PLC FAIL Detetction

Hi experts!
I need a example if PLC fails the output energize, I see an example of rslogix 5000 in which watchdog timer is used to detect if PLC fail its shutdown the system. In GE Funac 90-30 CPU 352 how can i used, I see there are many FATAL staus Bit but i can not understand which one i used? Plz guide me
Regards

What are you trying to detect and what are you using to detect it?

Hello SIr;
Have a great day. I am busy in other project so can not touch with the site. Now i am returning and finalize the logic after playing with GE9030.
I want to detect if PLC halt or programme execution faultor watch dog timer(I understand from that if ur programme gets a more time than the execuation/sweap time then it energise or de enrgise may be i am wrong) how i used?
Actually i want the failsafe system regarding to the PLC. which bit i used who makes my system safe no damage to the engine or output devices.
Thanks

When the PLC stops executing its program, all outputs are turned off.

What external device are you planning to use to monitor the PLC's internal fault bits?

Sir;
I control the output devices like actuator, solonieds relays etc,
I want that if PLC active i energise a master relay which contacts control the critical safties. If PLC fail by any reason the master realy de energise and all the critical outputs are off . In other words i add a backup saftey for the engine which include ESD, Fire Detection, PLC FAIL.
One of my friend send me drawings in which he used to detect PLC FAIL as follows.
Module IC693APU300 at terminal 2 and 3 connected to the speed measurment probe.
At terminal 13 apply -ve of the supply.
at 14 apply +ve supply. at terminal 17 and 18 connect electromagnetic relays for high speed
From terminal 19 he detect PLC FAILURE signal. I can not under stand how he make logic?
Even PLC ststus bits available i want to use these instead of that logic. Thats why i ask which bit i use to just detect if PLC fail by means of execuation or internal hardware failure.
Also want to check the condition of backup battery in the PLC?
Regards

As steve says.. all outputs go off on fault

Just output always to a relay. When it goes off....fault.

Thanks;
Can u explain which status bit i use from the PLC table to detect it. like i use ALW_ON for read the data contineouly.
Also check the battery status?
Regards

DO I understand that you want to check if a PLC is working and then use the PLC as a safety device.
This is not a good idea.
But,
if uou want to create a watchdog system - a better method is via an external timer hard wired from a PLC output.
The external timer is electrically held in and timing out to shut down power.
The PLC periodically resets the timer via one output.
That output needs to energize to reset the timer.
This will check the output state does change from open to closed.
To ensure the output is working you need to perform that the output physically goes from closed to open as well.
And No you should not use the same timer.

Principle of safety:
What checks the device being checked -

stick with Safety relays - it is easier.

robo77 said:

As steve says.. all outputs go off on fault

Just output always to a relay. When it goes off....fault.

Click to expand...

Use your always on bit to energise an output which is connected to a relay. This relay is de-energised when the plc fails. Use its N/O contacts to do what ever you intent to, beacon, siren.....

But carful, don't use it for safety related purpose

the issue with checking a PLC output is -
IT HAS TWO FAIL STATES NOT ONE...

If a wire breaks the output is permanently OFF.
If the OUTPUT Internally SHORTS
The Output remains permanently ON
Dont forget the chance of forcing output states as well.

so you need to Check Both Modes constantly.

This also only checks that PLC OUTPUT an NO OTHER.

Assume that output never fails -

But the output controlling the danger does.
HOW DO YOU KNOW OR TELL IT IS FAULTY

iant said:

DO I understand that you want to check if a PLC is working and then use the PLC as a safety device.
This is not a good idea.
But,
if uou want to create a watchdog system - a better method is via an external timer hard wired from a PLC output.
The external timer is electrically held in and timing out to shut down power.
The PLC periodically resets the timer via one output.
That output needs to energize to reset the timer.
This will check the output state does change from open to closed.
To ensure the output is working you need to perform that the output physically goes from closed to open as well.
And No you should not use the same timer.

Principle of safety:
What checks the device being checked -

stick with Safety relays - it is easier.

Click to expand...

This is what we do.
We use an external timer, specifically a "watchdog" function timer. The timer needs external 24vdc to operate.
The timer resets only after the trigger input is turned ON and then OFF within the set time of the timer.

The PLC pulses a discrete output attached to the trigger input of the timer, say at a 2 sec ON, 2 sec OFF cycle.
The watchdog timer is set for just over 2x the pulse length - in this case set for at least 5 seconds.
If the PLC fails the state of the output does not matter - the fact that it ceases changing resultes in the timer timing out and deenergizing.