Upgrading of PLC


Thread Starter

Sanjay Sharma

The author is currently working in captive power plant. PLC 2/30 is currently installed in boiler controls for starting, stopping and FSSS controls of boiler. It is also communicating with DCS mod 300. As DCS is being replaced by our instrumentation guys with latest DCS in market, we're also thinking in going for upgrading/replacement with new PLC.

My query is whether we can go for upgrading to PLC5 series on same 1771 platform so that existing backplanes can be used. This will reduce my cost also. If yes, what are min. changes to be done? I/O's digital--512. no remote I/Os are there. No additional software change is there.

Trevor Ousey

PLC5 hardware is quite expensive, the processor is at least twice the price of a comparable ControlLogix with reduced features. If you wish to keep the 1771 rack and IO modules, make it a ControlNet rack (or RIO) off of a ControlLogix system. This will give you more flexibility for communication and hardware. We have done this several times with PLC5s and the bonus is you can just go back to the PLC5 if there are problems with your program and you have ran out of commissioning time.

The 1771 hardware is not obsolete, it still has little more life, but
Rockwell will increase the pricing on it to encourage end user to migrate as they have in the past, and have longer deliveries.
Have you considered removing the PLC's altogether and migrating the software into your new DCS ?

If you are upgrading from ABB's MOD300 to AC800M you can probably junk large parts of the PLC digital logic as these are handled much better by a modern DCS.

We've had very good results doing this on our MOD300 upgrades.




Michel A. Levesque, PEng.

In short, yes you can easily migrate from PLC-2 to PLC-5 (or CLX), you have three alternatives.

First Alternative, is a straight CPU swap. If you have a 2/30, replace the first rack's 1771-ASB with a PLC-5 CPU (check the chassis for proper revision). If you have a Mini PLC-2, just replace the PLC-2 with a PLC-5 CPU.

Second Alternatively, replace the PLC-2 CPU with a ControlLogix CPU and rack and a 1756-DHRIO card to get your Remote I/O hooked up without going to ControlNet.

Third Alternative, replace the PLC-2 CPU with a ControlLogix CPU and rack and a 1756-CNB(R) card and repalce all your 1771-ASB's with 1771-ACNs for full ControlNet.

Good Luck.

Jeremy Pollard

Great response Trevor. If I can add that depending on the life of the
process, an EBAY PLC-5 may be the way to go... if you are looking at more than 5 years, then Trevor's suggestion has legs!!

Cheers from: Jeremy Pollard, CET The Caring Canuckian!

Control Design www[.]controldesign.com
Manufacturing Automation www[.]automationmag.com

3 Red Pine Court, RR# 2 Shanty Bay, Ontario L0L 2L0
705.739.7155 Cell # 705.725.3579