ML1400 Modbus communication

Rob735

Member
Join Date
Jan 2011
Location
Mississauga
Posts
38
Hello Gentlemen,
I am hoping someone here may be able to help me out. I have a project where I am using an AB ML1400 to communicate through MODBUS via a 1763-NC01 module to 12 refrigeration compressors.
I am wondering if there is a way other than cycling power to the PLC to “reset” the message blocks or the cascade so it will start like the first time it was triggered.
I have the system running and working well. There are 14 message blocks that communicate to each machine that are triggered through a cascade effect. One must be DN to trigger the next in line. Thus only allowing 12 messages at any point in time to be in the communication buffer. I do know that with the ML1400 the buffer only has room for 14 message commands. Which is kinda where my issues begin.
Everything works well until I have a machine fault or needing to be taken out of service. Once the repair is made and the machine is brought back into service the problem is occurring. When the machine is taken out for service or faults I have a service toggle switch on the HMI indicating “IN SERVICE” or “OUT OF SERVICE”. Out of service stops communication to that machine to prevent slowing of communication on the others. However it completes all messages in the cascade and has them all ready to start at the same time when brought back into service. Which is then triggering half of them to start and causing issues with over loading the Communication message buffer.

So again is there a way other than cycling power to the PLC to “reset” the message blocks or the cascade so it will start like the first time it was triggered?

Thank you for all the help sorry for the lengthy message just trying to provide as much info as I can.
Anything else you may need to help diagnose just let me know
 
Attached is a zipped RSS file which has a subroutine which sends/receives to 4 Powerflex drives using Modbus. (You can ignore the rungs which do a MOV of 255 - it's just our way of indicating a communication problem on our display.) Note that finishing the last communication or the MCR turning off resets the chain of communications. I hope this helps.
 
Thank you Bernie. I will give this a try. It will be a fair amount of work but if it works then I will be programing my message blocks this way to start in future.
 
I really like using this method. We've had up to 15 drives hanging off the RS485 cable. The polling rate is great.

And I don't trust multiple message buffers. One at a time is comfortable for me.
 

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