Which Modbus function codes are supported?

Use 0x01, Read Coils, and 0x05, Write Single Coil. The gateway exposes coils rather than holding registers.

Which values are valid for Write Single Coil?

The documented values are 0xFF00 for ON and 0x0000 for OFF. Other values return exception 0x03, Illegal Data Value.

Set the Unit ID to 0x01, which is decimal 1.

The gateway supports the following Modbus function codes:

Code	Name	Description
0x01	Read Coils	Read coil status (status registers)
0x05	Write Single Coil	Write a single coil (command registers)

The gateway returns the following Modbus exception codes when an error occurs:

Code	Name	Description
0x01	Illegal Function	Function code not supported
0x02	Illegal Data Address	Coil address out of range
0x03	Illegal Data Value	Invalid value (neither 0xFF00 nor 0x0000)

All Modbus TCP communication uses the standard MBAP (Modbus Application Protocol) header:

Byte	Field	Value
0–1	Transaction ID	Set by client (echoed in response)
2–3	Protocol ID	0x0000 (Modbus)
4–5	Length	Number of following bytes
6	Unit ID	0x01
7	Function Code	0x01, 0x05
8+	Data	Function-specific payload