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Weird BRX / Encoder problem

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  • Weird BRX / Encoder problem

    New BX-DM1E-36ED23-D PLC, new TRDA-20N1000RZD encoder (totem pole output), Rihno 60W 24V power supply.

    Configured a HSIO quad counter using X0, 1 and 2 for A, B and Z respectivly
    Conected encoder to said PLC inputs (1C to 0V) and powered both encoder and PLC from the same power supply.
    For initial test all power devices were (servo drive etc) were turned off.
    Turned system on but none of the LEDs on X0, 1 or 2 ever turned on.
    Got out my voltmeter and found that I was getting a voltage change of about half a volt (or roughly about a diode drop) on X0 and X2, nothing on X1.

    Next took encoder off and moved to bench with bench power supply and a 'scope. Encoder was drawing what seemed like a reasonable current, specs say 60mA max, but don't give a typical, anyway it was less than 60mA. Got a nice neat square wave on A, but still only about the half volt I saw on my voltmeter, B was dead, Z same about half volt.

    OK, I have a bad encoder, ordered a new one. After spending another $115 I was feeling a bit paranoid so I first checked new encoder at the bench with current limited power supply and 'scope, all good. Then I decided to check PLC, don't know why I decided that, because what could be wrong with it? Well, surprise, X0, 1, and 2 are dead, open, zero current flows when 24VDC is applied (all other inputs are OK, drawing between 6 and 8mA at 24v and their LEDs come on) but how could that happen?
    I can't think of any reason the encoder should have a boost converter in it (and I never saw more than about half a volt coming out of the encoder) even if the encoder was putting out its full supply voltage to its outputs, the BRX should be fine with that, because it is only a 24VDC supply.

    Well, what if the problem was the BRX in the first place, but while manufacturing defects can happen, what are the odds of the three inputs I was using, but only those three, being bad? Well, every now and then someone does win the Lotto. But what sort of fault could there be? the inputs are supposed to be optically isolated so it would seem the worst that could happen would be a short, but the encoder is suppose to be protected against a short to 0V (remember PLC 1C was connected to 0V) and they are open now which shouldn't have bothered the encoder.
    Or maybe, inputs could have been shorted to each other? Encoder specs. explicitly say protected against "short circuit between each output and 0V terminal" they say nothing about shorting the channels together. We sill have the very long odds of just the three inputs I used being a problem.
    Now, I can imagine inputs might have been shorted "downstream" of the "FULL BRIDGE RECTIFIER" and when I tried to test them by applying 24VDC it might have cleared the short by blowing up the rectifier.

    So, anyone ever see anything like this?
    Any Ideas how there could be faults in two different devices?

    New encoder is working on X4,5 & 6, fortunately I don't need all the inputs so I can spare 3, but on principal I'm not thrilled about losing them.
    Last edited by Tinker; 03-13-2020, 08:24 PM. Reason: another thought

  • #2
    So, if while testing the PLC only, you were off one pin (parallax) and were putting common on X0, instead of 1C...

    Unlikely, sure, but that's all I've got. I don't see that killing the encoder either. If that was what happened, X0-X3 are still good.

    It's a better guess than you were connecting to the analog inputs WX0-3. They both have a 1C, but only an input set for current would load the encoder at all.


    • #3
      We’re happy to do a failure analysis on the PLC. At the very least we should be able to speculate what might have happened.


      • #4
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