Mitsubishi A series PLC

technolog

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Hi. We're a used machinery merchant. We have all sorts of machines through our shop. We currently have a tube bender that's not working correctly. It has a Mitsubishi A172SHCPUN PLC. The PLC is in Run mode but it also has the Error LED lit - see photo. We're hoping that getting rid of the Error LED will cajole it into working correctly.



I know very little about Mitsubishis, basically just enough to get an upload. We're using GX Developer v8.1. When I go to Diagnostics/PLC Diagnostics it says there is no error - see capture.

When I got an upload, there was an error that mentioned the comments being bad - is this relevant? Another dialog popped up at the end of the upload too (see captures) but that didn't seem to be anything to worry about.

Can anyone assist in finding why the Error LED is lit and then eliminating the error?

IMG_20200810_103235.jpg Capture.PNG Capture2.PNG Capture3.PNG
 
It is unlikely that the comments have been stored in the PLC, It is also possible that it was programmed in GXIEC however, it is more likely that it was GX Developer. All it is telling you is that there is probably no comments stored in the PLC so all you will get is the code.
It's a long time since I used the A1 series and they are now obsolete and only spares are probably from sellers of used equipment. I am a little confused about the error light but it may be that there had been an error but has cleared, I cannot remember if the error light stays on even though the error has been cleared, It might require a re-boot, is the battery recent as a system this old without having the battery replaced is a recipe for disaster.
 
It's possible that the program is decaying - sounds strange but I've had it a few times with A series.
As the battery goes flat, it starts losing parts of the program.
If you look through the program and you find some parts in yellow with incomplete rungs then that is what has happened.
 
Interesting one that, however, If GXIEC IDE was used to program it you can get the same thing, I once uploaded a A1S in GX Developer and had the same thing, there seemed to be un-connected ladder but there was no error. It turns out that GXIEC when compiled did not always complete the branch instructions like MPP (I think is one of them), I contacted the supplier and he provided the source file in GXIEC, I downloaded it to a spare PLC then uploaded it into GX Developer and the same result it appears that the branch instructions are missing on some of the logic but the logic actually works as intended but will not translate into ladder, there are a few oddities in GXIEC for example, if you put a single contact driving a timer in GXIEC download it, upload it in GXDev, then the ladder is the same, however, if you put a number of contacts before the timer for some unknown reason the raw code puts these contacts onto one of the reserved bits then uses the reserved bits to drive the timer.
There were a number of small non-fatal bugs in GXIEC. I believe it was written by an outside contractor.
 
Yes the same here, never used the motion plc, I have had a Q series that showed an error, I cleared the error on-line, however, the error light stayed on and showed no error in the diagnostics, I re-booted the PLC & the error light cleared. maybe this could be the same in the A series, however, as this is an old PLC I think it makes sense to change the battery before doing any type of re-boot, although the documentation suggests the plc be powered down I used to go to one site where there were over 15 PLC's we were given a yearly contract to replace the batteries, as we could not power down the systems we would replace them while powered up and never had a problem (although probably not recommended unless you believe the battery is completely dead), I have known Mitsubishi PLC's to work for 20 years or so without replacing the battery, most of them though are always powered on for life & rarely powered down thus the battery only loses it's power naturally.
 
I had worked in the past with the A , A1S and A2S series and I remember that the only way to turn off the error LED was to reset the PLC with the key.

The error can be caused by an error in an instruction, for example an overflow or an index out of range.

If you look in the D9000 special registers there is one that stores the last error code and another that stores the error line number.

Update :

D9008 saves the error code
D9010 stores the error step number
 
Last edited:
I believe as there is no error shown on the diagnostics there may not be an error code in those registers, I assume that the errors shown in the diagnostics are read from them. however, it may be the case that the error has been cleared and the processor needs to be stopped, reset & restarted.
I tried this on the simulator (Q Series) I created a maths fault, reset the fault and the error light stayed on, the diagnostics said there was no error and to clear the fault light I had to do the above procedure, I can only assume it is the same for the A Series. The one thing that is not apparent is has the error been reset using the programming software ?, will it happen again at some point for example divide by zero or index out of range.
 

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