Clarifying terms often used in automation

mikas_m · Aug 8, 2007

Hello to all,
I'd like to start a topic in which often used terms can be explained.
English is not my native language, so I have hard time understaning some terms.
For example, terms "latch" and "latching".
I have found that latch is basic memory unit that can hold one bit of information. Isn't that also a flip flop? What is the difference? Also, I have found that term latching can mean "set", while unlatching can be understood as "reset".
What really is a latch when we're talking about PLCs and binary logic?

Thanks

dogleg43 · Aug 8, 2007

Since PLC programming is relay logic, terms from the hardwired world have become part of the programming language. If you look in an industrical control catalog for relays you will find actual "latching" relays that operate like they do (in most) in PLC's. You probably won't find one named a "flip-flop" relay.

There are also the terms "seal-in" or "3-wire control". This refers to using a normal relay and using a contact from it to cause it to remain on after the start pulse is no longer "On".

Ken M · Aug 8, 2007

Hi Mikas

In English the original use of the word "latch" was a type of fastening on a door or gate. Some door fastenings allow the door to be closed by pushing it shut, and opened by simply reversing that and pulling it open. The same action, reversed, was used to control the door, push or pull. However a latch on a door meant that pushing it shut resulted in a bar falling in to place to keep the door shut. Pulling the door to open it wouldn't work as the bar held the door closed. You first have to release the latch (the bar) then pull the door.

This analogy has been used in many areas including automation. Latching a bit usually means constructing some logic to set it true, but the bit stays true even when the original cause is removed. You have to perform another action to unlatch it. this is the equivalent of removing the bar.



.....A........B................C

----] [-----] / [-------------( )

.........|

.....C.. |

----] [--/

'A' is the trigger which causes 'C' to be set. But even if 'A' then goes false, 'C' will be latched until 'B' is true.

Regards

Ken

jtn · Aug 8, 2007

Ken M said:
Hi Mikas

In English the original use of the word "latch" was a type of fastening on a door or gate. Some door fastenings allow the door to be closed by pushing it shut, and opened by simply reversing that and pulling it open. The same action, reversed, was used to control the door, push or pull. However a latch on a door meant that pushing it shut resulted in a bar falling in to place to keep the door shut. Pulling the door to open it wouldn't work as the bar held the door closed. You first have to release the latch (the bar) then pull the door.

This analogy has been used in many areas including automation. Latching a bit usually means constructing some logic to set it true, but the bit stays true even when the original cause is removed. You have to perform another action to unlatch it. this is the equivalent of removing the bar.

.....A........B................C
----] [-----] / [-------------( )
.........|
.....C.. |
----] [--/

'A' is the trigger which causes 'C' to be set. But even if 'A' then goes false, 'C' will be latched until 'B' is true.

Regards

Ken

I usually call that "sealing in" a bit. What I consider a latching bit is when the bit stays true by itself when the logic in front of it goes false, and stays that way until an unlatch command is given. In AB these command would be (L) for latching and (U) for unlatching. I personally try not to use (L) and (U) unless the particular application just begs for it, I find it harder to troubleshoot.

A flip flop would be when a block of logic causes a bit to go to one state when the logic comes on, then after the logic goes false and then comes true again it will cause the bit to go to the other state. The state of the bit alternates each time the logic in front of it goes true.

jacekd · Aug 8, 2007

jtn said:
I personally try not to use (L) and (U) unless ...

Please, not again

rsdoran · Aug 8, 2007

jacekd said:
Please, not again

I agree, any instructions a plc has should be used when appropriate.

As mention plc's were designed to replace relays and relay logic; so in relay logic you "sealed a latch in" and this language/concept transfered to ladder logic.

To quote Phil's Learn PLCs which is linked above:

[font=Arial, Helvetica, sans-serif][size=-1]
Picture the remote control for your TV. It has a button for ON and another for OFF. (mine does, anyway) When I push the ON button the TV turns on. When I push the OFF button the TV turns off. I don't have to keep pushing the ON button to keep the TV on. This would be the function of a latching instruction. [/size][/font]

[font=Arial, Helvetica, sans-serif][size=-1]The latch instruction is often called a SET or OTL (output latch). The unlatch instruction is often called a RES (reset), OUT (output unlatch) or RST (reset). [/size][/font]

AB the output unlatch is OTU. Anyway this concept did not derive from relay logic, but derived from the more advanced capabilities that semiconductors (electronics) offered us.

An example of that is the "FLIP-FLOP" http://en.wikipedia.org/wiki/Flip-flop_%28electronics%29

Clarifying terms often used in automation

mikas_m

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dogleg43

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Ken M

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jtn

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jacekd

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rsdoran

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