Forcing Analog Output...?

TRW0930

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Join Date
Aug 2007
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New Brunswick
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5
Hello all...I'm a newbie with PLCs. What I'm trying to do if possible is to force an analog output to control a damper motor manually...we want to test all the motor in our HVAC prior to the COLD weather...can this be done? I thought I knew as I can force discrete I/O but analog is a different story....any ideas would be great...:unsure:
 
Temporairly disable the rungs which set the value for the analog output. Create a rung to write the value of a suitable register to the analog output. Then modify the value in that register.
 
Yes, it can be done. The details vary with the PLC brand and model, but essentially when enabled by a manual contact (bit) you load a value from 0 to 100% of the data range into the appropriate register. The value can be from an HMI or a fixed preset or whatever.
 
While you can't force an analog value by just typing in a number, an analog output is just a group of discrete outputs and you should be able to force it as such. Whatever value you want to use, convert it to binary and force all 16 bits of the output word to the appropriate values. (I haven't actually tried this but it seems like it should work.) However, if you want to be able to modify the value on-the-fly, it may be easier to create alternate logic or use an HMI as others have suggested.
 
well, there are "forces" – and then there are "forces" ...

we might not all be speaking the same language here ...

when most PLC people say "force" they're talking about some type of BUILT-IN function ...

some brands – and types – of PLCs have such a BUILT-IN "force" feature for analog values ... other PLCs do NOT ...

but ...

regardless of whether the system has a built-in "force" feature for analog outputs, it should still be possible to "make-the-analog-output-do-whatever-we-want" ... most of the suggestions so far are related to that NON-built-in type of action ...

party on ...
 
Not sure if this will shine any light on my issue but I'm using Contrologix...:site:

Also I notice a [NOP] instruction rung 0 then rung 1 has only a OTE tagged AHU_3_Damper and then rung 2 has another [NOP] instruction. I'm thinking I might need to force rung 1 off then apply my force to the analog output...? Does this make sense?
 
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Good point Ron, I assumed the OP was talking about the right-click-on-the-bit-and-select-Force-On kind of force, but it's not totally clear. I was thinking RSLogix but he doesn't specify.
 
OK - now we've got enough details to make some progress ...

the answer to the basic question is:

yes, the RSLogix5000 software WILL allow you to "force" an analog output – using a handy BUILT-IN "force" feature ...

the thing that confuses many people (even some with several years of experience) is that the value that you see in the TAG LISTING (in other words, on the "Ladder Display") may not be the same value that is actually being "forced" at the output of the module ...

see if the following picture makes sense to you – and if you need more help, then ask more questions ...

main idea: to force an analog value, you find the tag in the Controller Tags listing – and type the "force" value that you want into the "Force Mask" column ... specifically, you can't just right-click the tag name in the window display ...

naturally you still have to "Enable" the force to make it take effect ...

and welcome to the forum ...

force_analog_output.JPG
 
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Thanks...I'm thinking this forum will be a great help and very informative. Now prior to forcing the output, I assume I'll have to disable the writing to the tag by the plc or will the force do this for me? Again thanks for the help.
 
Now prior to forcing the output, I assume I'll have to disable the writing to the tag by the plc or will the force do this for me?

no, you will NOT have to disable the logic which is "writing to the tag" ... the "force" action is applied "downstream" of the tag's value - and the force will "override" the signal going over the chassis backplane to the output module ...

notice in the figure that I posted, the MOV instruction is still writing a value of 12.3 into the output tag ...

BUT ...

the "force mask" value of 34.5 is what the output module is actually being commanded to send to the devices in the field ...

in other words, it doesn't matter WHAT value happens to be present in the output tag ... the value from the "force mask" overrides that value - and the output module obeys the "force mask" value instead ...

(note that all of this depends on the forces being "enabled") ...

going one step further ...

or will the force do this for me?

no, the force will NOT change the value in the tag - BUT - that won't keep the force from working ... you're missing a basic idea of how the signals move through the PLC ...

here's the secret handshake: the TAG and the OUTPUT MODULE are NOT the same thing ... specifically, when you force an output, you can have ONE value (12.3) in the tag – and you can have ANOTHER completely different value (34.5) being commanded to drive the output module's circuitry ...

again, the "force" is overriding the TAG value – before it gets sent to the OUTPUT MODULE ...

going even one more step further:

IF (that's a big IF) you can easily disable whatever is writing into the analog output TAG, then you could do that - and skip using the BUILT-IN "force" idea completely ... this is exactly what some of the other responders were talking about ... in other words, if you can disable whatever is writing into the output module's TAG, then you can just type in your own value into the TAG - and the output module should be commanded to send that particular value out to the devices in the field ...
 
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