Learn PLC's

I am new to PLC programming but what I have learned I like. But some time I have trouble going form a problem to the relay ladder logic. There is no trobule going from the RLL to the program. Does anyone have any pointers or hints that I could use.

thanks
lena

Get Phil's book.
Here is a link that you may find usefull.
PLC Book
beerchug

Can you rephrase and ask that question again? I didnt understand the specific problem?

Is it you have trouble looking at relay logic and diagnosing problems but no problem converting it to a plc?

LENA2383,

The first thing you need to do is lose the intermediate hardwired step in your thinking - it's a crutch that often doesn't work.

You are using a PLC. In terms of control, a PLC is NOT a real-time, hardwired system.

Do this from a "computer point of view".

Think in terms of individual function-modules within a system.

The process itself is a single module.
Within that module are a few smaller modules.
Within each of those modules are yet smaller modules.

And the modules "talk" to each other. And they play nice.

PM from Lena

LENA2383 wrote on Today 11:15 AM:
For instance say that I got this problem right here

The alarm is triggered when liquid level switch closes.

The light flash whenever the alarm condition is triggered and has not been acknowledged even if the alarm condition clears in mean time.

The light will operate in the steady state ON mode when the alarm trigger condition still exist but has been acknowledged. (Acknowledged switch is a PBNO switch).

Use a PBNC to reset the timer and the alarm.

I find myself having trouble going the the RLL. I want to thank you for trying to help me.

Lena

Click to expand...

My Reply:

You should have made the reply on the site, would get alot more answers. Tell you what, I am going to copy this and make it a quote on your post. Watch the forum and respond when asked questions or attempt what is suggested. The people here will do their best to help but you have to cooperate and work to help yourself.

I will attempt a reply there but others may see and understand it better so may offer better advice.

Click to expand...

My response to the question:
After re-reading this post several times....do you need ladder code for ALL THAT? Have you made any attempt? IF you have can you show what you have done so far. What brand plc are you training on...ie is it AB or ?

Now for ideas, the liquid level switch we know will be an input...it can be in the form of XIO or XIC, that depends on device and how you represent it. Lets say its an XIO, when the level gets to a certain point this switch turns on and sends a signal to an input.

NOW this level switch is telling US that the liguid is TOO HIGH or TOO LOW, again depends on application. An ALARM is needed, therefore we turn on an output...this part is simple
This will turn on the alarm lite as long as the level is too high/low.

BUT WE need it to flash!!! How do we do that? Timers maybe? Built in Status files that cycle on/off for predetermined periods? Depends on you and brand plc used.

Now we come to the ACKNOWLEDGE stage! What will it do...in this scenario it seems to STOP the FLASHING but then makes the output alarm lite STAY ON. Was something done to create a holding circuit?

Now we want to CLEAR the timers used to flash the alarm lite and unlatch the holding circuit using a PBNC...would that be similar to using a PBNC as a STOP?

Think about this and make an attempt then show us what you have done.

I agree with Terry, dont think in terms of relay logic BUT I think in this case you are using the term RLL as Relay Ladder Logic to mean ladder logic used with plcs. I think you may have figured out some aspects of HOW TO write code to make the machine do what it should BUT have a problem with writing code to setup alarm/problem conditions...is that a fair assessment?

Looks like a standard alarm routine I use. Input comes on, latch input with reset button (n/c) in the line. Use latch to drive light via an internal 1 second clock pulse. Use latch to drive acknowledge line. When acknowledge button is pressed, latch acknowledged bit. Use acknowledge latch around 1 second pulse to stop it flashing. Also requires reset (n/c) in acknowledge latch line.
Expand to wherever you wish. I use bit counters and all sorts of things to total ons vs acknowledges to start and stop flashers, change pages on screens etc etc.
beerchug