Fenecon MasterBox2V0 Relay
The Fenecon MasterBox2V0 Relay is a six-channel digital I/O relay module integrated into the FENECON Energy Management System (FEMS). It is used to switch external loads such as SG-Ready heat pumps, heating elements, combined heat and power units, and other consumers based on energy management signals. This template reads the live status of all six relay channels via Modbus TCP (addresses 105 to 110) and writes switch commands to the corresponding setpoint registers (addresses 1 to 6). Monitoring actual relay states alongside commanded states lets you verify correct switching in real time.
- Protocol
- Modbus TCP
- Connection
- Ethernet, Modbus TCP
- Registers
- 12, read and write
- Brand
- Fenecon
- Last updated
Registers (12)
6 read, 6 write
| Name | Unit | Access | Address (Hidden until you enter your email) | Function (hidden) | Type (hidden) |
|---|---|---|---|---|---|
| relay_1_a | Read | ||||
| relay_2_a | Read | ||||
| relay_3_a | Read | ||||
| relay_4_a | Read | ||||
| relay_5_a | Read | ||||
| relay_6_a | Read | ||||
| relay_1_b | Write | ||||
| relay_2_b | Write | ||||
| relay_3_b | Write | ||||
| relay_4_b | Write | ||||
| relay_5_b | Write | ||||
| relay_6_b | Write |
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Use cases
- Monitor all six relay status registers to verify that SG-Ready heat pump switching commands are executed correctly.
- Compare relay_1_a through relay_6_a read-back values against issued commands to detect contact failures or wiring faults.
- Alert on unexpected relay state changes outside scheduled time windows to spot unwanted activation of connected loads.
- Track relay switching frequency over time to identify excessive on/off cycles that may shorten contactor life.
- Report relay activation durations per channel to document heat pump or heating element operating hours for maintenance records.
Installation
Enable the Modbus TCP write-access app on the FENECON system by activating the FEMS App Modbus/TCP Schreibzugriff in the FEMS App Center and entering the license key provided with the app.
Connect the ModbusCloud Gateway to the same LAN segment as the FEMS controller, ensuring the Gateway can reach the FEMS IP address on TCP port 502.
In the ModbusCloud portal, add a new device, import the Fenecon MasterBox2V0 Relay template, and assign the correct Modbus unit ID (default 255 for FEMS systems) along with the FEMS IP address.
Verify the connection by reading relay_1_a (address 105) in the portal's live register view and confirm the value matches the physical relay state observed on the hardware.
Things to watch for
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. Keeping a relay closed continuously needs the write command re-issued well within that window, typically every 30 seconds. ModbusCloud writes registers at most once per minute, so keeping this 60-second window is borderline and not guaranteed.
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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