The REU uses the open discrete to select ILS audio. The DEUs use the ground discrete to set the ILS displays. The NAV control panel sends the 28v dc to the FCCs when an ILS frequency is active. It also sends 28v dc to the LOC antenna switches when an ILS frequency is active for switch operation.
The on-side FCC supplies a discrete during an AFDS approach mode. The discrete causes the MMR to not accept any frequency changes. The discrete also goes to the NAV control panel. When the discrete is set, It prevents any frequency change to the output data word that goes to the MMR. The MMR uses the air/ground discrete for flight leg count.