Line Monitoring for Digital Output

Eh? Go again?

It needs to be decoded from digital to analog discrete i/o. You need discrete output module or you may choose solenoid valve which features off/on led status.- thats the safest. I/o Wiring of field instruments could sometimes fail and deliver wrong inputs. The same would happen if you got the signal from the controller. The safest is direct on field instrument 1step proces.

It all depends on what you mean by "line monitoring". If you want a gadget like a pilot light, connect it in parallel with your solenoid.

If you want statistical data or trending data, buy a HMI software package like what you will find at www.Indusoft.com and monitor the PLC. You get 40 hours of trial and error time before the demo license shuts down.

Install a 110VAC light parallel to the energized solenoid, since this will not be energized except when the interposing (definite purpose contactor) relay is closed. I presume you will use a N.O. interposing relay, since this would cut down on 110 VAC power usage. If this is a critical application where you need to see the active solenoid itself on screen, then you should parallel the solenoid voltage with a suitable bridge circuit (convert to some DC voltage), then to ADC input (analog-digital converter) and then the digital version of the signal is back into the PLC/HMI. Just remember that the "more moving parts" you insert into the picture the more that can go wrong, go wrong, go wrong.....