Memory

HELLO,PLEASE BARE WITH ME AS I AM NEW TO PLC's AND THIS SITE.
I AM TRYING TO FIND A PLAIN ENGLISH EXPLANATION AS TO WHAT THE RELATIONSHIP IS BETWEEN MEMORY SIZE,PROGRAM SIZE AND SCAN TIME AND CAN ANYONE EXPLAIN THE PURPOSE OF INPUT/OUTPUT OPTO-ISOLATOR.

ANY INFO WOULD BE GLADLY ACCEPTED

Memory size is the amount of available memory that the PLC has, each instruction uses up memory, so the more memory the more program instructions you can have. Scan time is the amount of time it takes the processor to look at the inputs, solve the logic of each rung, and write to any outputs that need to be updated. If you have a large program that uses more instructions it will occupy more memory and increase scan time as there will be more logic to solve.

Opto isolation, isolates the PLC's circuitry from the outside world.

Hope this helps.

HNC alert!!!

Memory size and program size are siblings. Program scan time is an illegitimate half-brother.

If you don't keep those optos isolated they'll breed like fruitflies.

huh?

HNC alert....?

HNC alert!!!

Click to expand...

That was my first thought Steve, but if you look at Bungles profile you will see he was born in 1967, a little bit too old to be doing his HNC, I might be wrong though , he could be a 'mature' student.

I tend to think that he has not been a railway signalling technician for very long and at the moment he is doing some courses at work to familiarise him with the equipment that he will be working on.

If I am wrong here Bungle, then perhaps you will accept my apologies and put me right.

Paul

Bob K

you posted just before my reply, I would have put in an explanation as to what a HNC is if I had read you post first.

Search this forum for HNC and you will find out what HNC stands for.

This is a British desease, but not to worry it cannot survive the trip across the pond!

Paul

Hi Bungle

Since you work on the railways, lets use that to answer your question.

Take a merry go round coal train. The train arrives at the colliery and is loaded with coal. A PLC is loaded with the status of the inputs.

The train goes to a power station and the coal is unloaded. This is the outputs being set on the PLC.

Each section of track equates to a section of a PLC program. Some parts of the track are fast and the driver can go at 75 MPH. Other parts are slow due to bends or points (switch tracks). Parts of the PLC program are the same some taking up very little time to complete others a lot longer. Some times the train may have to go onto a different section of track that is really slow. A PLC is the same, the program is transfered to a new part that is complex and takes longer to carry out. The scan time equates nicely to the journey time.

Lets now assume the memory is like the track. The greater the distance between the colliery and the power station the more sections of track we need. Hence the larger the program the more memory is taken.

Opto isolators are like the relays that one signal box connects to another. They both form an electrical barrier

Hope that helps in you understanding

Thanks barnschop, nice to recieve something amusing and constructive.

its much clearer now...

PLUCAS,

Thanks - did a search and found the answer. These must be the same folks asking the unitary, modular and rack configuration questions that I see sprinkled throughout the postings.

It does cross over here as Tech School, Associates degree (2 yr college) and Bachelor of Science (4 yr college degree).