GX developer: what's the difference between a coil and a SET/RST?

As per the title. This might seem like a basic question to many, but I'm unclear about the difference between a coil ( ) and an application instruction [ ]?
See the example below. I want to switch on a fan via output Y2B3.
When I operate the rig and set M74=true , M420=true and I press the pushbutton X212, on line 1141 , Y2B3 momentarily gets SET. Does line 28 do a reset and "cancel out" the switching on of the fan?

Coil. OTE
Set OTL
Reset OTU.

Your confusion is not due to Set/Reset vs coil. It is due to the way the PLC operates. Even if Y2B3 represents a physical output point, it is not immediately updated when the rung executes. The results of the program scan get sent to the output modules in a block. All outputs on the module are updated from the data in that block.

Don't use the same Y2B3 tag for both auto and manual operation. Create a separate tag for auto operation at 28 and for manual operation at 1141 and 1170. Then combine them in another rung to turn on Y2B3.

Nova5 said:

Coil. OTE
Set OTL
Reset OTU.

Click to expand...

Thanks, would you mind writing these in full? I don't understand the initialisms.

Steve Bailey said:

Don't use the same Y2B3 tag for both auto and manual operation. Create a separate tag for auto operation at 28 and for manual operation at 1141 and 1170. Then combine them in another rung to turn on Y2B3.

Click to expand...

Thanks for this. I've had no formal training and I'm not sure what you mean by "tag".
Edit.... I think I understand you. You mean like a variable/parameter? (M123, M372, Mxxx, etc), right?

OTE output energized
OTL output latch
OTU output unlatch

Thanks for this. I've had no formal training and I'm not sure what you mean by "tag".

Click to expand...

"Tag" is somewhat of a generic term for a variable in the PLC.

Steve Bailey said:

"Tag" is somewhat of a generic term for a variable in the PLC.

Click to expand...

Yes, I edited my previous post when I realised what you meant.
Cheers again.

AlfredoQuintero said:

OTE output energized
OTL output latch
OTU output unlatch

Click to expand...

Thanks, AQ.

As an auxiliary to this topic, using a "SET" or Latch on a physical output is a questionable programming practice. Consider all potential safety hazards. Data can be maintained even if the PLC is dormant for a period of time. Just my .02!

A Coil (OTE) is only Active whilst the run of logic controlling it is 1(true). You should only have one rung of logic writing to a coil (OTE) in a project. - Think of a Switch that is only on as long as you press it.

An OTL sets that output to 1 (true) until you write another rung on logic to release (OTU) - Think of a light switch that clicks on until you click it off - Again to make logic simple you should only have one SET (OTL) and one UNLATCH (OTU) per output in a program.


AS cidcHASE says OTL and OTU to control everything is poor practice

To expound on what others have said...

In general, you have two ways to "hold" an output.

You can have a contact set with an OTE and then reference the output to a "contact set" under the set that energizes the output. This is called a "seal around" by old guys (web search if needed). If the controller loses power and starts back up, this rung will be FALSE (output will NOT energize) when the machine is started back up.

If your contact energizes a LATCHED output (OTL in Rockwell, SET in Mitsubishi which I think yoiu are using), that output will remain on until it is unlatched (OTU in Rockwell, RESET in Mitsu), even if the controller is powered down and back up again.

That is the main difference in approaches. Do you need to "remember" the output's last state if the controller loses power / is shut down? If yes, then latch.

I used to use latches more than I do now (was an integrator) and I always used a first-scan routine to clear latches that needed it. If those outputs just had their close-state sealed around by themselves, they would not need any first-scan setup, for the most part.

Thanks for all the replies.
The fans are new to our system and I was using SET/RST for the fans because that's what is used for most of the other motors/pumps. Having said that, some pumps are controlled using coils only, like this one. It's the only line in the code where this pump is controlled, based on water level.

Mas01 said:

Thanks for all the replies.
The fans are new to our system and I was using SET/RST for the fans because that's what is used for most of the other motors/pumps. Having said that, some pumps are controlled using coils only, like this one. It's the only line in the code where this pump is controlled, based on water level.

Click to expand...

that is the start/stop cursor pattern.

that is functionally equivalent to a set/reset pair*, with the exception that it will always return to 0 on a mode valance or power cycle.

*
[> D353 K800] => [SET Y2D5]
and
[<= D353 K240] => [RESET Y2D5]

drbitboy said:

that is the start/stop cursor pattern.

that is functionally equivalent to a set/reset pair*, with the exception that it will always return to 0 on a mode valance or power cycle.

*
[> D353 K800] => [SET Y2D5]
and
[<= D353 K240] => [RESET Y2D5]

Click to expand...

Yes, it's a neat piece of code. Widely used, I imagine.

In my original post, the operator wants to be able to control the fans in one of two ways: manually (fans on/off whenever they please) and automatically based on water temperature - if it gets to integer 300 (30 celsius) turn it on, when it's below 30c turn it off.