Company Confidential
24
Raveon Technologies Corp.
Serial Port Baud Rate
While the modem is transmitting, the user may continue to send more data into
the
M8.
Because the buffers in the M8 are full-duplex, the serial port data rate
and the over-the-air data rates are independent. The serial port baud-rates may
be set slow to accommodate legacy equipment, or set at high-speed to minimize
latency. The over-the-air data rate us usually 4800 baud for narrow-band
channels, and 9600 baud for wide-band, although faster or slower rates may be
used.
In Packet Mode, selection of the serial port baud-rate is important. As shown
above, if the serial port baud-rate is the same as the over-the-air baud rate and
the packets are short, the channel utilization is only about 50%. But, if the serial
port baud rate is set much higher, say 2-8X the over-the air rate, the channel
utilization becomes near 100%.
Because the M8 can handle serial-port data rate far in excess of the over-the-air
rate, the efficiency of the M8 in Packet Mode is approximately the same as other
brand modems that cannot operate in a Packet Mode
— with the added benefit
or ARQ, error-free data, and addressing.
Note that many Windows applications which use the serial port, such as
HyperTerminal, put large gaps between the bytes of data they send out the serial
port. If an application is not getting the desired throughput, verify that it is not an
artifact of the Operating System or the computer.
Flow Control
If large amounts of data will be sent with the
M8
, it may be possible to overflow
the internal data buffer. To ensure the transmit buffer does not overflow, enable
and use hardware flow control. Hardware flow control is enabled with the
ATCH
1
command. Note that the
M8
modem will always indicate the status of its
internal buffer using the
CTS
signal on the DB-9 serial connector. When CTS is
negated, the internal buffers are more than 80% full. When it is asserted and it is
“Clear to Send”, the buffers are less than 80% full.
You can modify this CTS threshold with the ATJF xx command. If you would like
CTS negated when there is 1or more bytes in the M8
’s buffers, set
ATJF
to 1
(
ATJF
1).
Packet Size
The over-the-air packet size may be set with the
ATTT xx
command. Once the
modem receives one full packet of data into via the serial port, it will
automatically key the transmitter and send the data. Factory default is 80 bytes.
The M8 will also automatically send all of the data in its buffer when there is a
pause in the incoming data stream, regardless of the ATTT setting.
Key-On_Data
When serial data is entering the
M8
’s RS-232 port, the
M8
looks for pauses in
the data as indication that it is time to send a packet of data over the air. The
