LL programming help requested

sinha_nsit · Apr 17, 2008

This is apparently very simple application, but I have not been able to write a ladder program for it so far:

Input: two maintained type ON/OFF switches

Output: A lamp

Objective: The lamp should switch ON when both switches are ON. It should switch OFF when both switches are OFF. Any other combination, i.e., one switch ON and the other OFF, should not change the previous status of output.

The difficulty: How to retain the previous status, which may be both ON or OFF for the same combination of input conditions?
Thanks.

cjd1965 · Apr 17, 2008

It is simple. Series the 2 switches in a normally open fashion to SET an output, and series the 2 switches in a normally closed fashion to RESET the output

Cheers

Dirt · Apr 17, 2008

Something like this?

jacokapp · Apr 17, 2008

Or you can set the output on with (I1 and I2) on and then only reset with not(I1 and I2).But the one give above is better.

A I 1.0
A I 1.1
S M 20.0
AN I 1.0
AN I 1.1
R M 20.0
A M 20.0
= Q 0.0

Ken Moore · Apr 17, 2008

Dirt, I believe there is an error in your ladder. Either switch can turn off the lamp in your example.

Dirt · Apr 17, 2008

Ken Moore said:
Dirt, I believe there is an error in your ladder. Either switch can turn off the lamp in your example.

Please check ladder logic again, to set the coil for M 0.0 you need to have both switches off.

Edit: Maybe I'm blind, but I don't see the fault.

DEVO · Apr 17, 2008

I agree the that logic Dirt gave will require both switches to be off before turning off the lamp.

sinha_nsit · Apr 17, 2008

Thanks a lot.

Both methods work nicely, but "set output" / "reset output" method is more straightforward and easy to comprehend.
Thanks again.

rdrast · Apr 17, 2008

sinha_nsit said:
Thanks a lot.

Both methods work nicely, but "set output" / "reset output" method is more straightforward and easy to comprehend.
Thanks again.

It may be easy to comprehend, but be aware that if the internal flag bit used to control the output is retentive, then you may have the device unintentionally turn on after a power cycle.

Yes, I realize this is only for a lamp, but if the principle is applied to a device that can move, there is a potential safety hazard.

When using latch/unlatch (or SET/RESET) commands to control devices, it is generally a good practice to use the "FIRST SCAN" bit/OB of the processor to force them all into a safe state on program warm/cold restart.

S7Guy · Apr 17, 2008

I would prefer the set/reset solution. The LL solution isn't quite correct, because the lamp will still be on for one scan after both switches are switched off. Of course, this is just a student exercise and no one would notice the difference if the light stayed on an extra scan, but in real life there are many cases where the logical state of a device could be used elsewhere in the program, and the inconsistent state of the device against the switches could cause problems.

robertmee · Apr 17, 2008

That's easily correctable by moving the unlatch rung ahead of the lamp output rung.

sinha_nsit · Apr 18, 2008

Error of one scan time will usually be there in many LL programs, specially those involving timers. it requires to be judged whether this would have any undesirable effect.

LL programming help requested

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