MELSEC-L Serial Communication Module
The MELSEC-L Serial Communication Module (LJ71C24 / LJ71C24-R2) enables reliable serial communication between MELSEC-L PLC systems and external devices such as barcode readers, printers, inverters, instruments, and host systems.
It supports multiple communication methods including MC protocol, non-procedural communication, bidirectional protocol, and predefined protocols, making it suitable for packaging, printing, converting, and general industrial automation systems.
Key Features
- Supports RS-232C / RS-422 / RS-485 serial communication
- Flexible protocol handling: MC, non-procedural, bidirectional, predefined
- Buffer-memory-based diagnostics for detailed error detection
- Extensive error codes for communication, protocol, and modem functions
- Stable operation with built-in timeout, handshake, and monitoring controls
The MELSEC-L serial communication module provides stable data exchange, detailed diagnostics, and efficient troubleshooting, ensuring reliable communication in demanding industrial environments.
Find the Error Code List and Troubleshooting
A) Error code ranges (CPU / CC-Link / Motion)
| Error Range | Detected By | Meaning | Corrective Action |
| 4000H–4FFFH | CPU module | CPU detected an error (detail is in CPU manuals) | Refer to CPU User’s Manual; correct CPU fault then retry communication |
| B000H–BFFFH | CC-Link module | CC-Link detected an error (detail is in CC-Link manuals) | Refer to CC-Link module User’s Manual; verify network wiring/parameters |
| D000H–DFFFH | Motion controller module | Motion controller detected an error (detail is in motion manuals) | Refer to Motion controller User’s Manual; check motion system diagnostics |
B) Predefined protocol / protocol-communication errors (7Dxx)
7D00H–7D02H (protocol basic errors)
| Error Code | Description | Typical Cause | Solution |
| 7D00H | Protocol execution error | Protocol No. setting error | Check and correct protocol No. setting; verify selected protocol exists |
| 7D01H | Protocol parameter error | Protocol parameter setting invalid | Check protocol parameters; correct invalid items and rewrite settings |
| 7D02H | Auto negotiation error | Modem/line negotiation failed | Check line connection, dial method, AT command/response strings; verify partner readiness |
7D10H–7D21H (cable/handshake/timeouts/format/conversion)
| Error Code | Description | Confirming Method | Solution |
| 7D10H | Cable disconnection error | Check cable continuity and connector seating | Reconnect/replace cable; confirm shielding/grounding |
| 7D11H | Protocol simultaneous execution error | Check if same protocol is called by multiple tasks/channels | Do not execute same protocol simultaneously; retry after completion |
| 7D12H | Transmission monitoring timeout | Check handshake/partner response timing; review timeout setting | Increase timeout; verify wiring/handshake; confirm partner device responds |
| 7D13H | Receive wait timeout | Confirm partner actually sends data; monitor RX status | Increase receive wait; verify device sends reply and timing |
| 7D14H | Response monitoring timeout | Check partner response time and communication path | Increase response monitoring time; verify partner settings/wiring |
| 7D15H | RTS monitoring timeout | Check RTS/CTS hardware handshake behavior | Check RTS/CTS wiring and handshake settings; increase monitoring time |
| 7D16H | CTS wait timeout | Confirm CTS input status; inspect cable/pins | Correct CTS wiring/handshake configuration; adjust wait time |
| 7D17H | Packet format error | Compare packet framing/definition with partner device | Correct framing/format and protocol definition; verify delimiters/headers |
| 7D18H | Error check code error | Check checksum/LRC/CRC type and calculation rule | Match checksum/LRC/CRC settings with partner device |
| 7D19H | Conversion variable setting error | Check conversion-variable mapping and storage area settings | Correct conversion variable settings; verify storage device area |
| 7D1AH | Data is missing error | Check actual received length vs expected length | Verify partner sends complete message; correct length/format/timing |
| 7D1BH | Value range error | Check conversion limits / parameter range | Correct value range; adjust protocol parameter or conversion rules |
| 7D20H | Data length setting error | Check configured data length vs actual message length | Correct data length setting (protocol / conversion variable) |
| 7D21H | Decimal point position error | Check decimal position setting used for conversion | Correct decimal point position setting to match actual data format |
C) Protocol setting / registration / parameter errors (7Exx — key actionable codes)
| Error Code | Description | Typical Cause | Solution |
| 7E00H–7E06H | System error (C24 OS detected error) | Internal OS/system-level fault condition | Follow the manual’s standard procedure (*1): confirm power, wiring, environment; retry; replace module if persistent |
| 7E6AH | Protocol setting data error | Protocol setting data invalid/corrupted | Check protocol setting data and rewrite protocol settings |
| 7EC0H | Protocol function No. error | Protocol function number invalid | Correct protocol function No. setting |
| 7EC1H | Text string setting error | Text string format/length invalid | Correct text string settings (length/format/terminators) |
