FIELDBUS COMMUNICATION 1. Modbus RTU Modbus is a master-slave protocol and only one device can transmit on the line at any time. The master (which in most cases is a PLC) manages the exchanges and only it can take the initiative. It interrogates each of the slaves in succession and no slave can send a message un- less it is invited to do so. The master repeats the question when there is an incorrect ex- change, and declares the interrogated slave absent if no response is received within a given time period. If a slave does not understand a message, it sends an exception response to the master who may or may not repeat the request.
The Modbus protocol provides full control and status information of the Softstarter, reading as well as writing of parameters. Through the fieldbus it is possible to start and stop the mo- tor, read out currents and frequency, get information of faults and protections and read de- vice identification. See chapter 8 in the PSE Softstarter Installation and commissioning manual, document 1SFC132057M0201, for fieldbus related settings. Before the Modbus RTU can be taken in operation following parameters must be set in the Softstarter: – Parameter Fieldbus interface type set to internal Modbus RTU – Parameter Fieldbus control shall be set to On if the motor shall be controlled by Modbus.
Otherwise if only reading values from the Softstarter is required, set this parameter to OFF – Parameter Internal Modbus RTU baud rate shall be set to a value matching existing Mod- bus network. Supported baud rates are 9.6, 19.2, 38.4, 57.6, and 76.8 kBaud – Parameter Internal Modbus RTU frame format shall be set to a value matching existing Modbus network. Supported frame formats are 8O1, 8E1, 8N2 and 8N1 – Parameter Fieldbus address shall be set to an available Modbus slave id. In the examples (section 4) the fieldbus address is set to 47, but this parameter can be set to any value be- tween 1-247. Caution!
The motor may start unexpectedly if there is a start signal present when doing any of the actions listed below. – Switching from one type of control to another (fieldbus control/hardwire control) – Reset all Settings STATUS Approved PAGE 3/22 © Copyright 2019 ABB. All rights reserved. SECURITY LEVEL Public DOCUMENT ID. 1SFC132367M0201 REV. B LANG. en