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Modbus Map

The device works as a Modbus TCP slave.

Coils

Read: FC01
Write: FC05 / FC15

Address Description Format
0 Relay command 0 = OFF, 1 = ON
1 Relay timer enable 0 = timer disabled, 1 = timer enabled

Input Registers

Read: FC04

Address Description Format
0 Actual relay state 0 = OFF, 1 = ON
1 Remaining time Seconds
2 Relay timer enabled state 0 = disabled, 1 = enabled

Modbus 3xxxx notation:

Modbus Poll address Zero-based address Description
30001 0 Actual relay state
30002 1 Remaining time in seconds
30003 2 Relay timer enabled state

Holding Registers

Read: FC03
Write: FC06 / FC16

Address Description Format
0 Relay runtime Seconds

Default value: 300 seconds.

Allowed range:

Limit Value
Minimum 1 second
Maximum 3600 seconds

Values outside the allowed range are clamped by the firmware.

Modbus Poll Setup

Turn Relay On or Off

Function: 05 Write Single Coil
Address: 0
Value: ON / OFF

Enable or Disable Relay Timer

Function: 05 Write Single Coil
Address: 1
Value: ON / OFF

When Coil 1 is ON, the relay turns off automatically after the configured runtime. When Coil 1 is OFF, the relay stays on until SCADA writes OFF to Coil 0.

Read Relay State and Remaining Time

Function: 04 Read Input Registers
Address: 0
Quantity: 3

Expected result:

  • 30001 / Address 0: relay state.
  • 30002 / Address 1: remaining time in seconds.
  • 30003 / Address 2: relay timer enabled state.

Read Configured Runtime

Function: 03 Read Holding Registers
Address: 0
Quantity: 1

Write New Runtime

Function: 06 Write Single Register
Address: 0
Value: 120

Example: writing 120 sets the relay runtime to 120 seconds and saves the value to EEPROM.