Multiprog: 4. The Latch

Multiprog: 4. The Latch

1. Getting Started
2. The AND Structure
3. The OR Structure
4. The Latch
5. The One - Shot

Now You have a grasp on the basic principles  of ladder logic, you are ready to start combining them into bigger  structures that can do more interesting things. The first of which we  are going to show you is a latch. Image 4.1 shows an  example of this.  There are two new concepts at work here – The fact that the B  contact  is different, and the fact that X is both a contact, AND a coil in the   same rung of the ladder.

         Image 4.1 An OR Structure(Above) And A Latch(Below)

The B contact in the latch is what is known  as “normally  closed” or “NOT B” – The contacts you have seen so far  have all been normally closed contacts, these are closed when true, and  open  when false. This is because the default state for a contact when  unpowered is open - hence, normally open : Power can only flow when they  are energised, which will then close them.
However, normally closed contacts, allow power through as a default  state. They  block (Open) when the statement is true and the contact is  energised. In order to set a contact to normally  closed or normally  open, use the drop down menu on the contact dialog – See  image 1.14.

         Image 4.2 Setting a contact to normally closed

Now, back to the latch, you can see that  this reads as “X is  set, if ((A OR X) AND (NOT B)).” This has the  interesting property, that while  B is false, if A becomes true, then  goes false again, X will stay true (latch)  until B becomes true. If B  is already true, A will have no effect on the state  of X. If you are  familiar with electronics logic gates, this is principally similar to a  set-reset flip-flop (With Q' missing).

This sort of construct is extremely useful for catching short pulses  and  storing the fact that they have happened until they can be dealt  with, at which  point they can be reset. There are all sorts of other  uses as well, but that  list is as extensive as the imagination of the  wielder, so won’t be written here.

Hopefully you are beginning to see that a little logic can  go a long way, dont stop reading yet though! There are still a lot more  basics to cover before you are ready to rush off into the wide world of  PLC programming. The next one coming being tutorial five, the One -  Shot.