quick question

i have to do a paper for school on plc's and the question i am to answer is "what is a OSR, instruction and where is it used?"
if some one could help or tell me where to get the info that would be great. "thanks" in advance for any help.

OSR = One Shot Relay

A PLC operates on a time base (schedule) called a "scan".

During the scan the PLC reads the Inputs, then it does the calculations based on the current state of the Inputs, the Outputs, and other things called "internal relays". Those "internal relays" are virtual. Then the PLC adjusts the Outputs according to the results of the calculations.

An OSR, when included in a program, is activated when a particular "cause" occurs. Now here is the crux of the OSR... it is activated for one, and only one, scan... even if the "cause" is present for many scans.

In order to activate that particular OSR again, the "cause" has to go away and then return again.

When activated, an OSR is a "trigger" for a subsequent action. When activated by the occurrance of the initial "cause", the OSR becomes (relatively speaking) the "cause" of action for some subsequent activity.

In some PLCs the "One Shot" is used only as a control element in the code. In that case, the occurrance of the "cause", before the "One Shot" element, activates the One Shot, which then passes a "Go Signal" to the following code in that rung (rung = line of code)... for one, and only one, scan. That One Shot can be used to turn on an internal relay which can then be used anywhere else in the program for any number of subsequent activities.

In others, the OSR is an Internal Relay. In that case, the OSR Internal Relay can be used in any number of sections of the program to "cause" any number of subsequent actions.

The difference between the two methods is... nil.

But, since the OSR is activated for only one scan, the subsequent activity is liable to occur only once - during that scan, unless some kind of latching occurs to keep the particular activity going until some "cause to stop" occurs.

Of course, in many cases, it is expected that the particular activity occur once, and only once, when that particular intial "cause" occurs. In that case, no latching should occur.

As an example, when a particular event occurs you might want to perform a certain mathematical calculation... once... and only once... but only when the event initially occurs.

Make any sense?

In some PLCs this is also defined as a One Shot Rising (OSR). Meaning a leading edge One Shot on the the Off to On transition.

HLeap... You are correct. One Shot Rising.