emulate off-delay timer with on-delay

B

Thread Starter

Bud Buyer

Is there a standard ladder logic work around for emulating an off-delay timer with an on-delay timer (where the timer is enabled when its enable circuit goes false)?
 
V

Venancio Bravo

There are a lot of different ways to perform what you are asking for, but if you want to keep it simple, use the following algorithm, it uses only basic instructions. Many PLC does not include the TOFF instruction, among its functions.
decode to ladder, using this code:
SOR = START OF RUNG
EOR = END OF RUNG
BST = BRANCH START
BND = BRANCH END
NXB = NEXT BRANCH

SOR BST XIC I:0.0/0 NXB XIC B3:0/0 BND XIO T4:0/DN OTE B3:0/0 EOR
SOR XIC B3:0/0 OTE O:0.0/1 EOR
SOR XIO I:0.0/0 XIC B3:0/0 XIO T4:0/DN OTE B3:0/1 EOR
SOR XIC B3:0/1 TON T4:0 1.0 5 0 EOR
SOR END EOR

Good Luck.
I am issuing a new page over the PLC programming matter. You can find a lot more info, about cases as the one you described, at:
http://www.intelligentes.com/PLC/Courses/Nivel 1/AUTOMATAS PROGRAMABLES 1_5_4.htm
Some info is in Spanish, yet, a lot more in English, I will translate everything step by step.

V Bravo
[email protected]
http://www.intelligentes.com/PLC/Courses/Nivel 1/AUTOMATAS PROGRAMABLES 1_5_4.htm
 
S

Steve Myres, PE

Use the input condition to set a bit. Then use the open condition of the bit to enable an on-delay timer. Then if the bit is false AND the timer is expired, the off-delay is complete. The specifics of your PLC may make it unnecessary to check for the false condition as well as the timer completion.
 
What I usually do is something like this:
A Txx B
---|/|-----(TIMER)---------------( )--

A OUT
---| |---------------------------( )--
A B |
---|/|---|/|--+


Where A is the signal that you want off-delayed,
Txx is the on-delay timer (whatever syntax in your controller), B is the output of the timer,
and OUT is the final Off-delayed signal.

So, the final output is ON when A is on, then when A goes Off, the Timer starts. Keep the output On until the timer times out and goes On.

Probably a lot more ways to do this, but this is what I did with the TI-545.

Regards,
Matt
 
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