Setting up Modbus on a Raspberry Pi PLC with CODESYS

This guide continues Setting up CODESYS for Raspberry Pi PLC in industrial automation. Before you start you need CODESYS V3.5 SP19 Patch 3 or later on a Windows PC, CODESYS Control for Raspberry Pi SL 4.11 or later on the PLC, and a working login (the PLC answers at 10.10.10.20 on its side Ethernet port by default). Newer CODESYS versions call the Modbus roles client and server; older ones say master and slave. The menus are the same.

Which CODESYS device do you need?

A Modbus client in CODESYS can have up to 64 servers below it.

Modbus TCP client (master): read a remote device

1. In Devices, right-click the PLC and add an Ethernet adapter if the project does not have one. In its settings, choose the network interface of the PLC (eth0 is the side port).
2. Right-click Ethernet > Add Device > Fieldbuses > Modbus and add the Modbus TCP Client (Modbus TCP Master in older versions). Keep Auto-reconnect enabled so the client connects again after a network cut.
3. Right-click the client and add a Modbus TCP Server (Modbus TCP Slave) for each remote device. Set its IP address, port 502 and the Unit ID that the device expects (often 1, or 255 for some TCP-only devices).
4. In the channel tab of the server, click Add Channel and choose the access type, for example Read Holding Registers (function code 03), the trigger (cyclic, with its cycle time in ms), the start register offset and the length.
5. In the I/O mapping tab, give a variable name to each register, or map it to a variable that already exists in your program.

Modbus function codes and data types in a channel

A Modbus register has 16 bits. A value with decimals usually comes as an integer with a scale (215 for 21.5 °C) or as a 32-bit float in two consecutive registers. Read the register map of the device: it gives the address, the scale and, for 32-bit values, the word order.

Use the mapped values in the program

Suppose a temperature transmitter sends the temperature × 10 in holding register 0. Map the register to iTempRaw (INT) in the I/O mapping tab and scale it in Structured Text:

VAR
    iTempRaw       : INT;     // mapped to holding register 0 (temperature x 10)
    rTemperature   : REAL;
    bOverheatAlarm : BOOL;
END_VAR

rTemperature := INT_TO_REAL(iTempRaw) / 10.0;
bOverheatAlarm := rTemperature > 75.0;

CODESYS updates the mapped variables with the bus cycle task, and each channel sends its request at its own cycle time. A cycle time shorter than the time the device needs to answer only fills the bus: start with 100 ms or more and lower it if the process needs it. This code is an example for this post and has not been run on a PLC.

Modbus TCP server: let a SCADA read the PLC

1. Right-click Ethernet > Add Device and add the ModbusTCP Server Device (ModbusTCP Slave Device in older versions).
2. In its configuration, keep port 502, set the Unit ID that the master will use and the number of coils, discrete inputs, holding registers and input registers that the PLC exposes.
3. In the I/O mapping tab, link those areas to variables of your program. The master reads and writes them at the configured addresses.

Write the register map down for the SCADA team: address, data type, scale and units of each value.

Modbus RTU on the RS-485 port

On the Raspberry PLC, the RS-485 ports are /dev/ttySC0 to /dev/ttySC3, depending on the version (list them with ls /dev/ttySC*). CODESYS on Linux names serial ports COM1, COM2 and so on, and takes the device file from the [SysCom] section, entry Linux.Devicefile, of /etc/CODESYSControl_User.cfg. Check which COM number points to the RS-485 port of your PLC before you configure the master.

1. Right-click the PLC > Add Device > Fieldbuses > Modbus and add the Modbus COM port. Set the COM port number, the baud rate, the parity, the data bits and the stop bits. They must be the same in every device on the bus.
2. Right-click the COM port and add the Modbus client (Modbus Master, COM Port in older versions). Set the response timeout.
3. Below the client, add one Modbus server (slave) for each device, with its address from 1 to 247, and configure its channels and I/O mapping as in the TCP client.

To make the Raspberry PLC an RTU slave, add the Modbus server device below the COM port instead, with the address that the master will use, and map your variables to its register areas.

Troubleshooting Modbus in CODESYS

FAQ about Modbus with CODESYS on a Raspberry PLC

Do I need an extra license for Modbus in CODESYS?

CODESYS lists Modbus TCP master and slave and Modbus serial master and slave as fieldbuses delivered with the CODESYS Control for Raspberry Pi SL runtime package. Without a runtime license, the runtime works for two hours and then stops.

What is the difference between Modbus client and master?

None in practice. Newer CODESYS versions use client and server; older ones use master and slave.

Can the PLC be Modbus TCP server and RTU master at the same time?

Yes. Each device in the tree works on its own, so a Raspberry PLC can read RS-485 devices and serve the values to a SCADA over Ethernet, as a gateway.

How fast can CODESYS poll Modbus?

It depends on the baud rate, the number of registers and the response time of each device. Set the cycle time of each channel to what the process needs, not to the minimum.

Can I test Modbus without the field devices?

Yes, with a Modbus simulator on a PC, or in the Modbus network simulator of the PLC Solution Library.

More about CODESYS and Modbus on the Raspberry PLC

• Setting up CODESYS for Raspberry Pi PLC in industrial automation
• Ladder programming with CODESYS on a Raspberry PLC
• Modbus TCP and RTU with Raspberry Pi PLCs: Python and Node-RED examples
• How to use PyModbus with a Raspberry Pi PLC
• Choosing an industrial controller
• OpenPLC guide for industrial PLCs

See all the models in the Raspberry PLC family.