MIcroLogix 1400 and RS500: OSR & OTL

Saulo35 · Jan 17, 2012

Hi,

I am reviewing somebody else's logic and I find the following:

|----(some logic)----[OSR StorageBit OutputBit]-----|
|----[XIC OutputBit]------------------[OTL SomeOtherBit]----|

To me, the OSR instruction is unnecessary. The following logic should produce the same results:

|----(some logic)----[OTL SomeOtherBit]----|

This is a MicroLogix 1400 Series B with RS500 8.30.01.

Your thoughts?

Thank you,

Saulo35

dcooper33 · Jan 17, 2012

Saulo35 said:
Hi,

I am reviewing somebody else's logic and I find the following:

|----(some logic)----[OSR StorageBit OutputBit]-----|
|----[XIC OutputBit]------------------[OTL SomeOtherBit]----|

To me, the OSR instruction is unnecessary. The following logic should produce the same results:

|----(some logic)----[OTL SomeOtherBit]----|

This is a MicroLogix 1400 Series B with RS500 8.30.01.

There is a substantial difference. Without the OSR instruction, as long as (some logic) is true, (SomeOtherBit) will latch every scan. This means that even if somewhere else in the program a OTU (SomeOtherBit) is executed, the bit will relatch on the following scan (or just stay latched, if the OTL follows rather than precedes the OTU).

The purpose of a OSR (or an ONS on other Logix platforms) is to have the truth of the output only last for one program scan, regardless of the input conditions. The OSR (One-Shot Rising) output bit will be true for ONE scan on a FALSE-to-TRUE transition of the preceding rung logic. There is also a OSF (One-Shot Falling) instruction which will turn on an output for ONE scan on a TRUE-to-FALSE transition.

Two good examples of uses for a OSR:
1. After an XIC of a PB or HMI input which performs math on an analog value. You wouldn't want the incrementing or decrementing to continue the entire time the button is pressed, so you use the OSR to ensure the logic only executes once per button press.
2. When you have an alarm that turns on an audible buzzer. Whenthe alarm condition is scanned by the PLC as true, you can use a OSR after the condition prior to latching a bit that turns on the buzzer. That way, the alarm can be acknowledged and silence the buzzer, even though the alarm still exists. (Similarly, I use OSR's with "Alarm Ack" buttons so operators can't defeat alarms by wedging something in to hold the ack button "on")

Anyway, that's my $.02 on one-shots. That and another 73 cents may even buy you a soda.

Welcome to the Forum!
Cheers,
Dustin

Saulo35 · Jun 12, 2012

Thank you very much Dustin and I apologize for my delayed reply. I was somehow convinced that the OTL instruction had an ONS or OSR "built-in" at its input. If that were true, then even if an OTU were executed, SomeOtherBit would not be latched again until (some logic) transitioned to FALSE and then to TRUE again.

Following your advice, I did not eliminate the OSR in the reviewed program.

Best regards,

Saulo35

MIcroLogix 1400 and RS500: OSR & OTL

Saulo35

Member

dcooper33

Lifetime Supporting Member + Moderator

Saulo35

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