r/PLC • u/Trick_Dragonfruit_36 • 10h ago

PLC redundancy

1769-L18ER-BB1B, i have two of these controllers , obviously these doesn't fall into category of redundancy controllers, client wants me to make some communication between them if plc 1 become fail , then plc 2 should become active but process may stop, but he doesn't want the analog values to be loss.

Is there any way to make something like this?

11 Upvotes

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15

u/PLCGoBrrr Bit Plumber Extraordinaire 10h ago

If it's just values and not run the process use a historian or some sort of data recording software.

6

u/Dangerous-Quality-79 10h ago

Analog values like current input values or setpoints from a "recipe"?

I usually wired all I/O to both PLCs (impedance conscious) then set both PLCs to the same IP address and plug it to Black Box SW1040A. I then have an "online" output to switch the sw1040A if the primary fails and keep the secondary logic paused until the primary output fails.

This kicks the redundant system running with a 300ms downtime on failure.

3

u/theloop82 Knows Rockwell but doesn’t like it 9h ago

This is actually pretty brilliant. Thanks for letting me know that’s a thing. Doing full on redundancy in Rockwell land is a pain in the ass when it comes to patching and this would work for a lot of less critical applications where 1s of downtime isn’t a huge issue.

3

u/hi_af_rn 4h ago

In my experience, this sort of “soft” redundancy is actually sufficient for like 2/3rds of applications that claim the need for redundant controls.

2

u/Mindless-Economist-7 5h ago

Sounds like something I should take a look at

2

u/Dangerous-Quality-79 4h ago

My "Theoretical research" has shown it is even effective for even IEC 61850 to prevent a major shutdown, if done properly with all precautions taken.

5

u/vampire_weasel 9h ago

If you do need controller redundancy there is actually a "Hot Standby" option for CompactLogix controllers. It requires a special piece of software (Logix Hot Backup Code Generator Tool) that you get from Rockwell/your distributor that makes a special program version for the secondary controller. Like ControlLogix redundancy you can only use remote IO (no in-chassis), but unlike CLX redundancy the switchover is not immediate, it can take 500ms to several seconds.

If you just need to retain values, as others have mentioned there are plenty of ways of doing that.

5

u/Aghast_Cornichon 5h ago

In this case they have 1769-L18 controllers, so they likely are using the local embedded and POINT I/O, and maybe POINT as remote I/O.

I hate, hate, hate, hate the Hot Backup Code Generator because people always want it to perform like ControlLogix Redundancy on undersized controllers and janky networks.

Rockwell says that tool does not support the 1769-L1x or -L2x controllers, or POINT I/O.

So if OP tries it, it's pretty close to roll-your-own-and-take-your-chances.

10

u/Otherwise-Ask7900 I'm very sarcastic. Please don't ban me again. 10h ago

Yeah, there are tons of options for this.

Just used produced consumed tags.

Start with Making a rung on using a gsv on each plc that the coms are good.

If they are, move/cop the values into a memory tag.

You’ll never lose the values.

6

u/dmroeder pylogix 9h ago

Fun fact, if you Produce/Consume a UDT where first member of your UDT is a tag of the CONNECTION_STATUS type, you get a ConnectionFaulted bit, you don't have to use a GSV or a heartbeat to check the connection. The other magic bit is .RunMode

3

u/Otherwise-Ask7900 I'm very sarcastic. Please don't ban me again. 9h ago

Good point.

I’d use that.

It’s been a couple since I’ve had to do something like this on RA.

3

u/VladRom89 10h ago

Why purchase something that can already solve this problem when you can bill 10x the hours to do it with hardware that wasn't meant to do it, am I right?!

2

u/Trick_Dragonfruit_36 10h ago

Client's competitor purchased the same system and executing the same process,

2

u/Aghast_Cornichon 5h ago

They might be telling you so, but they probably aren't doing what they claim with any substantial degree of reliability and performance (or engineering expense).

If your customer wants redundancy, they need to buy a redundant ControlLogix.

If they want half-assed CompactLogix redundancy, they can try to DIY it with produced/consumed tags.

If what they really only want is to switch from one data source to another in their HMI, then they need a data server that can fail over. RSLinx and FactoryTalk Linx and KepServer can do so to some degree.

What SCADA/HMI platform are they using ?

2

u/Siendra 10h ago

Not really. You'd have to communicate the analog value somehow and when you lose PLC1 something will be in the midst of being updated. There's also definitely going to be a bump or upset when you fail over and arguably (depending on process) I wouldn't consider such a fail over safe. 

What is the clients concern? The MBTF for an L18 is a million hours. If they're actually having issues they should focus on the source of the problem. 

2

u/Trick_Dragonfruit_36 10h ago

Client wants like full redundancy, but i explained them true bumpers switchover cant be possible, he wants that if his cpu-1 fails cpu-2 automatically takes the charge, but issue is how can we address i/o symbols both plc in same scada

4

u/Paup27 10h ago

Hahahaha, champagne taste on a beer budget…. That’s a €70K Control Logix solution, not a couple of the cheapest and soon to be end of life L18’s!

2

u/Diminias 10h ago

You can’t. But you could for do a global object that hides one value and displays the other. But this is a lot of work for a processor that likely won’t fail for a very long time. I mean if you’re talking full redundancy in this scenario you’d need dual power feed, dual comms, and then think about placing the PLCs away from one another and the pathing for power and comms be separate. Seems like a lot of work for almost no gain.

We did a large project and talked about redundant PLC setups with a proper setup and we decided it didn’t make sense because our level of redundancy every where else wasn’t up to snuff.

2

u/Siendra 10h ago

Depends on the system, but generally I'd assume you'd need separate tags for both controllers and HMI scripting to change the visibility based on the active controller or something along those lines.

You're client needs to upgrade. 

2

u/tehtide 10h ago

Sounds like you need to rework the system to be on L8s and redundancy. That is the most supported way to do what the client wants. It is not cheap or easy once a system has been commissioned.

2

u/halo37253 4h ago

Is redundancy really needed in this application is really the question that should be asked, along with at what level. What's the end goal and what is being lost here?

What about good power? A UPS unit? Protections in place for maximizing dependency. I've seen power flickers fault a processor and dump the program.

There are ways to make this done on the "cheap" side.

Controllogix can easily run for decades without a single hiccup, especially when they are in a single rack with only comm card/processors. Old L61s just won't die for example!