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Unread December 2nd, 2018, 03:50 PM   #1
JRoss
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Mitsubishi PLC died. Twice.

Hey everybody,
I have a strange problem that had me baffled. I built a small control panel with a Mitsubishi FX5U PLC, a panel PC, a simple KB Electronics DC drive, and a Rice Lake scale indicator. The whole panel is powered off of a 30 amp 120 volt single phase circuit. It controls a crush-test fixture. The drive moves the crushing plate up and down, the scale indicator measures load and reports it to the PLC using RS232 streaming, and the PC is the operator interface and also does the data collection and report generation. The fixture is old, the panel is a replacement for the 20yo panel that came with it.

The problems began when I first powered the panel. At first I had trouble downloading to the PLC as it wouldn't go out of run mode. I had downloaded a test program before installing it. But eventually that worked and I got the drive running and the basic functionality working. Then I went to test the scale indicator.

I had bench tested the scale indicator streaming function with my laptop but hadn't tried it with the PLC. I first hooked it up to my laptop to verify comm settings, and... nothing. After time on the phone with tech support we determined the port on the indicator was dead. So I ordered a new control board.

New board came in, plugged it into the PLC, and... nothing. So I packed the indicator and both boards and sent them back to the distributor who determined both comm ports were dead, and installed and tested a third control board. At this point I also ordered another indicator of a different brand.

New control board connected to laptop... nothing. New indicator... worked! Got data into the PLC! But then I started smelling smoke and the PLC died! Lights done and went out completely. I had a second one, put it in, and the PLC serial port module was dead, as well as the comm port on the new indicator. But everything else worked fine, and I didn't the rest of the day doing development and testing on everything else.

Went home and ordered a new PLC and serial port. Ended up getting a different style because original module was backordered. Received parts and on a whim hooked up the Rice Lake indicator on the bench. Worked fine with laptop. PLC talked to laptop, but wouldn't talk to each other until I used a different serial cable. Finally success! Left it running for about 12 hours over night.

Back to panel, installed new PLC and serial module, started testing for stray voltages, bad grounds, etc. Didn't find anything. Tested indicator with laptop. Worked. Plugged indicator into PLC, RD light flashing! Started getting laptop ready to go online. Smelled smoke, and PLC had died. Just confirmed serial port died too. Though this time the indicator escaped damage.

I took apart both PLCs and the damage is at different places on the board.

So PLC works fine in panel with no serial connection. PLC works fine on bench with serial connection. But in panel with serial connection? Dead twice over. Any ideas what could be going on?
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Unread December 2nd, 2018, 05:33 PM   #2
lfe
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It seems that the two devices do not have the same power supply common, maybe in the PLC you inverted the cables that go to L and N?

If serial ports are not isolated, this can cause strong currents to circulate through the communication cable.
It would be nice if you put a serial port isolator between the PLC and the indicator
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Unread December 2nd, 2018, 06:31 PM   #3
sparkie
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You have a ground loop, it appears. I don't know about modern controllers, but the older Mitsubishi ones do not have isolated rs232. It is time to go poking around with a meter and see what you can find.
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Unread December 2nd, 2018, 08:18 PM   #4
g.mccormick
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Get a 232 repeater/isolator?
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Unread December 2nd, 2018, 09:12 PM   #5
sparkie
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Quote:
Originally Posted by g.mccormick View Post
Get a 232 repeater/isolator?
This could fix it, but I would prefer to find and measure the problem rather than keep throwing hardware at it until I get it right.
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Unread December 2nd, 2018, 09:36 PM   #6
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I concur with two previous comments:

Ground loop is probably the cause (root cause probably related to the DC drive/motor).
A serial isolator will probably be a worthwhile workaround. I have used one from this site with good results:
https://www.commfront.com/collection...opto-isolators

I would closely inspect any analog connections between the DC drive and the PLC. I recall when I toasted a brand new analog channel on a PLC-5 it was because the analog connections to the DC drive were not isolated. I always install an analog signal conditioner/isolator when dealing with that type of drive now. This is just a WAG, I don't know your drive specifically.
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Last edited by OkiePC; December 2nd, 2018 at 09:40 PM.
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Unread December 3rd, 2018, 12:27 PM   #7
JRoss
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Aha, that makes perfect sense! That explains why it died within minutes of plugging in the serial connection both times.

I am using the built in 0-10V analog output to control drive speed. Never had any issues with that over the course of several days of testing, but can't hurt to protect it.

I have a serial isolator on order. Any particular recommendations for an analog isolator?
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Unread December 3rd, 2018, 01:42 PM   #8
sparkie
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I also didn't mention that you should be plugging your laptop into a receptacle that shares the same 0V/Earth/Ground reference point with the rest of the hardware. What you are looking for with the "analog" isolator is actually a signal conditioner. It is pretty much just a follower opamp circuit. In into your grounding points in that cabinet and start checking for a difference in potential with a meter and looking for anything on the grounding/neutral that can give you problems. You are likely to have all kinds of intermittent issues that drive you crazy until you find that problem.

Last edited by sparkie; December 3rd, 2018 at 01:45 PM.
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Unread January 29th, 2019, 01:51 PM   #9
JRoss
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OK, I've got this wrapped up. Took a while because some of the parts I needed were on backorder, and then I had to wedge it into my schedule.

I replaced all the burnt parts and installed a serial port isolator, and have had no more problems. Thanks everybody for all your help!
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Unread January 29th, 2019, 01:59 PM   #10
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Quote:
Originally Posted by OkiePC View Post
A serial isolator will probably be a worthwhile workaround. I have used one from this site with good results:
https://www.commfront.com/collection...opto-isolators
Hey Jeremy,

I know im a little late to the game but if you need anymore I can save you a bunch of money...
https://www.plccable.com/rs232-optic...-or-equipment/

Glad you got it working
Mark
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Unread January 29th, 2019, 02:18 PM   #11
jraef
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KB drives are known for being low cost, but also low isolation and protection from emitting Common Mode noise. The approach I would have taken would have been a shielded 1:1 drive isolation transformer ahead of the DC drive. That CM noise is likely stressing a lot of the components in your electronics, it's just that on the serial port they failed first.
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Unread January 29th, 2019, 10:35 PM   #12
JRoss
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Mark - Thanks, I didn't even think to check your website, though it's the first place I go for programming cables!

jraef - Thanks for the heads up. Do you have a recommended brand of isolation transformer? Or even a different DC drive that has built-in isolation?

I'm hoping to sell the customer on a second one of these, so if I can have some solutions in pocket ahead of time, it would be helpful.
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Unread October 16th, 2019, 05:06 PM   #13
tatitocordero
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I have a question: What software language the Mitsubishi PLC uses?
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Unread October 16th, 2019, 05:45 PM   #14
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Good practice read the manual before wiring something i think .
Sections: 3.3;
3.7.1 to 3.7.4
http://dl.mitsubishielectric.com/dl/...d53401(e)f.pdf
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