I am looking for some information on what determines\influences how an M series serial card queries Modbus slaves when operating in RS485 half duplex mode.
Does it just work through the devices and data sets sequentially with values being updated as it works through the devices?For example, if I have slaves that vary in their response time but none that exceed the timeout, I could easily take several seconds to work through all the data sets.With quick responses, I could work through quite a few devices in less than a second.
Does the control module scan rate influence the scan order or interval of the devices?
For example, if I reference a dataset register in a module that is executing at 500ms, will the serial card poll the slave every 500ms? If I also have a module, executing at 2S, referencing a different device, will the serial card schedule those queries every 2 seconds?
We are working to integrate some legacy networks and it would help to better understand how Modbus queries are scheduled.
Thanks!
In reply to alex:
Andre Dicaire
In reply to Andre Dicaire:
Found a document on line from Mynah that talks about optimizing Modbus TCP communications with the VIM. search for "VIM polling mynah"
This shows the VIM view of the configuration in diagnostics, with Devices configured under ports of the four emulated cards. Each Device has TCP level diagnostics indicating message retries and no responses. The document discusses optional configuration parameters to manage the communications and these are either set in the DeltaV configuration dialogs, or in some cases at the Dataset. You should be able to find this with a google search.