CLX redundancy with Devicenet Module DNB

[TEJ 0]

Dear Friends, In one of my project with process control industry, Iam using CLX Processor L61, ENBT module and DNB module in A4 rack. The same said modules are also available in Rack 2 i.e secondary A4 rack. The second rack is introduces for the purpose of redundancy system. The IO's and SCADA are in Ethernet module with Ethernet adapters in RIO panels. Here my question is that any sample code is available to achieve the redundancy with 1756-DNB module. Any clarifcations required, let me know to explain you better. Thanking you in advance.

[paulengr 44]

You are probably barking up the wrong tree. PLC redundancy sounds good but totally unnecessary in practice. CLX processors are PAC's (process automation controllers) and as such, designed to do a fantastic job out of the box. Unlike a DCS system, multiple controllers aren't a necessity because DCS controllers are designed with MTBF's close to 10 years but design lives only around 3-5 years (same as PC's and most DCS's are PC hardware). Even stock PLC's from the 80's are designed with MTBF's out to over a hundred years, giving them design production lives around 20-30 years. So you need to adjust your thought process a little bit when it comes to PLC hardware compared to DCS's. The redundancy is usually a bad move because you don't need it and you get nothing but more maintenance headaches and often LOWER reliability as a result. First, note that the PFD (probability of failure on demand) on the L61 itself is very, very large. A stock L61 is already SIL 2 capable. If you want even longer PFD's, go for the GuardLogix. This is two hardened ControlLogix PLC's that are internally wired with some extra hardware to do self-checking on the PLC's themselves and fault on a discrepancy on their outputs. It takes up 2 slots in your rack. The result is a single PLC that is SIL 3 capable. You need redundant power supplies, and if you truly want SIL 3, and you need to use the special safety I/O (DeviceNet modules are available with this rating). AB has TUV certified all the equipment and publishes their PFD's. You just have to poke through the literature to find it. If you want to use the redundancy modules, you need to buy another A4 rack. Put CNB cards and the redundancy cards in your existing two racks and take all your IO out. I believe that as of recently you might be able to use Ethernet/IP now but before, it was CNB or nothing. The new third rack must contain all your IO. Put your ENBT card in the third rack as well. Keep in mind though that the PLC "system" won't be rated even a smidge over SIL 2 because you are adding redundancy to the part of the system that is already very reliable but actually adding additional common mode failure issues if anything. If you want "manual" redundancy, there are some gotcha's. You can configure both PLC's as "master" for input cards with no problems. With output cards, one PLC must be "master". So you can't truly do this the way you expect. You can of course do wired OR gates (and you can wire your inputs in parallel), or better yet, valves/sensors in parallel which allows you to create redundancy down to the I/O level. If you are very careful about your electrical design, you could have true redundant controllers, I/O, everything down to the hardware level and achieve SIL 4 with a LOT of up front planning. I haven't ever seen a SIL 4 requirement in practice but I'm sure they are out there. In practice, the probability of failure on demand (PFD) requirements usually fall down at the valves which are usually barely SIL 2 in most scenarios. You end up achieving higher levels of redundancy only with series or parallel valve arrangements. The PLC and sensor end is almost never the limiting factor when it comes to redundancy.

[BobLfoot 302]

Paul has given you an excellent answer I'll just add one thing. If you go to ControlLogix Redundancy get the Rockwell Redundancy Guidelines and Follow them religiously. The introduction of one XIC BOOL improperly can take your scan time from 30ms to 300ms - Actual Life Experience.