MC Protocol Serial Troubleshooting & Codes

This page is a practical guide for errors returned by MELSEC serial MC Protocol targets. It is not a complete Mitsubishi error-code table. Use the PLC and serial-module manuals for formal definitions.

Library Status Categories

The C++ library reports transport and parser failures separately from PLC/module error responses.

Status category	Typical meaning	First checks
Timeout	No complete response arrived before the response timeout.	Check wiring, baud rate, parity, stop bits, station number, and whether the PLC module is configured for the same frame type.
Framing	Bytes arrived, but they did not match the selected response frame.	Check 1C/2C/3C/4C/1E selection, ASCII format, binary vs ASCII mode, and CR/LF settings.
Sum-check mismatch	A response arrived, but its sum-check did not match.	Check whether sum-check is enabled on both sides. If it is, check serial noise and wiring.
Parse	The response frame shape was recognized, but a numeric field or payload length could not be decoded.	Capture the raw frame and check whether the selected frame/profile matches the PLC setting.
Unsupported configuration	The request cannot be encoded for the selected profile, frame, or build options.	Select an explicit PLC profile, choose a supported frame helper, and check disabled feature macros.
PLC/module error	The PLC or serial module returned an error response.	Read the preserved PLC/module error code and use the sections below.

PLC and Serial-Module Error Families

Serial MC Protocol uses more than one error-code family. Do not interpret every code as an SLMP Ethernet end code.

Code family	Where it appears	How to handle it
CPU-side 4000-series and related PLC end codes	QnA extended 3C / 4C routes when the request reaches the CPU.	Use the SLMP Troubleshooting & Codes guide for practical checks.
7Fxx serial-module responses	Serial-module rejection before or around CPU forwarding.	Treat as target/module dependent. Check frame mode, profile, device family, route, and module settings.
1C NAK codes	Legacy 1C A-compatible / QnA-compatible frames.	Representative project-observed codes are listed below. Record the raw response and exact target settings before applying a meaning to another setup.
No response	The module ignores the request or cannot answer in the selected mode.	Treat as a transport/configuration problem first, not as an error code.

Observed Codes

Only project-observed cases are listed here. If you see a code not listed here, record the raw response, frame kind, ASCII/binary mode, station, sum-check setting, PLC model, serial module, and selected PLC profile.

Code	Observed situation	Practical check
0x4031	CPU-side device or route rejection observed on serial paths, for example unsupported link-direct access on a target setup.	Check the selected profile, route notation, mounted module, and whether the requested device family exists on that PLC.
0x7F22	Serial-module rejection observed for unsupported serial-MC device/command shapes, such as S device probes on a C24 path before CPU forwarding.	Do not treat unsupported device families as valid access paths. Recheck the profile support table and the serial-module MC protocol format.
1C NAK 0x02	Intentionally bad sum-check on a transmitted WR0 read.	Verify that sum-check is enabled consistently and recalculate the transmitted sum.
1C NAK 0x03	Nonexistent 1C command.	Check the command mnemonic and selected 1C format.
1C NAK 0x06	WR0 read with a zero point count.	Send a valid nonzero point count within the command limit.
1C NAK 0x07	WR0 read with an invalid device code.	Check the device family and its encoding for the selected profile.
1C NAK 0x10	Format 4 WR0 read with a mismatched PC field on the tested iQ-F / FX5 bench.	Check the station and PC header fields against the serial-module settings.
0x7E40	Invalid C4 command/subcommand and an unregistered monitor-read shape on the tested bench.	Check the command, subcommand, and whether a required registration step exists for the selected route.
0x7F21	C4 read/write using DX0, which was unavailable on the tested setup.	Recheck the selected profile, device family, and route; do not generalize one setup to every PLC.
0x7F24	Intentionally bad sum-check on a transmitted C4 read.	Verify the sum-check setting and recalculate the transmitted sum.

Evidence Scope

The decoder preserves error codes such as two-digit 1C NAK codes and four-digit QnA serial responses, but this page does not assign meanings to unmeasured codes.

Representative 1C NAK and C4/serial 7Fxx measurements are complete; there is no active generic error-code collection TODO. Future measurements should start only from a specific diagnostic need and must record the exact frame, target, module settings, and observed response. Add only observed codes to this page.