L-75 Processor Major Fault

[prakasha]

Hi Guys

Today we went through the worst situation that PLC went to major fault Mode due to Trap by one of the Timer with -ve Value Moved into Preset.

What is the best way to prevent or avoid this kind of acts while implement or developing the program. I know there is option Fault Handler Program can be written but is it possible to prevent plc being trapped by loop by writing the program in fault handler. How can we avoid this kind errors while developing the program. Plz advice me to come out from this

 

[Operaghost]

It is difficult to prevent bad programming. I've used this type of logic to create faults when I was trying to demonstrate faults, but not something I would have in an actual running application. The timer won't allow you to type in a negative number, so somewhere a negative value is being written to the timer. And there is really no way to prevent that.

Sometimes this can happen when using an HMI. If an operator accidentally writes a negative number directly to a timer preset. Ideally you would write to a separate DINT tag where you could test to see if the value falls within an acceptable range.

Since I cannot prevent this, I would add either a Fault Routine or use the Controller Fault Handler. You can add logic (GSV instruction) there to capture the fault code and if the code matches the one you are getting, you can trigger logic (SSV instruction) to clear the fault. But even if you do this, it doesn't fix that there is still a negative number in the timer preset. And of course that can cause problems with the process.

So as a general policy, don't allow HMIs to write directly to timer presets. If you have presets that should never change, you can add logic to move the setpoint into the preset.

OG

 

[JeremyM]

You could try validating the PRE/ACC values just prior to the timer call:

GEQ Timer.PRE 0 GEQ Timer.ACC 0 TON

Setting the TIMER tag's external access to Read Only can prevent something like an HMI from changing these values.

 

[prakasha]

Yes I was trying in the same way to prevent the -ve value being moved to the timer. I thought there could be a specific solution for this. thank you for your valuable suggestions.

 

[5618]

I normally use if Value < 0 then MOV 0 to Value before sending any Value to a Timer.PRE. I have sometimes used ABS (absolute)Value to Value to eliminate any potential negative and keep a value. If possible, HMI value entry should limit entry to above zero in the HMI application. Even if there’s no way to get a negative through normal operation, I still want to protect against any fat-finger from someone working online.

You could get fancier and add another register, confirm good value, revert and alert if bad, and then send to .PRE if good.