plc scan time problems...HELP

hi everyone...i am a university student who is undergoing the subject industrial control...we are required to learn plc....damn its hard!!!i have been scratching my gead for the past 3 months but still cant figure it out what is the problem with my lab"s plc...i am using omon cpm2a plc...its seem that everytime i program it with the cx programmer, it won"t ru nthe way i want it to..some of my frens said that maybe its because of the scan time....can anybody out there tell me what is scan time and what is RACE TIME.....PLZ HELP ME THANKS

Try the link on the top of this page "Online Tutorial" After studing the guide, feel free to repost more questions. More information about what you have done, and what you think the PLC is not doing, would be required for quality help.

Scan time is how fast the processor works. The lower the number, the faster the scan. Most processors scan every 10 milliseconds or faster.

Race time - its not called race time, we just call it a race. It is when two thngs can happen and sometimes this one make it first and sometimes the other makes it first. This will result in unintended results in your program. Most races can be solved by using timers and scan time does play a big part in the problem.

I suspect your problem is the basic layout of your program. Using timers and counters can help you locate the problem.

If you can attach a PDF of your work so far, maybe one of us can give you a few pointers.

"Race Time", usually called "Race", is a condition usually associated with real relays.

In real-time, you might turn on a relay and then expect to use the output from that relay immediately to affect something else, before the intial relay is turned off.

The problem is... it takes time for a real relay to transition from one state to the other. In that time, there is a period where the particular relay is not in either state... that is, both the normally open contact AND the normally closed contact are de-energized. It takes time for the switch in the relay to move from the normally closed contact to the normally opened contact. During that time, there is no signal from either of the contacts.

There are other "Race" conditions in digital logic devices. These have to do with data, states, conditions, being ready to be "read" when the time comes.

For example, if a particular function is building a word from a series of bits and it takes 16 scans to do so, then the word will not be ready until 16 scans after the word-building begins. If some other function is trying to access that word every 3 or 4 cycles... then the second function will very rarely get a proper result... because the second function is accessing the word faster than the word can be constructed.

Then, there is this other effect that some call "Race", but I prefer to call it "bit-chasing". It is strongly related to spaghetti-code.

A bit is turned on, somewhere in the program. The expectation is that that bit will cause a subsequent action in another part of the program. In the mean time, other bits are turned on or off. Then, at the point where the bit was supposed to cause the subsequent action... now, for some reason, either the particular bit is no longer on, or other unexpected conditions have precluded the subsequent action from occurring as expected.

That is usually a matter of poor "temporal-planning".

its seem that everytime i program it with the cx programmer, it won"t ru nthe way i want it to.

Click to expand...

Sounds very much like a program problem not a PLC problem.

With CX-Programmer, you can view the scan time when on line - it automatically shows up in the lower section of the screen below the ladder. I cannot paste a picture as I cannot go online at the moment.

Let us see your code and explain what you mean when you say it will not work as you wish.