Relay status registers 105 to 110 all return zero. Why?

All-zero results on the read registers typically mean the FEMS Modbus TCP API is active but the relay module has not been enabled in the FEMS App Center. Confirm that the relay I/O app is installed and activated. Also verify the Modbus unit ID matches the FEMS default of 255.

Written relay commands have no effect on the connected load. What now?

Check that the FEMS App Modbus/TCP Schreibzugriff is licensed and active. FEMS requires a valid write-access license before FC16 writes to addresses 1 to 6 are accepted. Without it, reads succeed but writes are silently discarded.

Relay closes on command but reverts to open after about 60 seconds.

FEMS has a built-in watchdog on write registers. If no refresh write arrives within the configured API timeout (default 60 seconds), the command is cancelled and the relay reverts. To keep a relay closed continuously, re-issue the write command at least every 30 seconds from your automation logic.

Is it safe to write to relay setpoint registers during live operation?

Use caution. The relay outputs may control loads rated up to 230 V / 10 A or 24 V / 1 A depending on the channel. Writing to the wrong channel or sending repeated rapid toggles can trip connected protection devices, interrupt critical loads, or damage relay contacts. Test in manual mode first and confirm the correct channel assignments before enabling automated writes.

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Registers (12)

6 read / 6 write

Name	Address	Function	Type	Access
relay_1_a Read-back status of relay channel 1, 0 = open, 1 = closed.	105	FC03	uint16	R
relay_2_a Read-back status of relay channel 2, 0 = open, 1 = closed.	106	FC03	uint16	R
relay_3_a Read-back status of relay channel 3, 0 = open, 1 = closed.	107	FC03	uint16	R
relay_4_a Read-back status of relay channel 4, 0 = open, 1 = closed.	108	FC03	uint16	R
relay_5_a Read-back status of relay channel 5, 0 = open, 1 = closed.	109	FC03	uint16	R
relay_6_a Read-back status of relay channel 6, 0 = open, 1 = closed.	110	FC03	uint16	R
relay_1_b Command setpoint for relay channel 1: write 1 to close, 0 to open.	1	FC16	uint16	W
relay_2_b Command setpoint for relay channel 2: write 1 to close, 0 to open.	2	FC16	uint16	W
relay_3_b Command setpoint for relay channel 3: write 1 to close, 0 to open.	3	FC16	uint16	W
relay_4_b Command setpoint for relay channel 4: write 1 to close, 0 to open.	4	FC16	uint16	W
relay_5_b Command setpoint for relay channel 5: write 1 to close, 0 to open.	5	FC16	uint16	W
relay_6_b Command setpoint for relay channel 6: write 1 to close, 0 to open.	6	FC16	uint16	W