Timer Enable function

[Prayder]

Does the timer enable bit stay enable once the timer is done timing?

 

[Ken Roach]

I'm going to guess that you're talking about Allen-Bradley logic controllers.

Yes, the /EN bit stays true as long as the pre-conditions before the instruction on the rung are true. This is true for the TON, TOF, and RTO timer instructions in relay ladder logic.

The A-B operating system has a "timer timing" bit that goes true when the timer is enabled, then goes false once the timer is complete, no matter what the preconditions before the instruction.

There's a good timing chart for the TON instruction in the Logix 5000 General Instruction Set Reference, in Chapter 3, page 117.

 

[Prayder]

Thanks... I am having trouble with a powerflex 525 drive. Right now I am using dueling timers to keep a pulse going to the drive when it is time to start because after having stopped the drive doesnt always see the start command for some reason.. thanks again.

 

[Ken Roach]

Don't futz with timers for this purpose. Do it right with discrete logic.

Have you read my advice on not inverting the START bit to run the STOP bit ? I've written extensively on the topic over the years.

What version of firmware do you have on the drive ? There was an early firmware revision that would sometimes ignore even properly configured START commands (1.003) and then another firmware revision that would sometimes incorrectly assert a START command after the STOP was removed (1.005).

The current firmware for the PowerFlex 525 is version 4.001, available from the RA Product Compatibility and Download website.

If you're using the Ready bit in your logic, you'll want to review the 3.001 release notes too.

 

[Prayder]

We are using the current version of firmware. I apologize still kind of new to the logic aspect. What do you mean when you say discrete logic and could you provide me some detail as to "not inverting the start bit to stop the run bit"?

 

[Prayder]

when powered.... is a vfd always either in a start or stop state? If I use the tt bit of a timer to stop a motor, the stop output will turn off once the tt bit is done. but will it go back to a start state or will it have to have the output to start before it actually starts?

 

[Doug-P]

Many of your questions are answered in the instruction set reference for whichever processor you're using.