Strange Modicon Problem

Hey all you Modicon experts out there. Have you ever had a PLC logic program tell you the output is on when in fact it is not? We have a Modicon 984-230 that is saying this output is on but there is no led on the card lit and there is no voltage on the output terminal. We had the same problem about 3 months ago which I fixed by changing an output card and reloading the program. Not really sure which one fixed it. My theory is the output cards are failing due to age and they must be dragging down the internal power supply. Can anyone tell me how to prove this? Any input is appreciated. Thanks.

Tech7

I've had this happen twice in the last 2 months on 2 different Modicon platforms. In both cases it appears that the output channel just went bad. On the first one, I replaced the card. On the second one, I forced an unused output on the card and it worked, so I just used that channel and rewired it. I'm new to Modicon though, so that's about all I got.

Depending on the output card...If it is a diagnostic output card then the card looks for a small current..(I think 250ma) over time this value sometimes changes..If the card is driving a small load such as a pilot light or small relay it might not meet the min current it then shuts down the output..But appears to be on the logic.
The work around for this is to put a pull up resistor on the load..
on the 984-685 i had you could check if they were diagnostic cards from the traffic cop.. They used 32 output addresses AND 32 Input addresses..The inpuit addresses were used for diagnostics..Eg. if your out put was 0001-00032 and the inputs from the output card were 10001-10032 then input 10001 would be on if output 00001 was on and working..

D

On mine I tried forcing outputs on four different cards and none would go on. The card I ended up changing was not the card I originally thought was bad. If there is an internal logic power supplying all output cards and one card crowbars the supply, then maybe all outputs are affected? I'm guessing. Anyone else have a clue?

The logic program is the correct state at that point of the scan.

If the output card does not match the logic, the possible problems are:
1) The coil is being controlled elsewhere in the program such that at the time the "state" table is written to the output card it has chnaged state from what you see with the programming software. This is an extremely rare event in a mature system, unless recent program changes have been made.
2) There is a problem with that specific point on the output card or even the entire card itself.
3) There is a problem with a different card in the rack, even an input card. The logic signal for the I/O cards is multiplexed so any one card could affect the others.
4) There is a problem with the Remote I/O network, adapter, processor, rack power supply, or cabling.
5) There is a problem with the rack itself.

It might very well be a bad output.

But then again it could be something as simple as a blown fuse. Some models of the output cards were internally fused.

Obsolesence is a problem that will continue to plague the 984 family of PLCs. Telemakingjunk is not supporting the 984 family and replacement parts are becoming increasingly more difficult to find. We are slowly replacing our 984s with CLX systems. Fortunately, there are others doing this as well and for the short term used replacements are availalbe as systems are retired. The long term outlook for 984 systems in not good however.

The 984 family was introduced almost 20 years ago. Many of the electronics are no longer available. What can a manufacturer to do when components are no longer available.

Isn't obsolesence also a problem for AB, GE, and Siemens (I know it is for the old TI equipment)?

I guess I just don't like bad-mouthing manufacturers for things out of their control versus things they are designing/selling today.

I've often wondered how AB has comitted to supporting the PLC5 for another 40 years played out... did they buy a huge stock of chips or what? Or does that commitment merely mean there will always be a processor or I/O to fit in the rack but the internals may be different?


Telemecanique ended up with the Modicon PLCs after a long and convoluted series of the line being sold, resold, merged, and finally aquired by Telemecaniques parent company. By the time it landed in their lap the 984 had been out of production for some time, so I don't see that they have a particular obligation to it. Its just a reality that has to be dealt with.


Sorry, didn't mean to derail the thread.

984s, we get our IO cards repaired. They use components that are still readily available. The problem is the more advanced cards, CPUs, HS counters etc. The other problem is that it also competes with Schneider's current range of PLCs.
If they wanted to keep the range supported they can. If an old card becomes outdated due to components being un-available, they could re-design it for new components. There is nothing that old components can do functionally that can't be replaced by new components. The key to this is resources. The company wants to concentrate on its new range rather than the old range. I do not blame them on this, although it does make it a bit harder for us.

Jim Dungar said:

The logic program is the correct state at that point of the scan.

If the output card does not match the logic, the possible problems are:
1) The coil is being controlled elsewhere in the program such that at the time the "state" table is written to the output card it has chnaged state from what you see with the programming software. This is an extremely rare event in a mature system, unless recent program changes have been made.
2) There is a problem with that specific point on the output card or even the entire card itself.
3) There is a problem with a different card in the rack, even an input card. The logic signal for the I/O cards is multiplexed so any one card could affect the others.
4) There is a problem with the Remote I/O network, adapter, processor, rack power supply, or cabling.
5) There is a problem with the rack itself.

Click to expand...

