MicroLogix 1000 inputs just stopped working?

mplucker

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Join Date
Jul 2015
Location
IL
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Hello everyone,

I'm trying to figure out an issue with a MicroLogix 1000 PLC (1761-L10BWA) and I really have no knowledge of these devices or what could be wrong. The unit runs a hot stamping press and today when I went to use the press, things weren't working as expected.

Normally, when the hand switches are depressed, the input lights turn on (I/1 & I/2), the press stamps, and then a few output lights flash on, and then repeat. Now, it appears as if inputs I/0 through I/3 are all not working. When I depress the switches, no lights turn on. I tested voltages with my multimeter and everything appears to be working when the switches are depressed. The odd thing is that I/4 still works and when looking at the controller, it seems like I/4 and I/5 are somehow isolated with their own DC COM.

Does anyone have any idea what could be wrong here? Could the PLC be on its last legs? Is it possible for me to get a USB comm cable, download the program, get a new ML 1000 and upload to that?

Any help is appreciated since this stamping press needs to be functional again.
 
Welcome to the Forum !

You are correct that inputs 0,1,2,3 gave their own DC Common against which the inputs are referenced, and inputs 4,5 have a separate one.

The reasons are because the first four inputs are usable by special high-speed circuitry inside the controller, and also because the designers wanted to give users options to use sinking or sourcing inputs on the same controller.

Check carefully the connection between the DC COM terminal grouped with Inputs 0,1,2,3 and the voltage reference it is connected to.

If you really do measure 24V between that specific DC COM terminal and an input terminal, and the LED still isn't coming on, then the input circuits are blown and the controller needs to be replaced.
 
Thanks for the reply.

As for the voltage reading, it measures at exactly 9.00V. The DC COM terminal traces back to a power supply that has a Vout adjust (SPD24301 Gavazzi AC/DC Converter). I've never touched the Vout from the power supply, so I assume it was set to 9V for a reason.

Could the 9V input voltage be the cause of this problem? I would fear that increasing it to 24V might cause some catastrophic failure. As it is, the PLC still boots and the PanelView display shows the output of the program just fine.

I would call the manufacturer of the stamping press, but they've since gone out of business and trying to fix these machines is pretty much up to the owner. Other companies that service these machines just tear out the electronics and replace it with their own stuff.

Since I've never programmed a PLC, is it possible to download the program from the EEPROM using a USB 1761-CBL-PM02 cable, get a matching unit, and upload the program to that? (This assumes that there's something indeed wrong with the PLC and not an electrical issue.) I have a software engineering and programming background, but I've never done anything with PLCs.
 
Look for another power supply that might be tied to the same system, and for sensors that might use 9 or 10 volts DC.

In general, the MicroLogix 1000 is designed to be powered by 20 to 25 volts DC.

The low threshold for an Input circuit to be true is 14 volts. Between 14 and 5 volts it's indeterminate, and below 5 volts it's false.

I would actually be surprised if a MicroLogix operated correctly at a DC power supply voltage of just 9 volts. We used to get a lot of complaints from OEMs who wanted to use them on 12V DC battery systems and they wouldn't work reliably between 11 and 13.5.

There's a 12V DC version of the MicroLogix 1100 controller, but not of the MicroLogix 1000.
 
One important thing; in logic controllers, "upload" means "from PLC to programming computer" and "download" means "from programming computer to PLC".

In general, the programs stored in flash memory in the MicroLogix 1000 can easily be uploaded using an RS-232 cable (the venerable 1761-CBL-PM02) and RSLogix 500 software, then downloaded into a new controller. The software for that specific model is free of charge, though it might be a timesaver to have a local integrator who knows the toolset come and do the upload/download for you.

However, the controllers did have an "OEM Lock" feature which will prevent you from uploading the program. If I was using a MicroLogix to control a stamping press, I would almost certainly lock the control program to prevent tampering.

I don't know your machine or jurisdiction, but in general modern machine safety codes won't allow the use of a general-purpose logic controller for metal stamping operations or for safety functions like two-hand and anti-tiedown.
 
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One more thing: disconnect all the wires from that DC power supply and then measure the output voltage.

9.0 V is very high (and oddly precise) for a "crowbar" power limiting circuit, but it's not impossible that something else is pulling down a nominal 24V DC power supply to just 9 volts.
 
Ken, thanks again for the assistance.

After tracing the hundreds of wires in this machine's electrical box, there is indeed a transformer that outputs 24V. Measuring the DC OUT 24V+ terminal on the PLC shows 24V, so it appears that it is powered at the proper voltage.

Taking your suggestion of measuring the switching DC power supply, the 9V does seem incorrect since I found that the model number, SPD24301, decodes to 24V output voltage, 30W output wattage. The AC input to this switching PS is correct at 118V, but the output voltage is only 9V. The switches connected to this PS require 20-30V to function, so this might be the problem and purchasing a replacement is an easy test (and an easy fix).

