What is a watch dog timer?

[darrentpi]

I keep hearing about watch dog timers or clock in some of the softwares, but because I am a newbe, I'm not understanding what it is or does!
Just curious, always like to learn
DM

 

[rsdoran]

This is used to view/edit the watch dog time-out value for the current PLC. The watchdog time-out value is used to halt the system if the program scan time ever exceeds its value. The program can reset the watchdog timer for the current scan in certain situations such as FOR-NEXT loops by using the RSTWT instruction.

Thats from the help files...basically the plc can have a SET scan time and this will halt the plc if it exceeds that time. Lets say you have an operation that MUST be completed within 10ms, you set this timer to 10ms, if it exceeds that value the plc stops.

 

[93lt1]

Ron describes the scan time watchdog timer which is built into most PLC processors.

I've also seen people use the term "watchdog timer" for monitoring a process or sequence to ensure that the process or sequence does not get "stuck". In other words, if a sequence should be completed in 35 seconds, the watchdog timer is enabled the entire time the sequence is active, and times out after 45 seconds and promts the proper fault handling logic.

 

[OkiePC]

I've worked with an older PLC that had a hardware watchdog timer with relay contacts wired into the machine e-stop circuit. It was on a card installed in the furthest remote rack of the system. The ISSC PLC would electrically reset this timer after each logic scan. If we wanted to change the preset, we would get our electronics guru involved, and he would change a capacitor on the card itself! If the controller froze, the relay would open. There are still servo systems that have this arrangement.

 

[PLucas]

DM


Have a read of this introduction to watchdog timers it might be of some help to you.

Paul

 

[stevez]

You guys think you all know what a "watch dog timer" is.You are all wrong.A"watch dog timer" is a timer that lets your dog know that it is time to watch.
Have a good weekend eveyone.
Steve

 

[Tom Jenkins]

The PLC scan watchdog timer is used to make sure the PLC isn't locked up or stuck in an infinite loop. It is handled by the CPU firmware and essentially sets a bit or an alarm message or allows a fault bit to come on if the scan doesn't cycle back to the beginning of the program within a specified number of msecs.

Watchdog timers are very commonly used to monitor the communications between PLCs and between SCADA software and the PLCs. The intent is to let you know when the communications drops out to a given PLC, so you know the data is bad and to alert you to a problem that needs fixing.

There are lots of ways of doing this. Most common is to use a bit that turns on for set time (usally a second or two) and then turns off for the set time. The master PLC or SCADA reads this bit, and if it doesn't change within the specified time, say 10 seconds, an alarm is set indicating the communications are failed.

I prefer to just move the real time clock seconds into a register read by the master. If the same value occurs in two or three consecutive read operations I set the comm alarm.

 

[OkiePC]

Most common is to use a bit that turns on for set time (usally a second or two) and then turns off for the set time. The master PLC or SCADA reads this bit, and if it doesn't change within the specified time, say 10 seconds, an alarm is set indicating the communications are failed.

We call this a "Heartbeat", not a watch dog...

 

[leitmotif]

Dog timer

You guys think you all know what a "watch dog timer" is.You are all wrong.A"watch dog timer" is a timer that lets your dog know that it is time to watch.
Have a good weekend eveyone.
Steve

The problem with that is there is no incentive for the dog to do the watching. Yeah the alarm goes off, dog looks, rolls over and goes back to sleep if no one watching.

Obviously the dog needs incentive.
SO
the timer keeps track of the dogs watching efforts. After X amount of time (or total time making rounds etc etc) he gets a goody dropped in his bowl.

Dan Bentler