FIELDBUS COMMUNICATION 5. Troubleshooting 5.1. Modbus slave does not respond to requests RS485 – Are the termination resistors placed at the end of the line? – Only 2 termination resistors in one segment? – Are 2 termination resistors placed at each bus segment? – Are bias resistors connected in each bus segment? – Is the line polarity correct? Are the lines by accident swapped? – Never place any termination resistors on a drop cable. – Is the maximum line length exceeded? Modbus Parameters – Check that fieldbus interface type parameter is set to Internal Modbus-RTU (Int on dis- play). – Check that you are using the correct Modbus address, baud rate and frame format.
– Note that the slave will not respond to broadcasts (requests to address 0), only act upon them. Modbus slave – Has the device a unique Modbus address? – Is the function code supported by the device? – Has the request a valid address? – Has the request a valid quantity of coils, inputs, registers? – Is the power supply turned on? Modbus master – Is the Modbus master in RTU mode? – Is the request to response timeout correct? – Is the Modbus silent interval between two telegrams > 3.5 character times? – Notice that the slave device will not give any response when it is addressed with a broad- cast (slave address = 0). – Check if PLC is in run mode. 5.2.
PSE trips on fieldbus fault (EF40) – The purpose of the fieldbus fault function is to detect any communication problems be- tween the Modbus master and the Softstarter. – Fieldbus fault will be enabled if the Fieldbus control parameter is set to On and Operation when fault parameter is set to “trIP”. In case the Operation when fault parameter is set as “LocC”, there will be a switch to local “hardwire” control. STATUS Approved PAGE 21/22 © Copyright 2019 ABB. All rights reserved. SECURITY LEVEL Public DOCUMENT ID. 1SFC132367M0201 REV. B LANG. en