Forcing outputs RS Logix 500

Hello, when trying to force two outputs nothing happened. The text ON appeared by the adress. It was also possible to force off, which also appeared by the adress. The key was in RUN on the plc slc 5/03.

What have i missed?

I´m sure that i was on the right output. _Because force probably was applied to my output (a OTE) instruction. But nothing happened with the relay. And as i read Rons wall of text, what the program shows isn´t connected to force. 'very disturbing...

Or have i missed something else? I´m totally green on allen bradley PLC:s.

Did you enable forces? Did the indicator light on the output module change state?

Forcing is a two-step process. You must add a force to the item to be tested. Then forces must be enabled.

Enabling forces is global in the controller. It will enable all forces in that controller, so before you enable, you must verify if there are existing forces other than your own. If there are, make sure you understand what effect they have, Otherwise you could be enabling other forces and you might not be aware of the impact or reason behind that force. Also, once forces are enabled, all future forces will be enabled immediately.

Likewise disabling forces is also global to that controller. If you disable forces, you could again be affecting other parts of the system that you are not familiar with.

This two-step process is intended to prevent people that do not understand how forcing works from affecting live systems. They must understand how forcing inputs impacts the system versus what affect forcing an output has. They are very different in how the system

Someone that doesn't know what they are doing can pretty easily add forces to their programs. But if they don't know how to enable it, then they are not affecting that live system. However, it cannot prevent people from posting messages asking why it isn't working. I would ask myself, do I fully understand how forcing works in this Allen-Bradley controller before I started forcing items and enabling and disabling. Forcing, and its effect on a system in A-B PLCs can be very different from other brands. Particularly in the way inputs and outputs are affected.

OG

I've had this issue on PLC5 systems before. Forces enabled in the program, forced on a direct output, get the "ON" text, and nothing happens. I've resorted to using branches to turn on the output that way.

LOL. Thank you for validating my previous post. If you don't understand how the forcing procedure works, it doesn't do anything.

OG

Operaghost said:

LOL. Thank you for validating my previous post. If you don't understand how the forcing procedure works, it doesn't do anything.

OG

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Your post did not explain anything other than "make sure forces are enabled in the program and understand how forcing works".


There's no need to be a dick about it.

My apologies. In reading back, I was being dickish. My point was simply that if you (anyone) don't understand how forcing works then you shouldn't be doing it. And that includes knowing why an output doesn't energize in logic when it is forced. That is exactly what I meant by knowing how forces work.

Forcing an input will have a direct effect on the logic. However, forcing an output has no direct effect on the logic. The PLC program did exactly what it was supposed to do. Think of a forced output like jumpering voltage to the output module terminal. The logic would have no idea that you have done this and would not show the output as energized. But the field device would energize. Forcing works very much like a jumper wire would work.

Your solution was to add branches instead, and that is frankly a far better solution.

Sorry for being a dick. I'm not trying to teach how to force. That should probably happen in a formal class, or from an experienced person at the facility that can explain all the nuances at that facility.

OG

Operaghost said:

My apologies. In reading back, I was being dickish. My point was simply that if you (anyone) don't understand how forcing works then you shouldn't be doing it. And that includes knowing why an output doesn't energize in logic when it is forced. That is exactly what I meant by knowing how forces work.

Forcing an output has no direct effect on the logic. The PLC program did exactly what it was supposed to do. Think of a forced output like jumpering voltage to the output module terminal. The logic would have no idea that you have done this and would not show the output as energized. But the field device would energize.

Your solution was to add branches instead, and that is frankly a far better solution.

Sorry for being a dick.

OG

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No worries.

My point wasn’t that the logic didn’t reflect the force status, but rather that when the output was forced, the field device itself didn’t reflect the force status. Adding a branch did.

hardaysknight said:

No worries.

My point wasn’t that the logic didn’t reflect the force status, but rather that when the output was forced, the field device itself didn’t reflect the force status. Adding a branch did.

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For an output, the Force status, "FORCE ON" or "FORCE OFF" should match the actual device. However the logic status may not reflect the actual device status. If I see a device that is not energized, but it says FORCE ON then I expect that device to be energized in the field.

OG

Operaghost said:

For an output, the Force status, "FORCE ON" or "FORCE OFF" should match the actual device. However the logic status may not reflect the actual device status.

OG

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That’s what I’m saying, the force status was ON, but the device didn’t actually get voltage from the output. No voltage from the output card. The card had voltage going to it. No blown fuses

Like it was a bug or something.

hardaysknight said:

That’s what I’m saying, the force status was ON, but the device didn’t actually get voltage from the output. No voltage from the output card. The card had voltage going to it. No blown fuses

Like it was a bug or something.

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Interesting. I've never seen that happen.

One thing about forcing outputs - if the output bit is checked in the ladder (IE - XIC O:3/6) while it is forced on the ladder will NOT see it on unless the conditions are true for the ladder to turn it on.


Let's say you force a solenoid valve to open an air damper and keep it open, but a flame or blower control looks at the output for that valve it would see it off and that operation would be effected.


When I get online with a PLC and see the FORCES ENABLED indicator on I shake my head. Then I find out how to get it running without any forces.

Operaghost said:

LOL. Thank you for validating my previous post. If you don't understand how the forcing procedure works, it doesn't do anything.

OG

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Well, how do you activate the forces globally, the second step?
Maybe there is a point in what you´re saying about the required knowledge, but it would be nice to know what I and others have missed.