Network Communication With The Incorporated Modbus Interface; Protocol Description - ABB AC 31 Technical Manual

1. Network communication with the incorporated MODBUS interface

1.1. Protocol description

Support
Mode
Number of connection points
Protocol
Transmission control
Speed
Maximum length
ABB Control - AC 31
1SBC 260400 R1001-b 06.99
Communication
Communication is of greater importance in wide area installations. The AC 31 system
possesses other interfaces, apart from the CS 31 bus, adapted to different communication
modes.
This chapter presents, in particular, the interfaces available with the 40 and 50 series central
units which possess three communication protocols on the same serial interface (Figure 7-1).
The MODBUS mode is the easiest and most powerful of the three for communicating with
other devices.
The MODBUS protocol, which is known worldwide, incorporated in the 40 and 50 series
central units is the MODICON MODBUS RTU protocol.
Numerous automation devices, such as PLCs, displays, variable speed drives or monitoring
systems posses a standard, or optional, MODBUS RTU interface and can therefore easily
communicate with the 40 or 50 series central units via the serial interface (RS232 or RS485) or
via the CS31 connection but only in MODBUS slave.
MODBUS slave via the CS31 connection is available from software version 1.7 (on 07 KR 51)
Opposite are three configuration examples:
Point to point connection with a PC (Figure 7-2)
G G G G
Network connection with a PC (Figure 7-3)
G G G G
Network connection between central units (Figure 7-4)
G G G G
MODBUS is a question / answer type protocol, or still occasionally called master / slave :
the master sends a request to the slave and awaits the slave's reply.
The master devices on a MODBUS
supervisory systems. The slaves on the MODBUS network are generally PLCs, variable
speed drives, etc.
MODBUS protocol description:
Serial line
1 single master
max. 1 slave with RS232 interface
max. 12 slaves with build-in RS485 interface
max. 255 slaves with repeaters
MODBUS (master / slave)
CRC 16
up to 19 200 Bauds
on RS485:
1 200 m at 19 200 Bauds
1 500 m at 300 Bauds
network are generally central units, displays or
CS31 connection
half-duplex
1 slave with RS485 interface
max. 255 slaves with repeaters
MODBUS (slave)
CRC 16
up to 76 800 Bauds
on RS485:
600 m at 76 800 Bauds
1 200 m at 19 200 Bauds
1 500 m at 300 Bauds
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