Micrologix 1000 Timers

Hello all:

I am taking a Micrologix 1000 programming class, and having the time of my life.

I would appreciate some suggestions with the currient lab I'm working on.

The scope of the lab is as follows:

Using an Off - On Key Switch, Hand - Auto - Hand switch, and a single start - stop switch, write a program that will:

1) Allow for Motor 1, Auto, or Motor 2, selection.

2) In the Auto Mode, motor starts will alternate.

3) Use a standard Start - Stop Station for motor starts.

4) When the Start Button is pressed a 20 second alarm sequence begins (1 light on solid, 1 light blinks 0.5 sec. on, and 1 sec. off).

5) After 15 seconds the motor starter pulls in.

I have a solid working model, but I am using three timers. There is an unofficial quest among class members to program with the least number of timers, counters, and rungs, and still have the program function as specified.

I would appreciate suggestions (not spelled out solutions :-] ) that would assist me in reducing the timer count, if possible, that could point me in the direction of my timer reduction quest.

Thank you.

Louis

IMHO, your quest to reduce the number of timers is misguided. Your goal should be clarity. The real world imposes enough constraints on your designs without you having to impose artificial limitations.

You've defined three (or is it four) intervals that need to be timed, the on-time for the warning lamp, the off-time for the warning lamp, and the delay before starting the motor. I'm not sure what you mean by that reference to a 20 second alarm sequence. Do you mean to keep the alarm light blinking for 5 seconds after the motor starts?

Suppose you come up with a method that uses fewer timers than the number of intervals to be controlled and then your instructor changes the specification for one of the intervals? Instead of a simple change, you could be faced with a substantial amount of rewriting.

How many real timers to you think a micorlogix has?

You must realize that most microcontrollers do not have more than 2 or 3 timers. The MicroLogix firmware emulates all the timers you see in ladder in firmware. In your case you must design your own timer.

Hint:

1. You need only one timer as long as it is free running.
2. You must save away the time when your timer starts and accumulate differences in time while the timer is on.

I do whole projects with only one timer for interrupts and one timer for calculating difference in time. There is also a thread that covers this topic in great detail.

Steve, unix systems, and windows have a system timer. This one timer is used almost everywhere a delay is required. This is normal programming practice.

PLC timers, counters and one shots are just things created in firmware. Therefore less efficient versions can be created in ladder.

Peter gave it all away but I am going to expound on it with my limited knowledge.

Other brands of plcs do not offer the options that AB has with its timers. The use of accumulated time, done, enable etc is not done in the same fashion.

Look at the GEQ, LEQ and other compare functions then think about how this could apply to 1 timers use.

I agree with Steve in a fashion though, it may not make the code any easier or look any better.

Overall I am of the opinion that plc programming/code with documentation should be accesible by the user for troubleshooting and be as easy to understand as hardwired devices. There are many situations where a standalone hardwired timer is used but what is going can not be understood (especially when amplifier types are used).

I am making this reply not to specifically offer advice on HOW to do anything as much as WHAT to do when you program, make it understandable. Think about the machine and those that may have to come behind you to understand what the plc is doing.

DO NOT dumb down the code...ie make it too simplistic, use the features available. Document/explain extensively any all actions within the plc.

I state the above because there are so many features available today that make the programs so much simpler its awesome. YOU have to document/tell what you are doing/thinking..sometimes even for yourself. Programming can be made shorter and simpler BUT others have to UNDERSTAND what you have done.

Hows that fer deviating on a tangent guys?