urgent help please

Hi Friends
I am in need of an urgent help now.
We basically built shelters for oil and gas industry.
I am doing the fire and gas safety PLC programming. My clients had come for doing the Factory Acceptance Test of my system.
Now they are asking for an alarm signal when somebody puts the PLC to stop mode.This is like a PLC STOPPED alarm.
How can i do it. Pls help me
PLC i am using is siemens CPU226 CN
I need to give a alarm output if the PLC goes to STOP/FAIL

Please help me ASAP
Thanks

you could use a output that is turned on and fails off. Much like a power fail alarm, tie this to a old fashion dialer to send the alarm for best results, often phone lines remain on even when power is off.

program ur output with N.c input and at the output use rellay and for the alarm use rellay N.C point

suchitradevi79 said:

Hi Friends
I am in need of an urgent help now.
We basically built shelters for oil and gas industry.
I am doing the fire and gas safety PLC programming. My clients had come for doing the Factory Acceptance Test of my system.
Now they are asking for an alarm signal when somebody puts the PLC to stop mode.This is like a PLC STOPPED alarm.
How can i do it. Pls help me
PLC i am using is siemens CPU226 CN
I need to give a alarm output if the PLC goes to STOP/FAIL

Please help me ASAP
Thanks

Click to expand...

The best way to do this is with a pair of hardware (solid state) timers, fixed preset of 2.0 seconds. One timer is delay ON, one time is delay OFF. A single digital output of the PLC is programmed to toggle ON and OFF, at a 1 second interval. The coils of both timers are energized by this single PLC output.
If either timer 'times out', it means that the PLC's output has stopped changing for 2.0 seconds.
In this way, the 'last state' of the output module has no impact.
And, if the PLC is self-aware of its fault condition, you can add that to the program of the single output.
Other approaches use what is refered to as a non-retriggerable one-shot timer. These timers are more expensive.
Since you mention safety, you can likely use a special safety relay, set up to monitor 'speed', and your PLC output can mimic a proximity switch on a motor shaft. I.E. 'zero speed' = PLC stop.

As a low cost "WATCHDOG" you can use a "nil speed switch"
as used to detect conveyor belt failure by getting pulses from a prox switch at the tail end pulley. Simply use a PLC output that turns ON and OFF instead of the prox switch.