The design uses a quad opto device (TLP521-4) conatined within a 16pin DIL package, again the design is quite simple and is primarily intended to provided a basic level of isolation between external projects and the GPC(b).
External DC inputs within the range of +5V to 30V should be sufficient to activate the internal LEDs and so will switch the current sink transistors of the opto into a conducting state.
The design was first tested on stripboard.
The prototype appeared as below and functioned well during soak tests.
The design was then transferred onto PCB for manufacture.
The new board arrived from the manufacturer ready to be populated.
Intended modifications for #1
1 - Remove Hole references.
2 - Tidy up tracks so that Input paths are the same.
3 - Include input reference on next to LEDs.