Hi friends! Need some advice on AC500 Modbus communication..

working on Modbus serial communication using MOD_COM_Mast. I have AC500 plc that needs to talk to three third party drives. I have to write& read from 1797,1798 ( drives Modbus address for speed ref and cw resp. for all three drives) and also only read from modbus address 2049(drive modbus address for speed feedback from three drives). I can read and write on 1797 and 1798 but can't read from 2049. I have attached the program for your reference.

I tried to consolidate three modbus address in single MOD_Com_Mast but the number of words that it can read is over the limitation, fc 3 can only do 125 words. I created a separate MOD_COM_Mast for modbus address 2049 but it overwrites in two of the first modbus address (1797,1798) and also when i am online it doesn't respond at all.

(1797-2049 is 253 words)

How do i resolve this issue.. need your help please.

Everybody's enjoying thanksgiving, but i am here trying to figure out this problem.

Please have a look at the program and let me know what i am missing..

I would be so grateful.

Thanks

modbus.zip

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Asked 2 years ago

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Not much experience with Modbus Master, but I have a few suggestions. Make all your reads and writes sequential. That assures that only one function block is trying to access the same com port at a time. Like you did with the first two, but also do with the 3rd one. Also, you then only need to increment the slave address in one place, not two like you are currenlty doing. Expecting this task to complete in 1ms might be a little ambitious. The function blocks might execute that fast, but you certainly won't get the comms that fast.

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