Modbus TCP

nXPlc speaks Modbus TCP on its Ethernet port with the standard CODESYS devices, the same as on any other PLC: no library of ours and no code to write. Everything is configured in the device tree and downloaded with the application.

The board can be a server, which other devices read and write, a client, which reads and writes other devices, or both at once.

The Ethernet node

Both roles hang under an Ethernet node:

1. Right-click on Device → Add Device… → Fieldbuses → Ethernet Adapter → Ethernet → Add Device.
2. Open the Ethernet node. In the General tab, with the PLC online, click … next to Interface and select eth0: IP address, subnet mask and default gateway are filled in with the board’s current values.
3. Leave Adjust operating system settings off: the board’s address is set by the parameters of the IO node, see Ethernet.

Server

1. Under Ethernet: Add Device… → Modbus → Modbus TCP Server Device.
2. In the General tab choose how many registers to expose and the port (502 by default).
3. In I/O Mapping map your variables onto the registers.
4. Build, download.

The two register groups go in opposite directions:

	In the PLC	Written by	Read by
Holding Registers	%IW (inputs)	the Modbus client	the PLC
Input Registers	%QW (outputs)	the PLC	the Modbus client

To publish a PLC variable to a client, map it onto an Input Register. A variable mapped onto a Holding Register is overwritten every cycle by what the client sent.

Registers are WORD (0…65535): mapping an INT works, but negative values reach the client as large numbers (−1 becomes 65535).

Client

1. Under Ethernet: Add Device… → Modbus → Modbus TCP Client.
2. Under the client: Modbus TCP Server, the remote device. Set its IP address and port.
3. In the channels tab add what to read and write: Read Input Registers, Read Holding Registers, Write Multiple Registers, …
4. In I/O Mapping map the channels onto your variables, then build and download.

Note

The board has no real-time clock: the time the client uses internally counts the seconds since power-up. It is enough for its intervals and timeouts, but it is not the date.

Trying it from a PC

Two Python 3 scripts with no dependencies, for Linux and Windows:

• modbus_client.py — reads and writes the registers of the nXPlc server:

  python3 modbus_client.py 192.168.1.50 leggi-input 0 10 --ogni 1
  python3 modbus_client.py 192.168.1.50 scrivi 0 123 456

• modbus_server.py — a server for the nXPlc client to poll. Input Register 0 counts up, Input Register 1 holds the seconds, and every write from the PLC is printed:

  python3 modbus_server.py --porta 502

Limits

IEC data memory	84 KB, shared with the rest of the application. Past it, the build stops with C0106 Out of global data memory.
Network connections	8 sockets in total. The CODESYS connection takes up to three, the server one to listen, and every connected client or remote server one more.

With a few hundred registers and two or three clients neither limit is anywhere close.