MC Protocol Serial — MELSEC over RS-232C/RS-485

MC Protocol Serial is the protocol a MELSEC serial communication module speaks over RS-232C, RS-422, or RS-485. Use it when the MELSEC PLC has no Ethernet port and only a serial module — typically a legacy Q or A series installation. When an Ethernet port or communication unit is available, prefer SLMP: it is faster and easier to wire.

PLC families	MELSEC iQ-R, iQ-L, Q series, A series, via a serial communication module
Transport	RS-232C, RS-422, RS-485
Typical link	19,200 baud, 8-E-1
Physical layer	Part of the setup — an MCU UART needs a level shifter

The link, not the library, sets the pace here. In the published benchmark a single round-trip took about 67 ms at 19,200 baud, against roughly 9 ms for SLMP over TCP. Size your polling loop for the wire, not for the client.

The implementation

Language	Install	Start here
C++ (Arduino/PlatformIO)	fa-yoshinobu/mcprotocol-serial-cpp	Getting started

This is the only implementation. It targets ESP32/RP2040-class boards and Arduino-compatible cores, and the same library builds host-side on Linux for bring-up tools.

Configure the PLC and the wiring first

The frame format, baud rate, and character format on the PLC module must match the client exactly, and RS-232C levels must be converted before an MCU UART is connected. Wiring an MCU's TX/RX straight to the module is the common mistake.

→ MC Protocol Serial PLC setup · Serial modules · Supported registers · Troubleshooting & Codes

Before you write anything

A wrong write on a running machine is not undoable. Read Bit Write Safety before your first write, and confirm an address range is safe for the connected equipment.

PLC Scope does not cover MC Protocol Serial. For a first read-only check, use the bring-up scripts in examples/linux_cli of the C++ library.