ABB FMBT-21, User Manual

Page: 47 / 92

Modbus/TCP – Communication
protocol
Contents of this chapter
This chapter describes the Modbus/TCP communication protocol for the adapter
module.
Modbus/TCP
Modbus/TCP is a variant of the Modbus family of simple, vendor neutral communication
protocols intended for supervision and control of automation equipment. Specifically,
it covers the use of Modbus messaging over TCP connection on an IP network.
The FMBT-21 adapter module acts as a Modbus/TCP server with support for the ABB
Drives and Transparent profiles. The adapter module also supports Modbus over UDP.
The only difference between Modbus/TCP and Modbus/UDP is that in Modbus/UDP
the transport layer protocol is UDP instead of TCP.
The supported Modbus commands are listed in section Function codes (page 48) .
Two simultaneous Modbus/TCP connections are supported, that is, two clients can
be connected to the adapter module at a time.
For information of the port used with Modbus/TCP or Modbus/UDP, see TCP and UDP
service ports (page 70) .
Further information on the Modbus/TCP protocol is available at www.modbus.org .
Register addressing
The address field of Modbus Requests for accessing Holding registers is 16 bits. This
allows the Modbus protocol to support addressing of 65536 Holding registers.
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Modbus/TCP – Communication protocol 47