Modbus

Modbus is the long-standing industrial protocol for reading and writing holding and input registers on PLCs, sensors, and instruments. Endpoint supports both Modbus RTU (over a serial line) and Modbus TCP (over Ethernet or Wi-Fi), and the libraries used are the same cross-platform packages that run on desktop and server. The protocol is lightweight and comfortably within Endpoint's resource budget.

Client

The .NET choices are FluentModbus and NModbus4. Both cover client (master) and server (slave) roles, Modbus RTU and Modbus TCP. On Python, the standard library is pymodbus (client and server, RTU and TCP). On C/C++ the common choice is libmodbus, which is available as a standard Linux package.

.NET

using FluentModbus;

// Modbus TCP client
var client = new ModbusTcpClient();
client.Connect("192.168.1.50");

short[] registers = client.ReadHoldingRegisters<short>(unitIdentifier: 1,
                                                       startingAddress: 0,
                                                       count: 10);
client.WriteSingleRegister(unitIdentifier: 1, registerAddress: 10, value: 1234);

Python

from pymodbus.client import ModbusTcpClient

client = ModbusTcpClient("192.168.1.50")
client.connect()

result = client.read_holding_registers(address=0, count=10, slave=1)
client.write_register(address=10, value=1234, slave=1)

C/C++

// Link with -lmodbus.
#include <modbus/modbus.h>
#include <cstdio>

modbus_t* ctx = modbus_new_tcp("192.168.1.50", 502);
modbus_set_slave(ctx, 1);
modbus_connect(ctx);

uint16_t registers[10] = {};
modbus_read_registers(ctx, 0, 10, registers);
modbus_write_register(ctx, 10, 1234);

modbus_close(ctx);
modbus_free(ctx);

Hosting a Modbus server

Endpoint can also act as the server (slave) side, which is useful when Endpoint sits in front of sensors that a plant SCADA system polls over Modbus. All three libraries above include a server implementation in the same package. See the Endpoint as gateway page for the broader pattern.