Today is...
Thursday, May 23, 2019
Welcome to the Modbus Community, about
the world's leading automation protocol.
MicroLogix 1100 fault with 08h code
Numerous 08h faults on numerous displaced 1100s
By Steve Van Kley on 22 May, 2008 - 1:20 am

I have 7 well sites on an Indian reservation in Florida where I'm getting 08h errors on many of them at the same time. They're all powered by DC from a solar panel array at each site. These wells are a mile or so apart but I had 4 of them (wells #1, #4, #5 & #6) fault out last week within an hour of each other with the 08h error code (internal software fault). I reset them from Logix500 and all was well for a week and then this am 2 of them (#6 & #7) faulted with the same 08h error.

Does anyone know how to reset this hard fault from the front LCD buttons, and more importantly, has anyone seen this issue before? I tend to think it's a grounding issue... because I can't think of anything else that will take out this many PLCs at one time.

By Curt Wuollet on 23 May, 2008 - 1:06 am

The MicroLogix are very sensitive to power issues and noise. I have an 1100 that will not come up running every time if the power drops, it will fault as you describe. It's an AC powered unit. And worse, the past couple times we've had a bounce, it has lost it's program. I have had many 1000s that fault for no good reason due to line noise even with the suggested filter, etc. and I've been replacing them with Mitsubishi FX units which ends the problems. AB is stonewalling on the issue even though they replaced the first batch for a known memory issue. I can't easily replace the 1100 with something else because it must communicate with a SLC on Ethernet/IP. With the storm season approaching, it will go away too as soon as I find another solution. It's a shame because I like the product, especially the 1100. But I can't be replacing them all the time, especially in critical applications. AB has made me look bad, and they aren't doing anything to help. While we're at it, does any other brand of "brick" PLC speak Ethernet/IP natively? That's the feature that sold me on the 1100.

Don't bother with flames, this is not opinion, it's simply what's going on and what I've had to do about it. If AB doesn't like it, I've got some MLs they can replace, it they've got their problem fixed.


By Michael Griffin on 24 May, 2008 - 10:42 am

In reply to Curt Wuollet: With the AB 1100, try putting a on-delay timing relay (e.g. Omron octal base type) in the CPU power circuit. When the voltage drops below the relay drop-out voltage, it will turn off cleanly. With a 30 second (it can be more or less as necessary) on-delay, power is not restored to the CPU until the voltage has had a chance to recover properly. When the power is restored to the CPU, it is restored cleanly,

The problem is often not the power going off or on, it is the power flickering or voltage recovering slowly. The relay will prevent power being restored to the PLC until the mains voltage has had a chance to stabilise. I had a similar problem with a Siemens S5 (with a very expensive CPU), and the problems disappeared after applying this solution. Line filters, etc. had been of no help at all. It wasn't the most elegant solution, but it did work.

The problems with the AB 1000s sound like a different problem, and I doubt the above suggestion (timing relay) would do anything for them. If it was me, I too would just replace them with something else.

As far as PLCs that support Ethernet/IP, that seems to be pretty much an AB exclusive protocol. The ODVA web site also lists some motion controllers (from Delta and Rexroth) and an odd device from Jetter, but those aren't PLCs. There doesn't seem to be anything on the ODVA list that directly competes with AB, which I suppose shouldn't be too surprising. If you use Ethernet/IP, you are pretty much also deciding to use AB. Perhaps someone should tell you all about the problems of using proprietary protocols and why you should stick to opens solutions.

By Curt Wuollet on 25 May, 2008 - 12:13 pm

Hi Michael
Yes, perhaps you could fill me in :^).  We were spec'ing a critical piece of gear, we had to use _somebody's_ PLC.  AB got the nod because we have "local support".  So, when we needed a remote for some analog loops to interface with a building systems and displays, the 1100 with "free" Ethernet/IP seemed like a great solution.

Actually, I should have known it was too good to be true, a reasonably priced processor that includes any useful networking without a $$$ add-on. I have replaced it with another 1100 to buy time.  I may have to install a SLC for 4 digital points and 4 analog outputs. The cost per point would be outrageous, but at least they put a decent power supply in the SLCs. The other end involves flame safety and HP gas and enough other high risk items that I really don't feel like swapping both ends out.

That would be the _best_ solution. I was cranky enough that the local AB seller said he'd try to get AB to exchange the bad unit under warranty, I'm not holding my breath.  I might try the timer approach, it's a crutch, but it's also a gimpy PLC.

Actually, the 1000s would fail with a retentive memory lost error so they may well be related. In any case the Mitsubishi FX replacements seem immune to the issue, whatever it may be. I had to scramble to get the FX timers to work like the ML timers, but it was worth it to have reliability.

An unreliable PLC should be an oxymoron. We had a whole plant full of PCs that rode that out without a whimper.



Wago makes a nice product that can speak ethernet/ip. The 750-841 is programmable and reserves a protion of memory for ethernet/ip comms.


I agree with Kevin, I just recently added 22 740-841 Wago units to control our plant hvac units/building automation. It was the first time I used the wago field bus controller (PLC). The programming software is super flexible and the Wago support staff is simply great. You can create your own function blocks or use the function blocks provided. Programming language options are: Ladder, FDB, IL, ST, FC or a combo of all. Before using the Wago products I used AB, Mitsubishi, and Direct Logic PLCs. I do like the AB products and will continue to use them on some projects, but the communication issues they have can be a pain.

Good luck,

What are the costs of the Wago units and development software. And if links exists, please post.

Thank you.

The Wago base PLC is <$500 and the cards are $100-300 range.

The 750 series is what you want to look for. The 750-841 is a modbus tcp master and Ethernet IP master (unconnected explicit messages). I like the 750-871 which has a built in 2-port switch.

Their PLC's use CodeSys 2.3 and they charge ~$900 for a site license with free upgrades and no maint fees.