RSLogix 500 Unlatch question

Good day all. I have a question pertaining to an unlatch instruction. Unfortunately I'm not able to share the program however, I have attached three rungs that I have the question about. These inputs are from level sensor boards. The bit addresses B41:04 for example are push buttons on the HMI. From what I have learned, the low level switch input instructions are telling the processor to go look for 1's in the input bit boxes. The question I have is this: when it goes and looks for a "1" in the bit box and it sees a "1", it will make that instruction true which should cause the processor to go write a "1" in the output bit boxes. If the inputs do not have a "0" in their bit boxes the heat enable pushbuttons at the HMI (B41:05 for example) will not actuate. Could someone that understands this please give me some feedback, I'm not certain how this is supposed to work. Currently three of these heaters wont turn-on when pushing the button at the HMI. Thank you.

ceilingwalker said:

Good day all. I have a question pertaining to an unlatch instruction. Unfortunately I'm not able to share the program however, I have attached three rungs that I have the question about. These inputs are from level sensor boards. The bit addresses B41:04 for example are push buttons on the HMI. From what I have learned, the low level switch input instructions are telling the processor to go look for 1's in the input bit boxes. The question I have is this: when it goes and looks for a "1" in the bit box and it sees a "1", it will make that instruction true which should cause the processor to go write a "1" in the output bit boxes. If the inputs do not have a "0" in their bit boxes the heat enable pushbuttons at the HMI (B41:05 for example) will not actuate. Could someone that understands this please give me some feedback, I'm not certain how this is supposed to work. Currently three of these heaters wont turn-on when pushing the button at the HMI. Thank you.

Click to expand...

The last 2 (rungs that is) indicate that the condition is true which means that even if you press the button as soon as you release it it will automatically turn off by the logic posted.
One more thing. Even if the button doesn't get a "0" through the logic, you'll still need to press it or write a "1" to it to activate it.

Let's review the action of OTE, OTL and OTU instructions.

       Logic to output is TRUE      Logic to output is FALSE



OTE    Write a '1' to output box    Write a '0' to output box



OTL    Write a '1' to output box    DO NOTHING



OTU    Write a '0' to output box    DO NOTHING

According to your description the B41 locations are from an HMI request to turn on heaters. These rungs look for, possibly, an unsafe condition, power is on but there is a low level. So the request is turned back off by the PLC by unlatching (OTU) the request.

bernie_carlton said:

Let's review the action of OTE, OTL and OTU instructions.

 

Logic to output is TRUE Logic to output is FALSE

 

OTE Write a '1' to output box Write a '0' to output box

 

OTL Write a '1' to output box DO NOTHING

 

OTU Write a '0' to output box DO NOTHING

 

Click to expand...

After seeing this it dawned on me, this is exactly what I was taught at the PLC Bootcamp. Sorry, I forgot all about it until you jogged my memory. Thank you.

bernie_carlton said:

So the request is turned back off by the PLC by unlatching (OTU) the request.

Click to expand...

That's the other problem I have, there is no OTL. The circuit is sealed using overtemp and low level condition's, but there is no OTL.

When the HMI, in response to a button push presumably, writes to the bit it it like an OTL. As a troubleshooting tool, this is one of the presumptions if you see something used as a contact or turned off with no turn on instruction. Another possibility is a network communication. Another is that the bit is affected as part of a complete Word or File type instruction.

In the HMI's I have used a button which can turn ON a bit can be in 3 types. 'Momentary' which turns the bit on when you press the button then back off when you release the button, 'Set On, which can turn the bit on but not turn it off, and 'Toggle' which turns the bit on then off on alternate pushes.

From this logic the HMI button is probably not a 'Momentary' type. But we can't tell from this logic which of the other two it might be.

After seeing this it dawned on me, this is exactly what I was taught at the PLC Bootcamp. Sorry, I forgot all about it until you jogged my memory. Thank you.

Click to expand...

Bernie should have you all straightened out - but if you ever forget again, just look on the front cover of your book ... it's right there ...

we covered a LOT of stuff - and I know you can't remember it all - but you really need to go back through the book again from time-to-time (from cover to cover) for a refresher ... and the most basic stuff is right ON the cover ...

And don't forget that while it is common practice to have an OTL and OTU used in pairs, it is not required that you do so.

OG

This is an example of a place where extensive rung comments are appropriate. (Actually they're appropriate everywhere.) I understand that the poster did not write this and is just trying to understand. These are the people, including yourself 6 months from now, for whom the comments are made.

And I get tired of complaints about comments not being stored in PLCs. Just recognize that this is so and make appropriate plans for the full source of the program is available for anyone who has to access the PLC. Whether that means a networked centralized, controlled storage or a flash drive inside every cabinet. make and carry out the plans upon receiving the first PLC controlled machine.