Forcing slc 5/04 analog outputs

phlebotomist

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Mar 2008
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Just another newbie! To aide in calibration, I am trying to force an analog output on a SLC 5/04 using RSLogix500. I am able to enter a new value in the data output table (with the forcing enabled the field is red). However, the 'forced' value reverts back to the calculated value almost instantly. The analog output is assigned to an SCP command. Can you only force digital I/O? If a trend the data point, the forced value(s) are logged but not maintained. Please advise.
 
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Correct, you can only force digital I/O. However, you can edit the SCL instruction, or test edits with a MOV instruction and move the value into the output you desire. Either way the goal is to get a different value into the output,than the SCL instruction is calculating.
 
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there's more to this story ...

Greetings phlebotomist ...

there are quite a few very common misconceptions going on here ...

first of all, you CAN force an analog output in RSLogix500 ... the "trick" is that once you're on the Output Force Table, you have to enter the desired value in BINARY format ... this is a little annoying – since we normally think of analog values in a "decimal" format instead of "binary" ... but, as I say, the forcing CAN be done ...

going further ... this next part surprises a LOT of technicians – even some with many years of experience ... in an Allen-Bradley system, when you "force" any output (either digital or analog) the value stored in the Output Image Table (in other words, the value stored in the bits/boxes) does NOT change ... specifically, the "force" is applied "downstream" of the bit table ... more specifically, the force affects the real-world output – but it does NOT affect the value stored in the PLC's data tables ... note that even the Rockwell software programmers for RSLogix5000 (that's 5000 not 500) get this particular item wrong ...

in the figure attached below, notice that the "value" stored in the Output Image Table (24967) calls for an output of 16 mA ... (I'm using a very common 1746-NO4I module in my lab) ... at the very bottom of the figure, the binary value that I've entered into the Output Force Table corresponds to 12484 – which gives us an ACTUAL output of 8 mA ... and that's what I'm reading on the milliamp meter that I have connected ... notice that the bit patterns in the middle and at the bottom don't match up ... I'm just showing both formats for convenience ...

so ... actually the MOV approach recommended by aswilson is probably the easiest way to make the VALUE change on the Output Image Table – if that's what you're really interested in doing ... and - if everything else is "normal" then this will also change the output of the module ...

just remember that even if you DO force the actual output to some particular value, that will NOT affect the status/value stored in the output bits ... so – be careful how you interpret the results of any tests that you make along these lines ...

if you'd like to dig deeper into this fascinating subject of how forces DO and do NOT affect a system, you can find more details in the Email Quizzes area on my website ... if you're not familiar with my quizzes, you'll find a link to them on the Sample Lessons page ...

I hope this helps ... and welcome to the forum ...

analogforce.jpg
 
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