@Controls Guy, thank you for the suggestion, but can you explain what you mean by reorganizing my logic to slice of consecutive scans? I'm not sure that I understand that one.
There is probably a lot of the logic that doesn't need to be run each and every scan. Do-More has blocks of code that called Programs and Tasks that are, first, selectively executed, and second, can be interrupted before completion each scan once they've run for so many microseconds. That allows you to execute your code no faster or more often than needed to do it's job, freeing up resources to execute more time-sensitive code more promptly, or to reduce scan time.
You can do the same thing in your DL260 by selectively executing SBR's or JMP's. Plus, if you execute half the code each scan, you'll cut the logic portion of the scan by 50%, so even after you add back the new comms time between the increased number of scans, you won't have doubled the effective scan time of any particular rung of code.
I may suggest to the customer to swap to a Do-More at some point, but they just went through a big upgrade and I don't think they'll be open to spending more money to change something that's been working fine for the last 11 years. I'd have to re-write and test almost 1000 rungs of code (although I understand that it might be a little easier in the do-more environment).
Well, it's not the same thing now that's been working fine for 11 years. It's not really logical for them to say "It's been running the equipment OK for 11 years so we can add new comm load without limitation and be surprised if it doesn't still work."
You might also consider adding a second ECOM, though if it's scan time limiting the bandwidth, that's not going to help. You could also add a comms concentrator in the form of a new PLC or maybe you can do it with some of the hardware you have there now. Just have one device poll the PLC for all the registers needed by any one device (realtime writes like button pushes should probably still go direct to the PLC) and have everyone else talk to that device. Fewer requests for larger blocks (and sequenced by this concentrator device) will be more efficient for the PLC.
Finally, I agree with ADC PE and Franji1 that no clients should be set to poll as fast as possible (probably ever, but certainly in this scenario).