RXD1 will receive a single byte of data that is shifted as an
asynchronous serial stream.
RXD1 will return 0-255 if there was data present. or -1 ($FFFFFFFF) if there is no serial stream available. The hardware UART is used, so the CPU is not tied up, and bytes are buffered up to 256 bytes being received by an interrupt routine.
ARMmite--
Pin labeled IO0 on the schematic, UART1 of the LPC2103.
ARMexpress LITE
Pins labeled IO5
Data is positive true. Baudrates can be upto 115.2 Kbaud.