Jump to content

modbus polling and U90 debug

Recommended Posts


I am trying to use the M91-ra22 as a modbus master and using i to poll a 3rd party device (ciat connect2 HVAC controller)

I have set up the com port initialisation and the timed polling logic to access different sections of info on the connect2.

This works fine if I'm not trying to access the M91 using the dbug function of U90. As soon as I do the connection to my laptop gets severed and I then have to reset the PLC in order to resume comms.

The polling logic is user initiated and uses cascading timers (4 of) set at 2 seconds each.

I am connecting to the PLC using a 232/485 converter to get onto the 485 bus. this works fine on its own.

Any ideas how I can access the PLC and it still poll the 3rd party device simultaniously??

Link to comment
Share on other sites


Do you have a good sense of humor? If yes:

Your question seems like "How can I drive my car 100 km/h forward and in the same time 80 km/h reverse?" :rolleyes:

Seriously - you want to run the same port, in the same time, two totaly different protocols! Sorry, this is just not possible.

You can use V120-22-RA22 instead. This model has exactly the same I/O and pinout (the I/O card inside in fact is the same), but has graphical display, about 20 times more memory and is about 20 times faster. For you the key is, that it has two independend COM ports, so what you want to perform is absolutelly normal with it.

Link to comment
Share on other sites

  • MVP 2014

What Emil says is of course true. However, if you are stuck with the M91 there are a couple of other things that may help.

1. Whilst you can't poll modbus and connect U90 ladder in debug mode at the same time, you can include the logic in your program that allows you to manually suspend the Modbus polling so you can connect U90 Ladder. If you re-trigger the modbus initialisation with a ---|P|--- coil (placed in parallel with the power-up bit), then you can re-start the Modbus comms without needing to cycle power.

2. to debug the modbus comms you can use a PC-based modbus program to listen on the RS485 network. At the basic level you can see the Hex data from the PLC and the slave, but you might be able to find more advanced software that can do some decoding of the Modbus packet.

Link to comment
Share on other sites

  • 1 year later...

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...