Schneider Electric 890USE17700 - Manual (Page 134)

Data Exchange
134 890USE17700 April 2004
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
Message Data
Frame Modbus messages are embedded within the frame or packet structure of the
network in use. A Modbus over TCP/IP network uses both the Ethernet II and
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
Message
Structure The Modbus protocol uses a 16-bit word. A Modbus message begins with a header.
A Modbus message uses a Modbus function code (See List of Supported
Commands, p. 135) as the first byte.
Following is a description of the structure of a Modbus message header:
Invoke Identifier Protocol Type Command
Length Destination
ID Modbus
Message
two-byte field that
associates a
request with a
response two-byte field
value for Modbus
is always 0 two-byte field
value is the size of
the rest of the
message one-byte n-byte field
first byte is the
Modbus
function code