I have a micrologix 1500 (processor 1764-28BXB) and I am having problems with one of my outputs that controls a 24vdc horn. This horn is also controlled by a pushbutton in parallel with the output from the plc, causing voltage to be applied to the output pin when the button is pressed. If the plc is not powered up and the button is pressed, the plc turns on for as long as the button is held. What causes this and how can I prevent this other than having to install a diode between the output pin of the plc and the output of the button?

Cool. You could use a relay or a relay card for your 1500. Do you have to be able to turn the light on when the processor is off?

Why not just use a NC contact on the pushbutton and put it in series with the output such that when the button is pressed to energize the horn, the circuit to the horn from the PLC is opened. We do this all the time.

Are you using the FET output for your horn? If so all solid state devices have leakage. Never short a solid state device. ( damage may occur). As finfin said use a slave relay or one of the PLC's relays if available.

Long ago I got in the habit of wiring everything through the PLC, even if all the button did was turn on a single output. The only thing I wire around the PLC anymore are over-rides which must still function if the PLC does not. Everything else goes through the PLC, even if it means another IO card.

So wire the push button to an input. program the branch into the logic, and operate the horn only from the PLC output.

I'm agree with alaric, but for now the cheaper solution is to use a diode in serie with output.