GPS Input Signal
On vehicle equipped with a GPS receiver, (Mk-I and II) the information from the receiver is transferred to and from the Navigation computer via a 4 wire bi-directional interface. The navigation computer will send a "request" to the GPS receiver for the current vehicle position. This request is sent over a two wire link. The two wires are referred to as RXD and RXDN.
The navigation computer receives the current vehicle position over another 2 wire link referred to as TXD and TXDN. Each two wire link is similar to the CAN line but is unidirectional. When a "data burst" is transmitted, it is mirrored on the other wire as a self-check of data transmission.
Do not confuse these signals with the diagnosis bus (TXD/RXD), the are only related in name, not in function.