Reciprocating timers are defined as timing functions where the output of one timer is used to reset the input of a second timer, each reseting the other. These types of timers are used in situations where a constant cycling of an output is required. For example, if a flashing light is required in the event of a control system failure, a program with reciprocating timers could be used to create the flashing output function.

When it comes to AB PLCs, things like that are probably better controlled using the free running counter in the status table (S2:4). Each bit in the word turns on and off at a different rate, but the rates are related in that each bit turns on and off half as often as the bit below it. I find that S2:4/6 turns on and off at just the right rate to flash a warning light, all that's needed is to put an XIC instruction addressed to a bit in that word in series with the OTE that turns the light on, and it will flash. Simple...

Excellent James, I printed off your comment I am going to put that into the simulator and try it! Keep your comment coming, I have 10 more modules left in my PLC class. Next stop "counters".

James could you explain XIC at 1st grade level.. I am looking at a circuit that is confusing me right now...I think a better discription of the XIC function might help. I understand that it scans for a open condition and will close and open if closed if XIO..

I think my problem is that im looking at XIC as a N/C contact ie:relay logic. Ive been in relay logic for years so when I look at it, in a ladder logic diagram Im looking at it with relay logic eyes..Its going to be a habit I need to break!?.

The best way to understand XIC and XIO is to understand that they look at
a BIT.

XIC (-] [-) is TRUE if the bit is a 1 and FALSE if the bit is a 0

XIO (-]/[-) is FALSE if the bit is a 1 and TRUE if the bit is a 0