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cchothani

one Modbus Master to Multiple Slaves (Serial - RS485)

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I have an application where I need to have one V130 PLC communicate to three V120 PLCs and read a bunch of MI registers. I have the V120 systems already supporting Modbus comms over RS485.

So, I setup the V130 to be a Modbus Master and talk to all three Modbus slaves. I setup the Com port for RS485 with the appropriate properties. Then I have a RHR command to read from the first PLC. And this works just fine. I can read the Modbus registers from PLC1. Then I have two more nets with an RHR command to read from PLC 2 and PLC 3, but these nets don't seem to execute at all.

What am I doing wrong? Is it even possible to have one Master talk to several slaves with the Vision PLCs? I saw a reference to some timing issues and that you maybe should add some delay time between RHR commands. Is that what needs to happen.

I need some help ASAP. I need to deliver this system in a couple of days.

Thx,

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One master can talk to several slaves but it can only talk to one at a time. The Function in progress bit in the Modbus config can not be on when you call an RHR block. You need to manage who's "turn" it is.

Post your code so we can see what you're trying to do.

Joe T.

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Take a look at the example project called V280_Ethernet_TCP_MODBUS_IP_Master.vlp, specifially ladder rungs 3-6. Note that you need to set a bit for Read and Write requests and then use a Direct Contact for the R.H.R & P.H.R functions with a Reset of the bit after. Note the inverted contact for Function in Progress in series.

What this does is allow the request to remain open until the Function in Progress clears and then the Read or Write will occur.

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