ABB AC800M interface to Modbus device


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We have a problem in connecting Modbus devices to AC800M (PM856 CPU). Infact we have used RS232 to RS485 convertor and the TX , RX & GND of Convertor are connected to RX,TX & GND of PM856CPU. The poll time in the Modbus setting of PM856CPU is set to 0.
We have used MBConnect & MBRead to connect & read from Slave device. For both commands the status is being displayed as 1, but we are unable to connect to the slave device.

Kindly suggest us to overcome the above problem.

When u enable MBConnect with polltime equal to 0(zero) it is just a logical connection. If u really want to connect your modbus master with your modbus slave put polltime greater than zero. and enable the MBConnect block. As soon as you enable connect block master will send FC8 funciton code to the slave before setting the status of connect block to 1. If ur slave supports FC8 function block it will reply to FC8 and master will set the status of connect block to corresponding status code. Here Status 1 means connection esatblished successfully.

Good luck

I've put polltime greater than zero (4000) and the connection is established (when I enable the MBConnection block, I see the LEDs on CP341 flash). It will read 2 register (with FC4 modbus function) how the parameters (Rdn) of MBread block I must create? The partner (slave) is a S7-300 PLCs with CP341.

Thanks to all.