Need a school question answered

In the text by Thomas Hughes "Programmable Controllers 4th edition"
When talking about PLC's it states the following:

" Application memory size can be estimated by multiplying the number of I/O points by 20 words of memory. "

My question is... why do you multiply by 20 words of memory?

If anyone knows the answer or can point me in the right direction that would be great. My Professor made it a special assignment for me to find out why. no one in the class knew and neither did he.

Thank you very much for your help.

I don't know for sure but my guess is that it is a "Rule of Thumb" type of estimation. In the old days you pay dearly for memory size on PLCs so it was important not to buy more than you need. I haven't seen any problem with memory size these days unless people insist on putting recipe information in a PLC.

-They got Professor teaching PLC these days?! wow...

Its a Process Control class dealing mainly with Ladder logic. Since PLC's consist of Ladder Logic as you obviously know, it is being touched upon.

That must be a rule of thumb derived from "typical" ladder logic.

Each I/O point is going to have a "typical" amount of instructions and holding registers associated with its function. Some are going to have a simple latch circuit, and some are going to have a large number of storage registers and math instructions.

Obviously the kind of programming you are doing will change that; if you have a sortation system or data logging system, you might use very little I/O but a lot of data memory.

This rule of thumb will apply only to older PLC styles; you couldn't apply this to a ControlLogix because of the very different (and much larger) memory structure.

I have been involved with plc's since 1988, and have never heard this rule of thumb before.

I can't say if it is accurate or not.

When using a 9030 rack system, I like to get the most powerful CPU the customer is will to pay for.

I have never run out of memory, but have heard of it happening.

What is the copyright of your book?

regards.....casey

I am with you kc9ih, but I have run out of memory before on a 9030 system. 87) I was not the original programmer, but I was called alot to modify the program to do different things.

David

That is not a mathematically derived formula, but simply a general guideline or rule of thumb. The author eaither based it on his own experience or anecdotal evidence from others.

Take this value with several grains of salt. For example, we do a lot of math in our programs and have a lot of analog I/O and a lot of networking to small PLCs. The 20 words time number of I/O points won't be close for the master but is too high for the small PLCs.

On the other hand, if you are doing a few time delays and a couple of sequence interlocks it will be way too much memory.

As Ken pointed out, the platform makes a big difference. I have done some large systems in an AutomationDirect.com with 14.8k and had lots of spare room. A similar system of equal complexity on a Siemens S7-300 ran out of memory on a 48k platform and we had to upgrade.

The author has some good credentials!

Here is a link to the book. The PDF files on the text and author are attached. All good intentions considered, a rule of thumb is probably as good a reason for this number as there is.