I am relatively new to the industry and I am not completely up on my terminology. My main question is I have a network of wireless sensors that are being written to modbus addresses. I then use a prosoft card to move it to a PLC-5. I want to know if there is a limit to the amount of information that I can send through this path. Currently I have 120 addresses in the modbus being used. I then use a block transfer to move that data into the PLC. I am being asked from a co-worker at what point will the system start to bog down and or crash. Sorry if that doesn't make sense but if you need more information just ask. If you understand what I am saying and I am saying it incorrectly then please let me know the proper terminology.

Thanks in advance!

 

To begin with Modbus could support about 240 to 250 addresses.

BUT, as your co-worker points out it would take quite a bit of time to manage an update from all devices.

For example, 120 addresses could take as much as 120 seconds or more depending on the physical layer in use. RS485 would be slowest because of the key up and key down issues. On the other hand there are fast RS485 devices that are keyed up and down with data as opposed to Request to Send signals.

 

no one can really answer this question for you. if the data is not time critical, it probably won't matter. i had an application took took almost ten seconds to poll all the devices and collect the data using a prosoft module with a slc500. it was quite adequate for what i was doing.

--
Bob
http://ilbob.blogspot.com/

 

I forgot to point out another RS485 limitation and that is you can only have about 20 RS485 receivers for one RS485 driver.