Schneider Electric 890USE17700 Modbus Commands Supported by the STB NIP, Message Data, Frame

Data Exchange

Modbus Commands Supported by the STB NIP 2212

Introduction Modbus is the protocol used by Modicon PLCs. Modbus defines the message structure that the PLCs understand and use, regardless of network type. The Modbus protocol describes the process that a controller uses to access another device, how that device responds, and how errors are detected and reported.

Modbus	Modbus messages are embedded within the frame or packet structure of the
Message Data	network in use. A Modbus over TCP/IP network uses both the Ethernet II and
Frame	IEEE 802.3 data formats. For communications with the STB NIP 2212, Modbus
	messages can be embedded in either frame type. Ethernet II is the default data
	format.
Modbus	The Modbus protocol uses a 16-bit word. A Modbus message begins with a header.
Message	A Modbus message uses a Modbus function code (See List of Supported
Structure	Commands, p. 135) as the first byte.
	Following is a description of the structure of a Modbus message header:
	Invoke Identifier	Protocol Type	Command	Destination	Modbus
			Length	ID	Message
	two-byte field that	two-byte field	two-byte field	one-byte	n-byte field
	associates a	value for Modbus	value is the size of		first byte is the
	request with a	is always 0	the rest of the		Modbus
	response		message		function code

134 890USE17700 April 2004