The trouble with timers

I seem to be running into all sorts of road blocks in my quest to find my timer answer. The senerio goes as follows..A sensor goes true(ON)for several minuets and a solinoid is supposed to turn on for only 5 sec. At the end of the 5 sec the solinoid will turn off and the timer will reset for the next time the sensor goes true. I am a beginner at this and practice is great if you understand how to find a solution. There is probably more than one way of acheiving this, I just haven't found it yet.

Here is the statement of your homework, rephrased a little to make it sound like ladder logic.

If the sensor is on, start the timer.
If the sensor is on and the timer is not done, turn on the solenoid.

That is another way of putting it. Normally, I use IF THEN statements-- If sensor S1 is on then Solinoid SV1 is on for 5 sec.

If it helps you then IF/THEN statements are just fine, but you need to construct it the right way.

IF S1 THEN RUN TIMER ELSE RESET TIMER

IF S1 AND NOT TIMER DONE THEN OUTPUT = 1 ELSE OUTPUT = 0

A non latching output can be thought of as an IF/THEN/ELSE (If rung= true then output = 1 else output = 0)

Depending on your PLC and the kind of timer instruction you use a timer may auto reset when the rung is false.


A better way might be to think of the output as a boolean equation, eg OUTPUT = (S1 AND TIMER). As you move on to more complex logic you might find it to be more useful.

To answer your question, I am using a TRI LOGIC plc. I beleive they only have ON Timers for their ladder logic.

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Hi Steve, Thanks for your input. I have that part of the ladder working correctly thanks to you. By the way, as I mentioned I'm using ladder logic and the plc is TRI LOGIC. One quick question, can I have the same output(OUT1) in both my ladder and the CusFn program (BASIC). How will they affect each other? If you aren't familiar with this, I'll understand.

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I'm not familiar with TriLogic, but I expect the result would be the same as if you tried to control the same coil on multiple rungs of ladder. Whichever line of code, ladder or Basic, was executed last before the outputs get serviced will be the line that controls the actual state of the output.

A key point you need to remember is that classic PLCs do not interrupt the logic scan to service individual outputs. Rather, they keep track of the output states and pass a complete set to the physical outputs in a single task. That's the reason for the warnings against multiple coil usage.