I have a power supply with 12V running into a the common on my PLC. Output 0 on the PLC is wired into a solenoid. The negative on the solenoid runs into the ground on the power supply completing the circuit.
When I power up the PLC and the the external power supply all outputs on the PLC are off and the solenoid is off. When I activate Output 0, my solenoid clicks on as expected. However, when I turn Output 0 back off, the solenoid stays on. The solenoid only turns off if I physically remove the ground wire.
The diagram below gives some insight. Power supply wired to Pin 17. Output 0 (pin 10) wired to solenoid. Solenoid is wired back to ground on the power supply.
If I give the command to turn output 0 on, the solenoid comes on. If I give the command to turn output 0 off, the solenoid stays on.
Link to my cheapo solenoid. http://www.amazon.com/gp/product/B008IG7SWQ/ref=oh_details_o04_s00_i01?ie=UTF8&psc=1