I am currently working on a project for a company without a drafting department. The person I am working with is trying to show connection points to and from the PLC within a wiring diagram. He is concerned with doing it correctly as he is going to be sending off to have it re-drawn in Autocad. Any suggestions (An example diagram would be great!).

Thank you,
Peter J. Arthur

 

At 12:16 06/11/00 -0500, Peter J. Arthur wrote:
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>I am currently working on a project for a company without a drafting
>department. The person I am working with is trying to show connection
>points to and from the PLC within a wiring diagram. He is concerned with
>doing it correctly as he is going to be sending off to have it re-drawn in
>Autocad. Any suggestions (An example diagram would be great!).
<clip>

In answering your question, I will make a few assumptions. The first is that you are trying to draw the PLC in an electrical ladder diagram, and you are familiar with drawing relay logic ladders. The second is that you are using sinking inputs and sourcing outputs. The third is that you are doing 'A' size schematics in JIC format. I will answer your question as best I can, but I have a different suggestion for you at the end of this letter.
I hope am interpreting your request correctly.

The details will depend upon the exact model of PLC you are using, but generally I see the following:

1) Power is required to run the PLC CPU. This is usually separate from powering the I/O. Draw the CPU as a small box with terminals on a page with the various other power related devices. The power may be AC, or DC depending upon the model of PLC. Power drawings are usually at the front of the drawing set.

2) Draw each input byte or word in a long narrow box vertically oriented on the right side of the page just to the left of your right power
bar. Leave room on the right side of the drawing to describe each input. Draw in the input terminals, and also the power and common terminals
associated with that input group. Label each terminal, and draw in your input devices.
The end result will look like an electrical relay ladder, except one end of each of the rungs will terminate in the input card.

3) Repeat the above for each output byte or word, except the output card goes on the left side of the drawing (power flows left to right).

4) Don't split bytes to fill up a page better. If you use one byte per page you get more room to draw in and can generally do a more detailed drawing. If you use one word per page, you use less paper.

5) Arrange the pages in order of I/O address. Some people like to show all the inputs followed by all the outputs. I generally like to show them in the physical order of installation (i.e. if a module has both inputs and outputs, I show them on consecutive pages).

6) Don't try to squeeze too much onto too few pages. PLC I/O generally constitutes the bulk of the drawing set for most machines.

7) If you are using a very small PLC with only a few I/O (half a dozen in or out) and the rest of the drawing is fairly simple it is
sometimes better to disregard everthing I have said above and draw it all on one sheet ('B' size if necessary). This can be a lot of work though if you are to avoid turning the wires into a tangle.
Just remember the people who will be reading this though. You are drawing an electrical schematic, not an electronic circuit. There are few things worse than trying to read a drawing done by someone who thinks he is dealing with chips and diodes rather than motors and PLCs.


If you are paying someone else to re-draw your schematics in Autocad, I would suggest sending them to someone who is familiar with doing
this sort of drawing. Since PLC I/O is very repetative, it can be drawn very quickly once you have a general "framework" you can re-use. Someone who has done this sort of drawing before can likely do it much more quickly (and therefore cheaply) than a general mechanical draftsman. They are also less likely to make mistakes and you will get a nicer package in the end. Most
people who design control circuits do their own drawings, so you might like to contact a small local company for this. They may be able to do some of the other work you need for this project as well.


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Michael Griffin
London, Ont. Canada
[email protected]
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