As for the other info about upload/download, thanks for the proper terminology (I'm used to thinking of it in reverse). Also, the OEM lock could very likely present an issue if it is indeed enabled. I would imagine on the hot stamping presses this company produced, they probably enabled it. But I'm going to get the USB cable anyway to see if that is indeed the case.

I'll update if I encounter any other issues related to the PLC.
 
Isolate the PS and recheck to eliminate a short circuit issue and also make sure the PLC's RUN light is on and that the mode wasn't switched. If you indeed have a 24V t-former, which I see a lot of, it is 24V AC. It would need to be rectified and filtered to be used by the PLC, this I haven't seen before. Usually, it's a 480/230 - 120/220VAC power t-former and a separate 24VDC PS if not available on the PLC itself, which again, I've never seen a PLC engineered where the onboard PS was solely used.
 
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I missed your post, loundeb2. I checked for a short circuits and didn't find anything. And the RUN and POWER lights are on, and the program is outputting to the PanelView, so it's working as expected. However, I/0 to I/3 are all not working. I ordered a replacement AC/DC Switching PS to see if that's the issue.
 
However, the controllers did have an "OEM Lock" feature which will prevent you from uploading the program. If I was using a MicroLogix to control a stamping press, I would almost certainly lock the control program to prevent tampering.

Ken, I received the USB comm cable, configured it with RSLinx Classic Lite, opened RSLogix Micro and I was able to click the Upload button and then go online. Does this mean there's no OEM Lock? I was able to also save the project to my computer and it generated a few files, including a .RSS file.

If this means that the OEM Lock was not enabled, and if the PLC is found to be faulty, would it be as simple as purchasing another matching PLC, download the program, and just start it up? Or are there other things that need to be configured in order for the program to run in the PLC?

Thanks
 
This really sounds like a power supply problem. Try this:
With the machine on, (that is, with 118 vac infeed to the power supply) try measuring AC volts across the output of the supply. If you measure a significant AC voltage, the capacitors have weakened and the supply needs to be replaced.
 
I'll let Ken answer that last question, but it is good news.

mplucker said:
...Measuring the DC OUT 24V+ terminal on the PLC shows 24V, so it appears that it is powered at the proper voltage...

A note to the wise on the 24VDC OUT connections on the MicroLogix 1000. As mentioned, this provides an option for DC supplying field devices directly from the controller and back to the I/O, but it appears not to have been used for this machine as you have found the external and apparently faulty Gavazzi PSU.

This BWA controller is AC line powered. You should have 120VAC supplying it on the L1 and L2/N terminals on the bottom. The 24VDC OUT terminals are supplied from an internal AC/DC rectifier. So once your AC supply to the controller is healthy, which is indicated by the PLC powered and running, then the 24VDC OUT supply should also be healthy. You have confirmed this to be the case by the above comment.

Can you confirm that there are no wires connected to the +/- terminals for this 24VDC OUT supply on the controller?

If the hand switches are only low power consumption or indeed they have dry contacts, then you could temporarily connect the 24VDC OUT supply up to the input circuits. This would get you going for now and test everything else is OK.

Disclaimer: You say you have checked for short circuits and the like, but I/We do not know what faulted your PSU that powered the input circuits, if that is what is wrong here. It could have been a fault on the input circuit devices or wiring. If you do connect the controller's 24VDC OUT supply to the hand switches, and there is a fault, you could damage this supply, at the very least, or potentially smoke the controller completely. Be sure the circuit is healthy before attempting this, if you wish.

Have you not got any kind of a 24VDC power supply unit knocking around to temp rig up? Even a test bench PSU?

Just one other "small" correction on terminology...

mplucker said:
...I received the USB comm cable...

Unless you've bought a hybrid cable or two separate components, the AB 1761-CBL-PM02 is not a USB cable. It is a serial cable at both the DB-9 and the 8-pin round end, and everywhere in between. If your computer only has a USB port then you would attach a USB-Serial adapter to the 9-pin end of the PM02 cable to make the connection.

But you've already done it, so just out of curiosity, what did you purchase and how did you connect, USB or real serial port on your computer?

Regards,
George
 
Can you confirm that there are no wires connected to the +/- terminals for this 24VDC OUT supply on the controller?

Hi George, the + terminal has no wire connected, but the - terminal is connected to a wire labeled 0V (which measures at 4.2mV).

Have you not got any kind of a 24VDC power supply unit knocking around to temp rig up? Even a test bench PSU?

I do have a 24VDC supply somewhere in my shop, but I haven't had much time to spend on diagnosing the parts since I have an event this weekend I have to prepare for. Next week I'll have some time to do more diagnostics.

Unless you've bought a hybrid cable or two separate components, the AB 1761-CBL-PM02 is not a USB cable. It is a serial cable at both the DB-9 and the 8-pin round end, and everywhere in between. If your computer only has a USB port then you would attach a USB-Serial adapter to the 9-pin end of the PM02 cable to make the connection.

But you've already done it, so just out of curiosity, what did you purchase and how did you connect, USB or real serial port on your computer?

