How does a force work

Hi,

I am new to PLC's.

I am able to download a simple program / ladder to the plc i.e.

1 row, examine if closed input & output energise

When I run the program and force the input on shouldn't the out put energise i.e. the LED on the output (and input card for that matter) light up?

Ami inputting the addresses wrong, when i type I it brings up a list of inputs which i choose one from and teh same with the output.

Do i need to wire up the input / output cards before i can even simulate energising the or forcing the on?

Thanks,
James.

1. Some of the Allen Bradley PLCs, besides forcing a single item, may require you to 'Enable Forces'.

2. Forcing an input ON generally WILL NOT cause the cooresponding input light on the input module to light.

Bernie you are going to make this learning process a hell of a lot easier, thanks a million...again!

Planning on taking a module begginners module on misubishi plc's in september and another on siemens starting after that...with your help i mighn't need them!

Thanks again...

Greetings James ...

as you go a little further into this subject, you'll find that all brands of PLCs do NOT handle forces in exactly the same way ...

some brands force the input and output "bits" (in other word the "boxes") ...

other brands (including Allen-Bradley) force the input and the output SIGNALS (specifically NOT the bit/boxes) ...

these apparently minor differences can make for major headaches when you're using forces while troubleshooting a system - particularly if you frequently move from one brand to another ...

you can learn quite a lot about this subject (at least for the AB brand) by working through the free E-mail quizzes on my website ... #115 and #215 should be the most helpful ... just scroll down past the videos on the Sample Lessons page to find the "quiz" link ...

hope this helps ...

Another thing about forcing in AB. If you force an output and have a contact of that output as a condition it WILL NOT take on the status that the force sets.

E.G - I force O:0/1 ON

If in a rung I have a contact of O:0/1. It WILL NOT necessarily be ON. It will have the status that any logic establishes for it.

The output table in AB in its 'data' mode will show the logical truth of the output bit as a 1 or 0. But if it is being forced it will show in RED and the status as shown the 'force' version of the data table (selectable by a button) will have effect (again subject possibly to any 'Enable Forces')

jimmycooker said:

Hi,

I am new to PLC's.

I am able to download a simple program / ladder to the plc i.e.

1 row, examine if closed input & output energise

When I run the program and force the input on shouldn't the out put energise i.e. the LED on the output (and input card for that matter) light up?

Do i need to wire up the input / output cards before i can even simulate energising the or forcing the on?

Thanks,
James.

Click to expand...

Am i inputting the addresses wrong, when i type I it brings up a list of inputs which i choose one from and teh same with the output.

Very possible - in your addressing you must type the write letter and numbers. Zeros and "O" may be used and from my experience when you mix em they do NOT work. I cursed my typist for doing this numerous times (unfortunately it was me)

Do NOT force. In your circumstances at home it probably will not be dangerous. In industry it can be very dangerous.

Reading between lines I think you are getting ahead of yourself. Get some single pole switches and some light bulbs 5 or 10 watt RESISTIVE - do not use neon lamps like I did another lesson I learned here.

Learning. Wire one input with a switch.
2. Write program to energize an output when switch is shut. 3. with switch OPEN Energize and see what happens.
4. Shut switch and see if output comes on.

Keep your learning stage simple. Use manual switches only so you know exactly what is going on with inputs. Use LED on unit to know waht the output is doing or wire a small lamp across it.

Dan Bentler