All relay coil reads return errors. What should I check?

Confirm the RS485 A and B lines are not swapped. The KMtronic board uses 9600 baud, 8 data bits, 1 stop bit, no parity. Any mismatch in the ModbusCloud Gateway serial settings will cause all reads to fail. Also verify the unit ID in the portal matches the DIP switch position on the board; the factory default is unit ID 1.

The portal shows the relay state, but the physical contact does not switch.

FC01 reads coil state as reported by the microcontroller, which may not reflect a welded or failed relay contact. Check the load wiring on the NO or NC terminals independently with a multimeter. Inductive loads such as motor coils or solenoids can cause voltage spikes that damage contacts; fit a suppression capacitor if not already present.

Why does a write to relay_1_b not always take effect?

The KMtronic 4Port uses FC15 (Write Multiple Coils) in the OpenEMS implementation. Some older firmware variants on KMtronic boards respond only to FC05 (Write Single Coil). If writes are acknowledged with an exception code, test with FC05 directly using a Modbus test tool to determine which function code the board firmware supports.

Is it safe to write relay states from an automated rule in the cloud portal?

Write registers (relay_1_b through relay_4_b) directly energise or de-energise relay contacts. Before enabling automated write rules, confirm the connected load can tolerate unexpected switching, particularly for HVAC contactors or motor starters where abrupt on/off cycles can cause equipment damage or trip protection devices. Always test with low-risk loads first.

Free Modbus template: KMtronic 4Port

The KMtronic 4Port is a four-channel relay output board controlled over Modbus RTU via RS485. It is used in building automation, industrial control, and energy management installations where software needs to switch discrete loads such as lights, ventilation contactors, motors, or alarm outputs. This template reads the live state of all four relays through coils 0 to 3 and writes target states using function code 15. Connecting the KMtronic 4Port to a ModbusCloud Gateway lets you log relay states, trigger cloud alerts, and audit switching history without additional hardware.

Name	Address	Function	Type	Access
relay_1_a Read-back state of relay 1, true means energised (ON).	0	FC01	bool	R
relay_2_a Read-back state of relay 2, true means energised (ON).	1	FC01	bool	R
relay_3_a Read-back state of relay 3, true means energised (ON).	2	FC01	bool	R
relay_4_a Read-back state of relay 4, true means energised (ON).	3	FC01	bool	R
relay_1_b Write coil to turn relay 1 on (true) or off (false).	0	FC15	bool	W
relay_2_b Write coil to turn relay 2 on (true) or off (false).	1	FC15	bool	W
relay_3_b Write coil to turn relay 3 on (true) or off (false).	2	FC15	bool	W
relay_4_b Write coil to turn relay 4 on (true) or off (false).	3	FC15	bool	W

KMtronic 4Port

Overview

Use cases

Installation

Things to watch for

Registers (8)

Working with Modbus in your projects?

Free: RS485 and Modbus RTU installation checklist (PDF)

More KMtronic templates

KMtronic KMtronic

More RTU templates

Alnor HRU-P

Alnor HRU

Wattsonic - Schmachtl Sun Energy Box

Heidelberg Wallbox Energy Control

connecdoor.controller

Eastron SDM72D-M