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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++ PlatformIO: fa-yoshinobu/mcprotocol-serial-cpp; source + CMake for hosts Getting started

This is the only implementation. The C++17 communication core is independent of Arduino and the transport. Choose the integration for your application:

  • ESP32/RP2040 firmware: use the PlatformIO package with your board's UART integration.
  • Arduino-ESP32: use the optional UART adapter for synchronous or asynchronous reads and writes, UART ownership, and RTS-controlled RS-485. This adapter is specific to Arduino-ESP32; it is not the RP2040 integration.
  • Windows/Linux host applications: build from source with CMake to use HostSyncClient and the host serial backends. These components are not included in the PlatformIO build.

The UART adapter guide covers configuration, buffer sizing, callbacks, cancellation, and explicit recovery. Its examples include keeping an LCD and buttons responsive while a read is pending.

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.