The cable shows USB-1761-CBL-PM02 and I purchased it from Amazon. It came with a mini-CD with some Chinese drivers, but it installed on my Win7 machine and I was able to access the PLC without trouble.

I've never done much on the electrical engineering side of things (just a few college courses back in the day), so if what I'm replying with doesn't make sense, then feel free to correct.
 
So after I hooked up the computer and went online with the PLC, the I/0-I/3 inputs started working again for some unknown reason. When using the machine, things don't work exactly as they should, but good enough where I can get my stamping work done. The machine is acting as if the switches aren't timed correctly since the pneumatics are not depressurizing at the right moments.

I think my assumption of the Gavazzi PSU being faulty is valid since all the hand and pneumatic switches in question trace back to this PSU. That, and the fact that specs show that it should output 24VDC and it's only at 9VDC between the + and - terminal (with all loads disconnected).

I'm still considering getting a backup MicroLogix 1000 and downloading the program to it so if anything does happen to the PLC, at least I'll have a backup. The electronics on these machines are no longer serviced (except for one company in CA, and I don't want to ship a 3000lb machine).
 
mplucker said:
...the + terminal has no wire connected, but the - terminal is connected to a wire labeled 0V (which measures at 4.2mV)...

4.2mV is negligible, but 4.2mV with respect to what? What are you referencing against the "0V" wire to get this reading? Is it connected to the - terminal when you test? Where are you landing your test meter probes?

I would be curious to know where that "0V" wire goes to? If the + terminal is not used then the - terminal is of no real use to any devices and should not really be wired. Unless they were grounding this output supply, which I would not do? But either way, with it not in use there would be no real need to do so?

mplucker said:
...The cable shows USB-1761-CBL-PM02 and I purchased it from Amazon. It came with a mini-CD with some Chinese drivers, but it installed on my Win7 machine and I was able to access the PLC without trouble.

Yes, you purchased a third party hybrid cable with the USB adapter and 1761-CBL-PM02 all-in-one. This is, of course, not an Allen Bradley manufactured product. I'm glad it worked, but when you specifically mentioned "Chinese drivers", I immediately made a mental comparison between the software world and the automotive world and winced. šŸ™ƒ

mplucker said:
So after I hooked up the computer and went online with the PLC, the I/0-I/3 inputs started working again for some unknown reason. When using the machine, things don't work exactly as they should, but good enough where I can get my stamping work done. The machine is acting as if the switches aren't timed correctly since the pneumatics are not depressurizing at the right moments...

If the "faulty" PSU is disconnected (I would hope it is?), while you await a replacement, then what is currently supplying the input circuits that have allowed them "start working again"? I was under the impression that you cannot use the machine until you replace the PSU? Have you still got it connected up?

That of course is still dependant on whether or not this PSU is actually faulty?

So let's try and determine that...

mplucker said:
...I think my assumption of the Gavazzi PSU being faulty is valid since all the hand and pneumatic switches in question trace back to this PSU. That, and the fact that specs show that it should output 24VDC and it's only at 9VDC between the + and - terminal (with all loads disconnected)...

The specs do indeed say that the PSU is rated 24VDC 30W and also say the trim pot is for fine adjustment between 24VDC and 28VDC and that the DC "out ON" LED comes on at 19.2VDC. This would happen as the DC output voltage climbs at power up. So you cannot adjust this PSU down to anywhere near 9VDC.

The PSU's rated input voltage is 100-240VAC, but can actually be used from 85-264VAC. If the output is measuring 9VDC, with a 118VAC line voltage, then it is most definitely faulty. It also mentions that there is an internal, non user replaceable, T2A fuse on the AC line input side.

mplucker said:
...The switches connected to this PS require 20-30V to function...

This would further strengthen the case that the PSU had to be operating at 24VDC.

So, and with me still not knowing what PSU is currently wired to the inputs (I'm assuming it's still the Gavazzi), then why would the inputs appear to be working, but yet, not quite right, timing-wise?

If you remember back to the good information Ken provided to you with regard to the ON and OFF state voltage levels required for the MicroLogix 1000 inputs - 14VDC ON and 5VDC OFF. The 14VDC is a "soft target" so, and with electronics being electronics, it does not necessarily mean that a lower voltage cannot switch the input to the ON state. If you are currently only switching 9VDC to the inputs, then it would normally not be high enough to switch them to the ON state. However, it is possible that the odd 9VDC signal is doing so, or is doing so intermittently.

This might explain the erratic behaviour of your machine's operations?

If you have some other PSU currently in use, supplying 24VDC for the inputs, then it would appear as though there is some other issue on the machine itself?

Remember, you still do not know what faulted the PSU in the first place? It could just be fatigue, but it could also have been a machine wiring or device fault. Although, nothing seems to be inoperable on the machine when it does work, right?

mplucker said:
...I'm still considering getting a backup MicroLogix 1000 and downloading the program to it so if anything does happen to the PLC, at least I'll have a backup. The electronics on these machines are no longer serviced (except for one company in CA, and I don't want to ship a 3000lb machine).

That would be very prudent.

Regards,
George
 

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