Gotchas

Each entry starts with the symptom you will see in the editor, debug sidebar, or node status. The fixes refer to the public slmp-connection, slmp-read, and slmp-write nodes.

slmp-read returns nothing

Symptom	Root cause	Fix
slmp-read produces no useful payload or errors immediately.	The selected slmp-connection has no PLC profile at runtime. The runtime requires one explicit canonical PLC profile.	Open the slmp-connection config node and select the correct PLC profile, such as melsec:iq-r.

Mixed word+bit write fails

Symptom	Root cause	Fix
One slmp-write node that writes word addresses and bit addresses returns a PLC error.	Some PLCs reject SLMP command 0x1406 for mixed word and bit block payloads.	Use one slmp-write node for word updates and another slmp-write node for bit updates.

G/HG rejected

Symptom	Root cause	Fix
G or HG addresses are rejected by slmp-read or slmp-write.	Module buffer access is not exposed through the high-level Node-RED node surface.	Keep G and HG out of high-level flows, or use a function node with the lower-level JavaScript API for raw module access.

DX/DY fails on melsec:iq-f

Symptom	Root cause	Fix
DX or DY fails when the connection PLC profile is melsec:iq-f.	The iQ-F profile does not support DX and DY.	Use X and Y for iQ-F, and remember that iQ-F X/Y text uses octal numbering.

LTN/LSTN/LCN/LZ reads return wrong data

Symptom	Root cause	Fix
Long timer, long counter, or long index values look truncated or are rejected.	LTN, LSTN, LCN, and LZ are 32-bit current-value families, not normal 16-bit word values.	Address them as LTN0:D, LSTN0:D, LCN0:D, or LZ0:D; use :L for signed 32-bit values.

Non-canonical PLC profile rejected

Symptom	Root cause	Fix
A hand-edited flow or environment-provided PLC profile is rejected.	The node accepts only exact canonical PLC profiles. Short names and aliases are not normalized.	Use one of the canonical profiles shown in the slmp-connection dropdown, such as melsec:iq-r.

X/Y works on one PLC profile but fails on another

Symptom	Root cause	Fix
An X or Y address works after changing PLC profile but points at a different I/O point.	melsec:iq-f uses octal X/Y text, while the other supported profiles use hexadecimal text.	Review X and Y addresses whenever you copy a flow between PLC profiles.

D50.3,count is rejected

Symptom	Root cause	Fix
An address such as D50.3,8 is rejected.	.bit notation is scalar-only and means one bit inside one word.	Use D50.3 for one bit, or use a direct bit range such as M1000,8 for consecutive bit devices.

Device-matrix flow is noisy as a first test

Symptom	Root cause	Fix
The first imported flow produces many skipped or failed entries.	slmp-device-matrix.json is a broad verification flow, not the smallest connection smoke test.	Import slmp-basic-read-write.json first and verify a simple D300 read before using the matrix flow.