User's Manual

Serial Port Operation
Crescendo VHF Half-Duplex User Manual Page 16 of 74
4. Serial Port Operation
4.1 Overview
The Crescendo radio has two DCE RS-232 serial ports with DB9 connectors. The serial port pin outs can be
found in Appendix A.3 on page 56.
The main port supports:
TX, RX, and GND.
RTS and DTR inputs.
CTS and DCD outputs.
While the auxiliary port supports:
TX, RX, and GND.
Both serial ports support over the air data transfer. In general, due to the presence of control lines, the main
port should be used as the main data port. The auxiliary port should be used for performance monitoring and
configuration.
Both main and auxiliary serial ports have internal byte buffers on transmit and receive interfaces. This
configuration is shown in Figure 2.
4096 byte output buffer
TX Line (input from DTE)
RX Line (output to DTE)
Input to RF /
internal processing
Output from RF /
internal processing
4096 byte input buffer
Figure 2: Buffering scheme on the Crescendo
This serial port buffering scheme has a number of ramifications on the Crescendo operation:
No preamble is required to account for radio turn-on time.
If the data cannot be sent, it will be buffered until the transmitter is ready.
If the data terminal equipment (DTE) is not ready for data, the Crescendo can buffer the data until
the DTE is ready.
If the buffer is full, additional data received will be discarded until space has become available.