student of plc's

Hi, I am new to this site and am in school learning plc's.
we are using rslogix software as a tutorial and love it so far. we are working on a program for a garage door opener. in the instructions we should use otl and otu with binary or intergers. I have have the otl in place and the door goes up like it should, but I cannot get the rung to unlatch. I 'm thinking it should have something to do with the binary or intergers am I correct in my thinking?? or am i barking up the wrong tree?? any input would be appreciated but I'm not looking for the answer just want to know if my thinking is correct.

thanks

Welcome to the fray.

I assume you have used the OTE instruction already. So you know that the OTE assumes the logical result of all the conditions in the rung. If the rungs logical result is TRUE (on, 1, active, take your pick) the OTE sets it's bit to 1. If the result is FALSE (off, 0, inactive) the OTE sets the bit to zero.

An OTL is a bit different. Think of it as the ON half of an OTE. If its rung is true the OTL sets its bit to 1. However, if the rung is false, the OTL does nothing. This infers that there must be another instruction paired with the OTL that handles the set to zero part.

I hope this is enough of a hint to get you going. If not, let us know.

Keith

Ok what I get from that is I need another branch in parallel with the otl. I want the the otl to go false when LS 1 opens so if I put a xio in series with the otu and have that in parallel with the otl it will set the bit back to 0 or off. the xio is the input for LS 1. does this sound anywhere near correct to you??

thanks for your input
Eric

what does the green mean? ...

Greetings Eric,

let me try to guess what’s confusing you ...




the input in the “unlatch” rung is TRUE - because it’s green ... so you’re wondering: “why doesn’t the OTU go active and light up green too?” ... if that’s what’s confusing you, then here’s the solution ...

there is only ONE bit/box in the processor with the address “O:2/0” ... the bit/box is shown (and its address is noted) in the Data File display window ... right now there is a zero stored in that bit/box ... the reason that the OTU isn’t green, is simply because the bit/box does not have a one stored in it ...

the reason that there is a one in the bit/box is because (get ready for this) - because the OTU is indeed active ... in fact, the OTU is actually writing a zero into the bit/box each and every time that the processor scans the OTU rung and finds it true ...


secret handshake: green around an OUTPUT-type ladder logic instruction does NOT mean that the instruction is “active” - instead it means that there is a one stored in the bit/box ...

try this experiment to nail it all down ... make sure that BOTH the OTL and the OTU rungs are false ... then go to the Data File and manually enter a one in the bit/box ... BOTH the OTL and the OTU will highlight green ... NOT because the OTL and/or the OTU are “active” - but rather because there is a one stored in the bit/box ... now put a zero in the bit/box ... the green around BOTH the OTL and the OTU will go away ...

does this help? ...

final note: has your class discussed the possible safety considerations of using “latches” to control the garage door? ... hint: suppose that the power to the PLC goes off (even for a second) ... what will happen when the power comes back on again? ... do you want the garage door to start moving again immediately? ... or do you want the operator to have to push the button again to get things started? ... major idea: there is a big difference between the ways that “seal-in” constructions and “latch/unlatch” constructions act when the PLC first powers up ...



and welcome to the forum ...

Ron Beaufort said:

final note: has your class discussed the possible safety considerations of using “latches” to control the garage door? ... hint: suppose that the power to the PLC goes off (even for a second) ... what will happen when the power comes back on again? ... do you want the garage door to start moving again immediately? ... or do you want the operator to have to push the button again to get things started? ... major idea: there is a big difference between the ways that “seal-in” constructions and “latch/unlatch” constructions act when the PLC first powers up ...

Click to expand...

My experience with PLC's shows that both novice and expert can misapply latches. This is also true of "one-shots". You really must think of all aspects of the machine/process operation. Getting the machine to work is easy. Getting it to work, and fail in a known, safe state can be the tricky part. Not to mention recovering from emergency stops, crashes, and full losses of power.

Good one, Ron

thanks for the tip, it helped alot I almost have the circiut working the way it should. I latches and unlatches when told. It is a simple program using a garage door. I now have to figure out how to make the circiut unlatch when the door is moving, like when the door is going down if I press the up button the the door stops. with my program it stops going down and goes up. gotta go do some program changes, again thanks guys

Eric