CommunicationABB Control - AC 31 Page 7-91SBC 260400 R1001-b 06.991.3.4. Example using the MODBUS functionAn example of MODBUS communication between two 40 series central units.40 seriesMODBUSmasterMODBUS protocolRS232 line40 seriesMODBUSslave 1- A program isn't required in the slave central unit.- A program example in the master central unit:Read the 6 bits I62.00 to I62.05 in the slave 1 central unit.The information is placed in O62.00 to O62.05 of the master central unit.The MODBUS address of variable I62.00 in the slave 1 is 992 in decimal.A delay should be defined to enable re-sending the MODBUS message in case of error. Thisdelay is called TIME_OUT and is generally 2 seconds.ERROR_CODEERRORREADY%O62.006992TIME_2SREAD_BITSSLAVE_1READYMODBUSFREISLAVFCTTIMEADDRNBDATARDYERRERNIn case of MODBUS communication between one master 50 serie central unit and differentslaves 50 series central units, different MODBUS functions can be used. It is possible to linkthem together to optimize the refresh time.Check example : Read 6 bits continuously I62.00 to I62.05 in the slave 1 central unit. Theinformation is placed in O62.00 to O62.05 of the master central unit. Write 20 words continuouslyIW02.00 to IW02.19 in the slave 2 central unit. The information is received from master centralunit in MW00.00 to MW00.19.MODBUSFREISLAVFCTTIMEADDRNBDATARDYERRERNSLAVE_1READ_BITSTIME_2S9926%O62.00ERROR1ERROR_CODE1 MODBUSFREISLAVFCTTIMEADDRNBDATARDYERRERNSLAVE_2WRITE_WORDSTIME_2S000020%MW000.00ERROR2ERROR_CODE2
