PLC Automatic Reset Capability

OK first I would need to know what kind of error code that is showing up. There are several in an 1100 that can rear their ugly heads. If it is a scan time error, you could be out of luck . You will need to reduce the # of rungs you have. If it is a comm or I/O error, that can be fixed. If you have I/O timing error you can change the time base so it won't count in seconds but in min or even hrs. But if you are waiting that long for a "put" whether it be in or out, I would look over the program and do some changes.Hope this is a start.

I have used Allen-Bradley's PLC5, SLC500 and ControlLogix but never the Micrologix, unfortunately. There are provisions where the processor can be made to go to a subroutine that can do the error reset itself but I don't know if Micrologix has it.

An alternative is to have one output pulse every second or so and send it to a timer. As long as the pulses are coming, the timer is held at the ready state. If the pulses stop (such as when the watchdog expires), the timer runs and, when it times out, does the manual reset automatically for you.

What I really do is to program the thing so that the watchdog doesn't expire. Someone mentioned reducing the number of ladders in the program. You can also think of ways to do things with less complicated instructions. The manual has a table of how much memory each instruction uses as well as the number of clock cycles it consumes (at least they used to have it). We used to challenge each other to come up with the shortest program to do the same thing. It's a good talent to have.

Vulcan,

I will be back in Manila on sept 21st, it's possible if you can call me at my Phil. tel # 0495127477 or my cell # +971502487606 or email me at: nparedes@yahoo.com

Its on PLC issue and HVAC application.

Nelh

"An alternative is to have one output pulse every second or so and send it to a timer. As long as the pulses are coming, the timer is held at the ready state. If the pulses stop (such as when the watchdog expires), the timer runs and, when it times out, does the manual reset automatically for you."

What I am doing is trying to prevent a possible need to have to manually reset the PLC in the future. The PLC program itself is correct and has no known bugs. The environmental conditions are close to ideal. Your external timer suggestion sounds like a possible solution. Could you please describe how I would go about doing this? Any recommendations for specific timers or brands to look at?

Thanks in advance.

It's simple really.

Make a rung in your program to switch an output on and off at one second intervals. Connect that output to a timer that resets everytime the output comes on (or off - depends on the timer you use). If the pulses stop, the timer runs through to its setpoint (say, 3 or 5 seconds) and triggers a second timer. The second timer's job is to switch off the plc for a few seconds and switch it back on again. The recycling of power resets the fault and allows the plc to run again.

If you can find a timer that will do both jobs, that would be better. I'm too lazy right now to go looking.

I've been reading up on the Micrologix 1100 and you'll have to set S2:1/8 (Fault override at power-up) and S2:1/12 (Mode behavior). The first one allows the plc to clear the fault when the power is recycled. The second one allows the plc to run the program immediately after power is restored.

Just one more thing about the Micrologix or the Allen-Bradley family of PLCs. The only way that the watchdog timer expires is if your program runs longer than the preset value set in S:3. That can happen if your program uses loops.

We used to have this problem when a loop that was supposed to only normally run for 5 loops had a condition that made it run a hundred loops! We fixed that by programming but it took a long time to find it. The number of loops was dependent on an analog input. When the analog input malfunctioned, it gave a fantastic number that made the loop run over and over again until the watchdog timer expired. If I were you, I'd take a look at your program again before making an external auto reset.

mx, further to Vulcan's comments, if there's a possibility of a loop causing a long scan cycle, you can insert a REF instruction (to be executed every so-many loops). REF resets the watchdog timer (among other things). Doing this does, however, mean that the WDT won't catch an infinite loop (which is what it's principally there for), unless you add extra instructions to count the loops & suppress the REF rung after too many loops.

That sounds simple enough, but I have yet another question.

The second timer's job is to switch off the PLC for a few seconds and switch it back on again.

My question is how is it going to do this? I thought that there might have been a reset input for the Micrologix 1100 PLC - so that the timer can send some sort of signal to the PLC telling it to reset, but from what I have read, this doesn't seem like the case... Unless I have missed something somewhere...?

I guess my main problem is how am I going to cause the PLC to reset?

how am I going to cause the PLC to reset?

Okay, to answer your question: one way (maybe the only way) to reset the PLC from a watchdog error is to recycle the power. My suggestion merely gives you a way to automate it. Just remember to set those two bits that I mentioned in my previous postl.

Go to the Koyo website and you'll find a universal timer. You can program it for any timer function you might need.

A watchdog timeout on an 1100 is a "Non-Recoverable Error" - you can't trap it.

If you really need the WDT function, and can't increase the timeout value enough, I think you'll have to go with Vulcan, and use an external timer (which you will be able to control).

Do you know whether the "first scan bit" (S:1/15) gets set after one of these events? If so, you could use it to disable the WDT until you're good & ready to let the ladder program run.

Are you sure you don't have any loops which could get "stuck" during a brownout (& trip the WDT)?

What does your max. scan time (S:22) sit at when running normally?

I've used 1000, 1200 & 1500 systems (tho' not an 1100), & haven't had a latch-up like this. [Edit: except when I've screwed up the ladder logic myself (own goal)].

What precautions have you got to limit the effects of the enviroment?

if too much enviromental conditions are hitting your PLC, you could get into more trouble than just a simple timer problem.

May I suggest a power line monitor, so when the power drops below a certain specified level, the PLC has its power cut off.

If the customer still requires the machine to operate during those periods, they could have a problem with stability or machine operating within specifications.

Not familiar with the PLC itself, is it 240VAC input or 24VDC input?

If 24VDC, place a diode in line and maybe a "SuperCAP" or a battery backed supply, so the thing remains powered during abnormal line conditions.

Ok, this may not get you around the manual reset, but prevention of the problem will always be better than trying to find workarounds after the problem occurs.

UPS.

If it is a 24VDC powered model, then you can probably hold it through a brown-out with some caps for that matter.

Another option is a power monitoring relay. Play around with the settings until you find the place where the PLC latches up,and set it just above that point (feed the PLC power from it's contacts). It is better to have the PLC go "offline" (which should be a comms fail alarm), and then reboot when power is restored, than to have it sit there alive but brain dead. Look at it this way: If there is insufficient power, then all the stuff that the PLC is monitoring/controling are "dead in the water" as well.

You'll possibly find its around 18V, as the internals would run at 15V, with a 15V regulator and 3V drop across that reg, it would stop operating at 15+3V=18V

I have used variable powersupplies on several PLC's including AB's and they drop out at around 18 to 20V

That's correct. The PLC currently locks up when the power drops below 18.3V.

Implementing a power monitoring relay is a potential solution, but that doesn't solve the problem for possible non-power related issues causing the PLC to lock up (if there are any).

The ideal solution would be to have a PLC that can identify that it is locked up and be able to reset itself - no matter what the cause. This would help eliminate any human intervention required to maintain the system.

If you can provide some contact details, I have a processor that I believe will at least match the specifications of your ML1100, and operate down to at least 10.4VDC.

pl. conact

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I think you are missing the boat here. The problem is not your inability to automatically reset the PLC, The problem is that your PLC needs to be reset. Why? Find the root cause of failure and resolve that. You are asking (how to reset CHECK ENGINE) light in your car. You should be looking for the reason why it is coming on, not how to reset it.

I must agree with the previous post - your money is much better spent on fixing the source of the problem.

Adding a timer will not fix a poor power supply, and if the power supply is questionable, you may make things worse by stressing other components by constantly cycling power.

If this is a critical system, adding a timer in as discussed, will actually make the system less reliable because they are in series.

If you want reliability look at adding a power supply in parallel, or even a UPS.

How many times has your processor failed - I have found them to be quite reliable