| 7EC2H | ASCII/Binary conversion error | Conversion setting mismatch or data type mismatch | Correct conversion settings and data format; align with partner |
| 7EC3H | Conversion variable No. error | Invalid conversion variable number | Correct conversion variable number (No.) and mapping |
| 7EC4H | Conversion method error | Unsupported conversion method | Correct conversion method setting |
| 7EC5H | Parameter write error | Parameter write failed / invalid parameter area | Rewrite parameters; verify parameter area and write conditions |
| 7EC6H | Device No. error | Head device out of range | Correct device number and device range |
| 7EC7H–7EC8H | System error (C24 OS) | Internal OS/system-level fault condition | Follow procedure (*1): verify environment/power; replace module if persistent |
D) Runtime / special feature errors (7Fxx — key actionable codes)
| Error Code | Description | Confirming Method | Solution |
| 7F00H | Character error | Check received characters / encoding vs expected | Align format/encoding with partner device; remove noise sources |
| 7F21H | Conversion setting error | Verify ASCII/Binary conversion and variable mapping | Correct conversion mapping and conversion rules |
| 7F22H | Data length error | Compare actual length vs configured length | Correct data length setting; adjust partner device message length |
| 7F23H | Decimal point position error | Check decimal position settings | Correct decimal point position in conversion settings |
| 7F24H | Value range error | Check converted value against permitted range | Correct range and scaling; verify partner value format |
| 7F25H | Data too long / too short | Check configured storage length vs actual received | Correct length mismatch; revise protocol definition |
| 7F26H | Not enough storage area | Check storage allocation vs data amount | Expand storage area or reduce stored data volume |
| 7F40H–7F42H | Transmission/Receive monitoring timeout | Check partner response timing, line stability, handshake | Increase monitoring time; verify wiring/handshake; confirm partner timing |
| 7F67H | Request received while already communicating | Check if requests overlap (tasks/channels) | Prevent overlapping requests; retry after completion |
| 7F6CH | Protocol used by another channel | Check protocol assignment per channel | Do not share same protocol across channels simultaneously |
| 7FC8H | Protocol setting data error | Verify protocol setting data consistency | Check and rewrite protocol setting data |
| 7FE6H–7FE8H | Remote password errors | Verify password config and lock status | Correct remote password settings; clear lock state if required |
| 7FEFH | Switch setting error | Check module switch settings and configuration | Correct switch settings; restart module to apply |
| 7FF0H–7FF5H | Simultaneous execution / illegal state / request errors | Check request timing and execution overlap | Prevent overlapping operations; correct state/timing; retry after completion |
| 7FF8H | System error (C24 OS) | Internal OS/system-level fault condition | Follow procedure (*1): verify environment/power; replace module if persistent |
E) Modem function errors (7FD0H–7FE5H)
| Error Code | Description | Confirming Method | Solution |
| 7F00H | Character error | Check received characters / encoding vs expected | Align format/encoding with partner device; remove noise sources |
| 7F21H | Conversion setting error | Verify ASCII/Binary conversion and variable mapping | Correct conversion mapping and conversion rules |
| 7F22H | Data length error | Compare actual length vs configured length | Correct data length setting; adjust partner device message length |
| 7F23H | Decimal point position error | Check decimal position settings | Correct decimal point position in conversion settings |
| 7F24H | Value range error | Check converted value against permitted range | Correct range and scaling; verify partner value format |
| 7F25H | Data too long / too short | Check configured storage length vs actual received | Correct length mismatch; revise protocol definition |
| 7F26H | Not enough storage area | Check storage allocation vs data amount | Expand storage area or reduce stored data volume |
| 7F40H–7F42H | Transmission/Receive monitoring timeout | Check partner response timing, line stability, handshake | Increase monitoring time; verify wiring/handshake; confirm partner timing |
| 7F67H | Request received while already communicating | Check if requests overlap (tasks/channels) | Prevent overlapping requests; retry after completion |
| 7F6CH | Protocol used by another channel | Check protocol assignment per channel | Do not share same protocol across channels simultaneously |
| 7FC8H | Protocol setting data error | Verify protocol setting data consistency | Check and rewrite protocol setting data |
| 7FE6H–7FE8H | Remote password errors | Verify password config and lock status | Correct remote password settings; clear lock state if required |
| 7FEFH | Switch setting error | Check module switch settings and configuration | Correct switch settings; restart module to apply |
| 7FF0H–7FF5H | Simultaneous execution / illegal state / request errors | Check request timing and execution overlap | Prevent overlapping operations; correct state/timing; retry after completion |
| 7FF8H | System error (C24 OS) | Internal OS/system-level fault condition | Follow procedure (*1): verify environment/power; replace module if persistent |