Company Confidential
36
Raveon Technologies Corp.
9.5. Serial Data Flow Control
If large amounts of data will be transmitted, and the serial port is operated at a
faster data rate than the over-the-air rate, 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 asserte
d and it is “Clear to
Send”, the buffers are less than 80% full. The ATCH command enables or
disables the RTS input signal.
9.6. Setting the Over-The-Air Data Rate
The M8 has programmable over-the-air baud rates. The over-the-air rate is
stored in register R2, and is programmed with the
ATR2 x
command, where
x
is
a number corresponding to the rate. There are many possible baud rates, but
not all rates may be used with all radio modem models. Consult the following
table.
Over-the-air
rate
Radio
Bandwidth
Number of
Modulation
Levels
Modem Channel
Bandwidths that support
the rates
800bps
(R2=0)
12.5kHz or
25kHz
2
W and N
1200bps
(R2=1)
12.5kHz or
25kHz
2
W and N
2400bps
(R2=2)
12.5kHz or
25kHz
2
W and N
4800bps
(R2=3)
Default setting
12.5kHz or
25kHz
2
W and N
8000bps
(R2=4)
12.5kHz or
25kHz
4
W and N
9600bps
(R2=5)
25kHz
2
W
19200bps
(R2=6)
25kHz
4
W
5142bps
(R2=7
)
12.5kHz or
25kHz
2
W and N
9600bps
(R2=8)
12.5kHz or
25kHz
4
W and N
Changing the over-the-air data rate will change the sensitivity of the receiver.
Higher-data rates require a stronger signal at the receiver to be properly
received. The default over-the-air data rate for
M8
radio modems is 4800 baud
for narrow-band versions, and 9600 baud for wide-band versions. These are the
optimal data rates for radio modems on wide and narrow radio channels.
