default 1000ms)


Scenario 2: When connection is not established

master tries for connection
if connection is not established within socketConnectionTimeout(default 2000 ms)

master produces error response


if connection is established

if response is not produced within timeout (default 1000ms)

master produces error response






Timeout (maximum) = socket connection timeout(default 2000ms) + request timeout(default 1000ms)



Limitations #

works with only ipv4, ipv6 is not supported
only 247 slaves can be connected at one time,

when more than 247 slave is connected, oldest slave connection is broken


Only single element can be read at once, reading multiple coils or multiple registers is not implemented, although reading multiple elements is specified in modbus/tcp implementation
Only single element can be written to at once, writing to multiple coils or to multiple registers is not implemented, although writing to multiple elements is specified in modbus/tcp implementation
works with dart 3.0 and above because it uses dart records

Steps to use this library #


make function async where this object needs to be created


make an instance of ModbusMaster class like this only.
final modbusMaster = await ModbusMaster.start();
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listen to stream of response using stream
modbusMaster.responses().listen(
(response) {
print(response);
}
);
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Send request to slave using these commands


read single coil of a slave
modbusMaster.readCoil(
ipv4: '192.168.29.163',
portNo: 502,
elementNumberOneTo65536: 11,
);
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read single discrete input of a slave
modbusMaster.readDiscreteInput(
ipv4: '192.168.1.5',
elementNumberOneTo65536: 11,
);
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read single holding register of a slave
modbusMaster.readHoldingRegister(
ipv4: '192.168.1.5',
elementNumberOneTo65536: 11,
);
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read single input register of a slave
modbusMaster.readInputRegister(
ipv4: '192.168.1.5',
elementNumberOneTo65536: 11,
);
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write single coil of a slave
modbusMaster.writeCoil(
ipv4: '192.168.1.5',
elementNumberOneTo65536: 11,
valueToBeWritten: true,
);
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write single holding register of a slave
modbusMaster.writeHoldingRegister(
ipv4: '192.168.1.5',
elementNumberOneTo65536: 11,
valueToBeWritten: 15525,
);
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close must be called at end to close all tcp connections and stop modbus master
modbusMaster.close();
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Example Code #
1. First start Modbus/TCP server i.e. slave device using pyModbusTCP library #
from pyModbusTCP.server import ModbusServer
import time
import socket

# get ipv4 address
host_ip = socket.gethostbyname(socket.gethostname())
modbusServer = ModbusServer(host = host_ip, port =502, no_block =True)

try:
print('Trying to start MODBUS/TCP SLAVE SERVER')
modbusServer.start()
print(f'MODBUS/TCP SLAVE SERVER is online at {host_ip}')

set_coil = True
while True:
if set_coil:
# coil address is coil number - 1, therefore 11-1 is written
modbusServer.data_bank.set_coils(11-1,bit_list=[True])
print('COIL-11 = SET')
else:
modbusServer.data_bank.set_coils(11-1,bit_list=[False])
print('COIL-11 = RESET')

print('WAITING FOR 5 SECONDS')
time.sleep(5)
set_coil = not set_coil

except Exception as error:
print(error)
print(f'Trying to stop MODBUS/TCP SLAVE SERVER at {host_ip}')
modbusServer.stop()
print('MODBUS/TCP SLAVE SERVER is offline')
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2. Check whether Modbus/TCP server (slave) is online or not #

Check whether socket for Server is open or not using windows powershell command
# use ip address of your device, which is printed by python script
netstat -na | Select-String "192.168.29.163:502"
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When socket is listening, netstat command shows this
TCP 192.168.29.163:502 0.0.0.0:0 LISTENING
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When socket is closed, netstat command shows nothing



3. Use dart code & ModbusMaster library to read value of coil of slave (pyModbusTCP server) #
import 'modbus_master_isolate.dart';

void main() {
testReadingCoil();
}

void testReadingCoil() async {
final modbusMaster = await ModbusMaster.start();

int countResponseReceived = 0;
modbusMaster.responses().listen(
(response) {
++countResponseReceived;
print(response);
if (countResponseReceived >= 3) {
// close modbusMaster when 3 responses are received
modbusMaster.close();
}
},
);

for (int i = 1; i <= 5; ++i) {
print('-> Request $i');
try {
modbusMaster.readCoil(
ipv4: '192.168.29.163',
portNo: 502,
elementNumberOneTo65536: 11,
);
} catch (e) {
// When read request is executed after close command, exception is raised.
print('EXCEPTION THROWN WHILE READING $e');
}
await Future.delayed(Duration(seconds: 5));
}
}
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4. Close python script by repeatedly pressing Ctrl+C in python interpreter #

Check whether socket is open or not using 'netstat' command given above.
If python script has finished but socket is open, use python statement to close modbusServer
modbusServer.stop()
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Recheck status of socket using 'netstat'.