Allen Bradley Timers

Does Allen Bradley have a certificate that proves the built in timer accuracy and repeat ability
?

That's a good question...

Another related question I've been wondering, along the same lines, is if / how is affected with increasing scan time.

I've seen programs when a TON set to time 1 second intervals was not in fact exactly 1 second, but slightly off.

I can't imagine any kind of certificate. These aren't laboratory grade timers with traceability back to a atomic standard clock.

If you re-trigger a timer from a NC of it's DN bit then you'll get up to 1 scan time more than the preset each time.

A trick to get better long term accuracy is, instead of restarting the timer at the DN bit, set the preset of the timer to more than the time you want. Compare in a scan for GEQ your desired time. If so subtract your desired time from the .ACC time. You'll get a more accurate pulse train in the long term.

Of course you can also use the free-running clock but it may not have a convenient time bit available.

The real time clocks are not too bad in the SLC and PLC-5. I think you will see about 10-30 seconds variance per week. I tried a home-made real time clock in a Micrologix1000 using the best tricks I know and could not keep the accuracy to within about 10 minutes per week.

The timer is a computer instruction. It is not a device. Its accuracy is entirely dependent on how frequently and how consistently the computer instruction is executed - so that means accuracy is affected by you the programmer and the PLC scan time.

Alaric said:

The timer is a computer instruction. It is not a device. Its accuracy is entirely dependent on how frequently and how consistently the computer instruction is executed - so that means accuracy is affected by you the programmer and the PLC scan time.

Click to expand...

Hit the nail on the head Alaric.

A-B timer instructions are not Timers at all. Basically they are "compute" boxes - when they
are scanned (variation here), they calculate how much time has elapsed since they were last scanned (it stores a reference internally each time it is scanned), by comparing to the internal clock.

It then updates the Accumulator value, stores the internal reference, and then checks to see if the accumulator is greater than, or equal to, the preset value.

If it is, it resets the TT flag, and sets the DN flag (I'll not confuse the issue by describing the TOF instruction).