I wanted to share something I found.

When Reading and Writing 100 Long words in sequence to a MicroLogix 1400, changing the Coms Servicing Selection and Message Servicing Selection has an impact on performance and write cycle.

I use the Default settings which will transfer only one message per read write cycle. If you write data and expect everything in the plc to change in the same cycle, it may not. I had to code where I write to the PLC and then keep reading until I see the update happen in the reply. It is very reliable but not instant.

Using the non-default setting changed the performance of the PLC but I'm pushing it to the limit as it is.

http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1766-rm001_-en-p.pdf

In one of the newer versions I tried to do the same thing and found that I now get an error if I try to read or write a block greater than ~30 long words. Somehow I think its related but I don't have a clue how to verify that.

I haven't tried the 3.27 yet.

If anyone has experience with this I would like to hear what you have found.

Thank you, Larry