Cant toggle bit or force in Studio 5000

[Bit_Bucket_07]

this is one of the MAIN stumbling blocks for technicians who habitually use FORCES while trying to troubleshoot a system ... suppose that their line of reasoning is something along these lines:

OK ... we've forced the lamp/pump/valve/etc. to be ON in the field – so that means that the rest of the logic SHOULD respond to that ON status ...

if they haven't had quite enough training along these lines, then they fail to realize that their reasoning is being misled – simply because there is STILL a ZERO in the output's bit/box – even though the output in the field (the actual device) is being turned ON ...

in simplest terms – the PLC's logic will NOT respond to the forced status of the output bit/box as expected – which can lead to a time and money extensive "chase of the wild goose" ...

on the other hand, when you force an INPUT, the forced status (either ONE or ZERO) will (generally) be reflected in the rest of the program's logic ... this can be very confusing to bleary eyes at 3:00 in the morning – and the machinery is down ...

takeaway: INPUTS and OUTPUTS behave differently when they are forced ...

for those who want more details – start here ...

http://www.plctalk.net/qanda/showthread.php?p=537414&postcount=1

and skip ahead to paragraphs 38 through 43 for a quick start ...

party on ...

Excellent description of the phenomenon, Ron.

Of course, it doesn't apply to all PLCs. Am I mistaken in remembering that the PLC-5 and SLC-500 didn't handle forces in this manner? I don't recall encountering this prior to working with the AB CLX models, but that could just be a failing of my memory due to oldtimer's disease.

I know that asynchronous IO updates were new to the CLX models.

 

[Ron Beaufort]

Am I mistaken in remembering that the PLC-5 and SLC-500 didn't handle forces in this manner?

since I've been retired, I'm not keeping up with things as well as I used to – but as far as I know, all Allen-Bradley systems handle their forces in the same way that I described in the thread that I linked above ...

I do know that some other brands DO work differently though – so that certainly does add some room for confusion to the mix ...

stay safe ... stay well ...

 

[Bit_Bucket_07]

since I've been retired, I'm not keeping up with things as well as I used to – but as far as I know, all Allen-Bradley systems handle their forces in the same way that I described in the thread that I linked above ...

I do know that some other brands DO work differently though – so that certainly does add some room for confusion to the mix ...

stay safe ... stay well ...

Yeah, I've worked with a number of different brands over the years, and I've never really used forces very often with any of them.

You keep well as well!

 

[daba]

I'm sure Ron had this covered in his links but to simply clarify what happens with forcing in all A-B processors....

It's all about the "flow" of data....

INPUT FORCES

Input forces are applied to the data entering the input data-table or tag. That's the only way your logic will see them. They will be highlighted on the ladder GUI.

OUTPUT FORCES

Output forces are applied to the data leaving the output data-table or tag, en-route to the output modules. They will not be highlighted on the ladder GUI, nor will you be able to determine if the output is on when forced by referencing the output bit.

"Forcing", in the wrong hands, is a hazardous procedure, not suited to your average tech at 3 in the morning.

 

[I_Automation]

"Forcing", in the wrong hands, is a hazardous procedure, not suited to your average tech at 3 in the morning.

I don't really like forces. For testing - OK, leaving them in - Bad Practice.



When I get online with a PLC and see the "Forces Present" and "Forces Enabled" buttons on I cringe.


As mentioned, if an output is forced and that output is checked anywhere in the program the program may not operate as it should if it sees the output OFF but after the scan it is turned ON, or is ON and forced OFF.

 

[alive15]

In rs logix 5k, for testing purposes, you can place an empty rung around all your inputs and see if your output comes on. You can't do it in 500 though, the software will throw up an error.

 

[I_Automation]

In rs logix 5k, for testing purposes, you can place an empty rung around all your inputs and see if your output comes on. You can't do it in 500 though, the software will throw up an error.

For this I always set a TRUE bit and unlatch a FALSE bit at the start. For SLC's B3:0/0 = FALSE, B3:0/1 = TRUE, and in case it is needed for an output B3:0/15 is NOP.

For testing as you say put XIC TRUE on the branch. To shut off a rung put XIC FALSE inline.

If it is testing I may want to do later online also I use an unused bit with the description Toggle Bit To [enable/disable/force/test] Feature and leave it in the program.

 

[alive15]

For this I always set a TRUE bit and unlatch a FALSE bit at the start. For SLC's B3:0/0 = FALSE, B3:0/1 = TRUE, and in case it is needed for an output B3:0/15 is NOP.

For testing as you say put XIC TRUE on the branch. To shut off a rung put XIC FALSE inline.

If it is testing I may want to do later online also I use an unused bit with the description Toggle Bit To [enable/disable/force/test] Feature and leave it in the program.

Yes, your method is even better with the binary bit. I've connected to old plcs that were not written by me, and I would see many of those binary bits you mention in parallel, all named along the lines of "toggle pb". I'm gonna start adding those myself, they are very useful for troubleshooting.