Emerson OpenComms-485 manual Modbus Packet Format

REV. 5 7/6/2007

Liebert Monitoring Group –Development

Modbus Packet Format

Each Modbus packet consists of the following fields:

•Device Address

•Function Code

•Data Field(s)

•Error Check Field

Device Address:

The address field immediately follows the beginning of the frame and consists of 8-bits (RTU). This bit indicates the user assigned address of the slave device that is to receive the message sent by the attached master device.

Each slave must be assigned a unique address and only the addressed slave will respond to a query that contains its address.

Function Code:

The function code field tells the addressed slaves what function to perform. Function codes are specifically designed invoke a specific action by the slave device. The function code range is from 1 to 127.

OC-485 Modbus server supports the following Modbus function codes.

Code	Function	Description
01	Read Coils	Read from 1 to 2000 contiguous status of coils managed
		by the server. Coils in the response message are packed
		as one per bit of a byte, 1=ON and 0=OFF. If the
		requested quantity of coils is not a multiple of 8, zeros
		are padded in the final byte.
02	Read Discrete	Read from 1 to 2000 contiguous status of input status
	Inputs	managed by the server. Discrete inputs in the response
		message are packed as one per bit of a byte, 1=ON and
		0=OFF. If the requested quantity of inputs is not a
		multiple of 8, zeros are padded in the final byte.
03	Read Holding	Read the contents of contiguous block of 1 to 127
	Registers	holding registers. Data are packed as two bytes per
		register; the first byte contains the high order bits.
04	Read	Read the contents of contiguous block of 1 to 127 input
	InputRegisters	registers. Data are packed as two bytes per register; the
		first byte contains the high order bits.
05	Write Single Coil	Write a single output to either ON(1) or OFF(0) mapped
		in coil section.
06	Write Single	Write a value into a single holding register;
	Register
15	Write Multiple	Force each coil in a sequence of coils to either ON or
	Coils	OFF.
		2