Jim since I've checked everything else, and I agree with all of your scenarios, I would have to go with point number 3 in this case. I just wish I had a good way to prove this besides changing cards and getting it to work. As stated these cards are 20 years old and are starting to fail at a regular rate. Fortunately this system is an abandoned part of a nuclear plant and is not safety related. It must, however be fixed. Unfortunately in a nuclear plant, you can't just rip it out and install something else in it's place. Months and many dollars in engineering costs, not to mention the NRC stand in the way. So, I will end up trying to purchase replacement I/O cards on the internet and hope I get lucky. Sorry, didn't mean to rant. Thanks all for the feedback.

Tech7

Jim Dungar said...

"The logic program is the correct state at that point of the scan."

Strictly speaking, I agree with you. However, in terms of the posted question, this seems to imply, or strongly suggest, that if a particular rung (the one you are looking at) consists of a particular output, control-relay or actual output, driven by an "Always On" bit, that you will always see that output being ON... because that happens to be that part of the program (that part of the program-scan) that you are looking at.

This is not necessarily so.

Especially if, for whatever reason, the particular output is controlled by some number of additional rungs. For those that are crazy about never using a particular output in more than one rung, there are plenty of good reasons to do so.

As a result, because status screen updates are woefully out of sync with reality, if a program does have a particular output controlled by more than one rung, the output in the currently displayed rung might flicker off and on.

What really counts is the absolute last controlling factor... which might be thousands of rungs beyond the rung you are looking at. Ultimately, the last controlling factor has control over the actual state of the output.

The status dislay will indicate the last state seen before the display update.

Jim also said...

"Isn't obsolesence also a problem for AB, GE, and Siemens (I know it is for the old TI equipment)?"

The "old TI equipment", at least the 505-Line, is being well covered into the foreseeable future by Control Technologies Inc. (CTI). They already have most of the boards covered and will soon cover the CPUs.

The only thing that bothers me about CTI is that they haven't given any indication that they are going to pick-up on the software... I hope they do. However, even if they don't, the TI-Software, as it exists, is well ahead of most PLCs out there now. I will, however, admit that AB has finally, just recently, after many years, finally caught up to and surpassed many of the basic capabilities existing in TI. They're close, but they still have a ways to go.

For those AB-Lovers... if you don't know TI... you can't comment. I was weened on AB... then I stumbled onto TI... NIRVANA!

Jim in your replys i see you didnt mention diagnostic outputs..does this mean you dont agree? As for AB with 20 year old equiptment..try getting an SLC 100 or 150..you can but you are going to pay!!

Oh i remember the TI..I was trained on them..god damn things they were!!

Darren, no I did not mean to slight the cards that do have the diagnostics, but they are not always available.

Tech7, don't forget you can swap identical cards from one rack to another also. Personally, I would stay away from e-bay and it's uncertainties, there are many reputable resellers that offer warranties.

Terry, you are correct that there is a lag in the screen update which may skew the actual status. This would be a concern for high speed "coils" like internals, or any external monitoring the looks at the Modicon State RAM table rather than the I/O image table. However, for typical "slow" responding output cards it is a reasonable troubleshooting tool for previously working programs.

Alaric, I am not trying to defend Schnieder electric and their decisions concerning the Modicon line and the mess they "bought".

If users are not aware of it, Schneider has announced that the entire 984 PLC and 800 series I/O families will enter obsolesence in June 2006. They plan to offer spare parts/repairs for another 8 years. This is due in part to some environmental proscriptions dealing with the sale of hazardous material in the modules themselves. This does not affect any of their other PLC families.

Jim, thanks for that info on parts/repairs for the next 8 years. I was not aware of that.

The local rep told us they were not being provided by Schneider/Telemacnique anymore and we would have to resort to used/repaired replacements - but then that doesn't surprise me,unfortunately the local Telemecanique rep doesnt know much about anything at all, including how to comb his hair. At the same time he was pushing us to upgrade to a Quantum .

We chose the CLX instead, and the quality of local support and the rep's knowledge had a lot to do with that decision.

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Jim Dungar said:

This is due in part to some environmental proscriptions dealing with the sale of hazardous material in the modules themselves.

Click to expand...

I imagine this is in response to the new EU directive about hazardous materials in electronic components - specifially and mostly lead solders. This is a perfect example of ill informed regulators mandating technology. Most low melting point non-lead solders will grow microfilament stress whiskers which can cause electrical bridging between points on high density boards. Tin solders are notorious for it. These whiskers can form long after the board has left the factory. Solders with just a small percentage of lead in them do not grow these stress whiskers. Manufacturers are struggling to find another solution. In the mean time, board failure rates could increase. Regulations have their place, but sometimes.... grrrr

Alaric said:

Jim, thanks for that info on parts/repairs for the next 8 years. I was not aware of that.

The local rep told us they were not being provided by Schneider/Telemacnique anymore and we would have to resort to used/repaired replacements - but then that doesn't surprise me,unfortunately the local Telemecanique rep doesnt know much about anything at all, including how to comb his hair. At the same time he was pushing us to upgrade to a Quantum .

Click to expand...

I was told the exact same thing up here in canada Alaric...I pushed for CLX however some "Bean counter" said it couldnt be done...So in went the Quantum..