ABB AC500 Introduction Manual page 15

Brief information about Modbus
Modbus® RTU (developed by Modicon in 1979)
Modbus® RTU is an open master/slave protocol, and can be easily implemented on serial interfaces.
Numerous automation systems have Modbus® RTU interfaces as standard or optional features, and are
thus easily able to communicate with the AC500 via its integrated COM1 and COM2 interfaces (RS232
or RS485). The Modbus® is used not only in industrial applications, but also in building installations, in
energy optimization systems, for long-distance data transmission and for linking up operator panels.
Communication
By polling, i.e. the master transmits a request to the slave and then receives the response. Both
interfaces COM1 and COM2 can operate simultaneously as Modbus interfaces. The Modbus operating
mode of an interface is set using the engineering tool.
Topology
Point-to-point via RS232 or multi-point via RS485. With RS232, a maximum of one master and one slave
is possible, while with RS485 one master and a maximum of 31 slaves can be operated. The maximum
cable length is 15 m with RS232 and 1.2 km with RS485.
Data transfer
Max. 187.5 kB/s. Each telegram has a 16-bit CRC appended. The telegrams permit process data
(input/output data) to be written and read, either individually or in groups. The data are packed in the
RTU format.
Transmission media
May vary. One widely used option is the RS485 bus physics, a twisted-pair, shielded cable with
terminators.
Diagnostics
Detailed diagnostic messages for rapid trouble-shoot-ing are shown on the CPU display or can be called
using the programming tool.
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G1
Introduction to AC500 1-13
AC500 / Issued: 07.2006