Company Confidential
26
Raveon Technologies Corp.
4. To change the radio channel, use the
ATHP
x
command while the modem is
in the command mode.
8.3. Data Transmission
To transmit data, send one or more bytes of data into the serial port of the
modem. When a full packet of data has been collected into the internal buffer of
the modem, or when there is a pause in the data, the modem will automatically
key its transmitter, and send the data over the air.
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 M8S 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 M8S can handle serial-port data rate far in excess of the over-the-
air rate, the efficiency of the M8S 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.
Busy-Channel Lock Out
If your system operation require the M8S modem to monitor-before-transmit, of if
you do not want the M8S to transmit on a channel that is busy, you can enable
“Busy-Channel-Lockout”, using the
ATBC 1
command.
ATBC 0
disables BCL,
and thus the modem will transmit whenever it has data to send out.
The factory-default is BCL disabled. Use caution when enabling it, as a CW
interferer, PC with poor shielding, or some other source of RF can stop the
modem from transmitting. The threshold where the M8S senses RF carrier, and
determines that the channel is busy is set by the
ATCD
command. This is
factory calibrated to an equivalent RF level of approximately -110dBm.
8.4. Addressing (Packetized Mode only)
Addressing Basics
One of the more powerful aspects of the
M8S
modem is its addressing scheme.
Incorporating addressing in the modem allows multiple radio systems on the
same frequency to co-exist, and not interfere with each other. Also, some user
application cannot tolerate receiving data that was not intended for it, and by
