KMtronic KMtronic
The KMtronic 8-channel relay board is a Modbus RTU controller that switches eight independent relay outputs over an RS485 bus. Used in industrial automation, building control, and energy management installations, the board lets the ModbusCloud Gateway read each relay's live coil state (relay_1_a through relay_8_a via FC01) and write new on/off commands to all eight outputs in a single FC15 transaction. Each relay switches up to 250 VAC/10 A or 24 VDC/15 A. The board's DIP-switch unit ID and direct coil mapping make integration fast and predictable.
- Protocol
- Modbus RTU
- Connection
- RS485
- Registers
- 16, read and write
- Brand
- KMtronic
- Last updated
Registers (16)
8 read, 8 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_7_a | Read | ||||
| relay_8_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 | ||||
| relay_7_b | Write | ||||
| relay_8_b | Write |
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Use cases
- Monitor the live state of all eight relay outputs to verify correct switching sequences in automation routines.
- Alert on a relay remaining energised beyond a scheduled window, indicating a stuck contact or missed off-command.
- Track relay switching frequency over time to estimate contact wear and schedule preventive maintenance.
- Verify that load-shedding commands issued from the portal are confirmed by the corresponding read-back coil states.
- Compare expected and actual relay states across multiple KMtronic boards on the same RS485 bus to detect address conflicts.
Installation
Set the unit ID on the KMtronic board by configuring the DIP switch near the RS485 connector; note the chosen address for the portal setup.
Wire the RS485 A(Data+) and B(Data-) terminals of the board to the corresponding terminals on the ModbusCloud Gateway, and connect a shared GND; use a 120-ohm termination resistor at each end of the bus.
In the ModbusCloud portal, add the ModbusCloud Gateway, import the KMtronic template, and enter the unit ID matching the DIP-switch setting; set baud rate to 9600, 8N1.
Trigger a manual read in the portal and confirm that relay_1_a through relay_8_a return valid boolean values before enabling any write automation.
Things to watch for
All relay coil reads return an error or timeout. What is wrong?
Check that the DIP-switch unit ID on the board matches the slave address configured in the portal. Also confirm the RS485 baud rate is set to 9600, 8 data bits, no parity, 1 stop bit. A mismatched baud rate or address is the most common cause of no-response errors on this board.
The gateway connects but all relay states read as OFF permanently.
Verify wiring polarity: A(Data+) and B(Data-) must not be swapped. A reversed RS485 pair produces framing errors that may silently return zero values. Check LED indicators on the board: an energised relay lights its corresponding LED, so you can cross-check against the portal readback.
FC15 write commands seem to be accepted but relays do not switch.
Confirm that the coil address range in the write command starts at address 0 and spans exactly 8 coils. Some Modbus masters index from 1 (coil 1 = address 0); an off-by-one offset causes the command to target undefined coil space and the board returns no error but takes no action.
Is it safe to write relay states automatically from portal automations?
Use caution. The KMtronic board executes write commands immediately with no interlock. Rapid or unintended FC15 writes can switch loads connected to motors, solenoids, or high-current circuits without warning. Always test write automations with the load disconnected first, and ensure downstream equipment tolerates unexpected switching before enabling live automations.
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