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Old Thu 07 August 2008, 09:32
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Ladder Logic

Sometimes it is easier to use some basic visual tools to help understand how a circuit works. Some of the questions that keep popping up on this forum about electronics show that, for most people, building the electronics is the most intimidating part of building a CNC router.

A "tool" can might help is Ladder Logic. Although Ladder Logic can be a very sophisticated method of describing a project, at its most simple level, it can be used to show the big picture.

If you look at the various levels of designing an electronic device, Ladder Logic would be the first, most basic level. After Ladder Logic would be the schematic. After the schematic would be the board layout. After the board layout would be the computer programming.

Instead of posting some Ladder Logic diagrams, which would be incomplete and confusing, just Google "Ladder Logic" and read some of the basic references. Wikipedia has a nice introduction to Ladder Logic.

After you've looked at some of the stuff on the Internet, you'll be ready to draw up some simple diagrams. Hopefully, those diagrams will help you understand how your controller works.

In one of the test controllers that I have right next to me, I have 120VAC circuits, 35VDC circuits, 24VDC circuits, and 9VAC circuits. That means that I would draw at least four ladders. The rails of the 120VAC ladder would have two rails, just like a physical ladder. The left rail would be the 120VAC HOT circuit. The right rail would be the 120VAC NEUTRAL circuit. Each device that required 120VAC for its operation would be a rung on that ladder. The same general design would be used for the other three ladders.

When the Ladder Logic design was finished, it would show how the E-stop button controlled everything in the control box. In fact, it would show each device in the control box as a rung of a ladder and it would show how each device was "controlled" (turned ON/OFF) by any control device on the rung of a Ladder.

When I'm called in to trouble-shoot a piece of equipment, the first documentation that I ask for is the Ladder Logic diagram. That shows me the basic layout of the electronics. Then I ask for the schematic which gives me the details of every component in the electronics.

One thing to keep in mind, as you read some of the Internet information about Ladder Logic, is that PLC (Programmable Logic Controllers) are used in industry to control many of the processes. Much of the information on the Internet assume that you're going to be using PLC modules. Just skim past that information. It is interesting to some of us, but not very applicable to a CNC router.
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