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SM35-J-RA22 analog input showing 32767 after wrong wiring


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Hello,

I have been programming on SM35-J-R20 that my company found after another company they bought. However, for one application including 4-20mA sensor, 10-bit analog input was not enough sensitive, so we bought SM35-J-RA22.
After four weeks of waiting for it, it finally arrived. I was so excited for trying it out that I accidentaly connected it wrong - I connected 4-20 mA output to pin 1 (+24V) and +24V to pin 9 (AN2). This seems to have destroyed my AN2, because now when it is not connected, it fluctuates between 0-2, and when I connect it to something, it immideately shows value 32767.
Without a doubt, this is my mistake, however, Unitronics spoiled me in this, because after countless occurences of mistakely swapping 0V and +24V, after fixing the wiring it always worked well without any problem, so that's why I got neglectful towards my wiring. With that said, it would be nice if Unitronics made a precaution to this similarly to the precaution with swapping 0V and +24V.
Thaknfully, this project involves only one analog input, so using AN3 instead fixes my problem. However, what can I do if I wanted to fix AN2? When I looked at circuits, none of them looked burned.

Thank you in advance,
best regads
Š.K.

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Glad you were able to use the other input. It seems I have made that mistake a few times (though not specifically with a Samba) and never seen a problem after correcting the wiring. Is it possible that there was more than 24 volts applied to the input?

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So you burned up the Internal input resistor.  I have never done that 😜

You can get your channel back by ordering some 500 ohm precision resistors, connecting them externally between the analog input and 0V, and put your analog input into 10V DC mode.

Joe T.

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SM35-J-RA22 has 37 ohm resistor at current input.

Applying 24V dissipate ~16W at small resistor.

Unitronics already made precautions in Installation Guide:

image.png

Looks like there are 2 groups of peoples:

1. Read and then do.

2. Do, then read, than say "wow/heh...".

 

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Dear Alex,

I could recite technical documentation as well as the installation guide as a poem, because I had to do some research for selecting the right PLC as well as learning how to work with it. Apart from the old PLCs, my company had me to recycle the wiring as well, and unfortunately for me, the cables are all blue, so they are easy to mix up.

So forgive me if you believe that I am the one from the second group, after all, unlike PLCs from Unitronics, we, people, do mistakes.

Sincerely,
Š.K.

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9 hours ago, AlexUT said:

Looks like there are 2 groups of peoples:

1. Read and then do.

2. Do, then read, than say "wow/heh...".

@AlexUT - you're getting Forum Fatigue.  Play nice.  It's humiliating enough when you realize you've blown up your own equipment.

8 hours ago, MiMoSVK said:

nd unfortunately for me, the cables are all blue, so they are easy to mix up

Been there.

kratmel's advice is wise.  We update the controls on a fair amount of old machinery and it's very common for European-made machinery to not use wire numbers, relying on painstakingly detailed drawings showing what wire goes to what terminal.  That goes away when you put new controls in so you have to make your own drawings and add wire numbers as you take it apart.

 

Joe T. 

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Thank you all. Yes, using numbering makes sense. I guess I should make my company order some so that something similar won't happen again.
I am happy that you think it is "just" a resistor. I thought I burned up some circuit that would be hard and expensive to replace. If I would ever need to use AN2 as well, I guess I would find which resistor got burned, unsolder it and replace it (or use Joe's solution if I fail to do so). Until then, I am not trying my bad luck anymore.
I will post some eventual update to this thread if I will be resolving this in the future.

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