The ATC/TCAS control panel sends control data through the ATC Mode S transponder to the TCAS computer. The TCAS computer sends this data to the DEU for display.
The function selector puts the TCAS computer in the TA only or TA/RA mode. In the TA only mode, the TCAS does not supply RA traffic symbols or advisories. It also does not supply RA aural messages. The function selector can also put the TCAS computer in the test mode.
The ND shows the relative altitude of the target above or below the symbol.