CAI 700 NDIR, Operator'S Manual

Page: 174 / 205

700 NDIR Analyzer 6-20-2017
174
Modbus Protocol
Modbus on TCP/IP Application Data Unit
MBAP Description
This section describes the encapsulation of a Modbus request or a response when it is carried on
a Modbus TCP/IP network.
A dedicated header, called the MBAP (Modbus Application Protocol) header, is used on TCP/IP
to identify the Modbus Application Data Unit. This header provides some essential differences
compared to the Modbus RTU application data unit used on the serial line:
1. The Modbus Slave Address field usually used on a Modbus Serial Line is replaced by a
single-byte Unit Identifier within the MBAP Header. The Unit Identifier is used to
communicate via devices such as bridges, routers and gateways that use a single IP
address to support multiple independent Modbus end units.
2. All Modbus requests and responses are designed to allow the recipient to verify that a
message is finished. For function codes on which the Modbus PDU has a fixed length,
the function code alone is sufficient. For function codes carrying a variable amount of
data in the request or response, the data field includes a byte count.
3. When Modbus is carried over TCP, additional length information is carried in the MBAP
header to allow the recipient to recognize message boundaries even if the message has
been split into multiple packets for transmission. The existence of explicit and implicit
length rules and use of a CRC-32 error check code (on Ethernet) virtually eliminate the
possibility of undetected corruption to a request or response message.