Worked Example

This section provides a practical example of sending Modbus TCP commands to a Brainboxes ED device.

Writing a Single Coil

In this example, we are sending Modbus commands to write single bits to an ED-527 which are either high or low.

To write a single bit as either high or low, use the Write Single Coil function code: 05

Value	Result
`0x0000`	Output bit LOW
`0xFF00`	Output bit HIGH

The ED-527 has its 16 output register addresses starting at 0x0001 through to 0x0010.

To set the bit high in register 1, the Modbus packet will look like this:

0017 0000 0006 FF 05 0001 FF00

Field	Value	Description
TrID	`0017`	Transaction ID
Prot	`0000`	Protocol (always 00 for Modbus TCP)
Len	`0006`	Number of bytes in the rest of the transaction
UI	`FF`	Unit Identifier
Fn	`05`	Modbus function code for writing a single coil
Addr	`0001`	Address of the Modbus register to set
Data	`FF00`	Data value (`0xFF00` = HIGH, `0x0000` = LOW)

Understanding the Fields

Transaction ID (TrID): A unique identifier for this transaction, useful for matching responses to requests
Protocol (Prot): Always 0000 for Modbus TCP
Length (Len): The number of remaining bytes in the message (Unit Identifier + Function Code + Address + Data)
Unit Identifier (UI): Used for routing in gateway scenarios; typically FF for direct connections
Function Code (Fn): Specifies the action to perform (05 = Write Single Coil)
Address (Addr): The target coil address
Data: The value to write to the coil

To set the same output low, change the data value to 0x0000:

0018 0000 0006 FF 05 0001 0000

Field	Value	Description
TrID	`0018`	Transaction ID (incremented)
Prot	`0000`	Protocol
Len	`0006`	Length
UI	`FF`	Unit Identifier
Fn	`05`	Function code (Write Single Coil)
Addr	`0001`	Address of register 1
Data	`0000`	Set bit LOW