Instead of leaving the relay contacts uncommitted, we provided two common connections to each set of DPDT contacts. And we set up the relay outputs so we could choose whether to supply power to devices when a relay was on or when it was off. In addition to providing electromechanical relays, we dedicated one output line to driving a small solid-state relay. That relay can easily control a 110-VAC load such as a motor or lamp. Some of the semiconductor devices we test are sensitive to light, so during testing we use the solid-state relay to turn off a lamp that lights the test head.
All designers know that circuits require testing and troubleshooting.
To make it easier to troubleshoot our circuits, we included an LED for each output line from the PC add-in card. So, our interface includes only four inputs. The comparator's output drives a line that goes to port C on the PC add-in card. Although we needed only a few input lines, we could easily run out of output lines in our applications. But by carefully examining our needs, we could find a way to use the lines we had.