Ken Roach
Lifetime Supporting Member + Moderator
You're getting it exactly.
Let's say your system logic to start the drive becomes true, and the controller solves the ladder rung with the START bit on it.
But then, in the instant before the next rung is executed, the I/O connection Requested Packet Interval times out and the Logix operating system grabs the current status of the Drive:O.Command word (all sixteen bits) and sends it to the drive.
The very next rung, which would have set the STOP bit to False, has not executed when the whole Output.Command word was copied out to the communications subsystem.
So the output word includes both the .START and the .STOP bit as true.
In the A-B world, STOP commands always win. It's that way on terminal strips, and over networks, and in every other instance where both Start and Stop can be present at the same time.
The classic indication of this network problem is when an operator tells you "I've got to punch the Start button several times before it takes effect". You can even see that in the habits of operators hammering on a Start button even long after the problem has been solved.
Let's say your system logic to start the drive becomes true, and the controller solves the ladder rung with the START bit on it.
But then, in the instant before the next rung is executed, the I/O connection Requested Packet Interval times out and the Logix operating system grabs the current status of the Drive:O.Command word (all sixteen bits) and sends it to the drive.
The very next rung, which would have set the STOP bit to False, has not executed when the whole Output.Command word was copied out to the communications subsystem.
So the output word includes both the .START and the .STOP bit as true.
In the A-B world, STOP commands always win. It's that way on terminal strips, and over networks, and in every other instance where both Start and Stop can be present at the same time.
The classic indication of this network problem is when an operator tells you "I've got to punch the Start button several times before it takes effect". You can even see that in the habits of operators hammering on a Start button even long after the problem has been solved.