Jump to content

one Modbus Master to Multiple Slaves (Serial - RS485)

Recommended Posts

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.


Link to comment
Share on other sites

  • MVP 2023

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.

  • Upvote 1
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...