Input leds on/off

Hey guys,
I had a question pop up the other day about the input leds on an AB AC input card. I almost never use AB but in the past I was thinking that on many PLCs the inputs are basically controlled by there own input power source...meaning the LED is on if the input device is passing power. I was sure that in some cases I have had a card pulled out of the rack with the input power on the LEDs would still be on if the device was on?? Or the CPU can be off and the input power be on and again the LED would be on... on many the input circuit shows normally an optic isolation between the input power and the card power... so my question was on the AB PLC if the power supply for the rack was off would the input LEDs remain off as well? I was thinking they would be on but I was proved wrong?? Am I remembering this wrong???

It depends on the Manufacturer, and the decisions they make when they design the Card.

My experience has been that Allen Bradley LEDs are on the PLC Back-Plane side of the Opto-Isolator.

If you Download the Manual from the AB Literature Library, they have a simplified Diagram that shows the wiring for their Cards.

Stu....

The vintage of the PLC will make a difference too.

Back in the 1970's and 1980's, neon lamps were the standard way to provide an indication of a 120V AC circuit, so they would have been on the field side of the circuit.

Ken Roach said:

The vintage of the PLC will make a difference too.

Back in the 1970's and 1980's, neon lamps were the standard way to provide an indication of a 120V AC circuit, so they would have been on the field side of the circuit.

Click to expand...

Good point Ken. I still remember my surprise when I shut off the Rack Power to a Modicon 984, and all the 120VAC Input Card Inputs stayed Lit. That was in the Mid 90s.

Stu....