Company Confidential
35
Raveon Technologies Corp.
over the air in the same order as it enters the modem. When the buffer is empty
and there is no more data coming into the modem, it will automatically de-key the
radio and go back into the receive mode. The modem will send a hidden end-of-
message signal to the receiving modem, thus avoiding any extra data bytes
“
dribble bytes
” from coming out of the user serial port. When the modem is
operating with very weak signals, the end-of-message signal may be obscured,
and missed by the receiving radio modem. In this case, additional noise bytes
may come out of the user serial port.
While receiving, the modem will also output the receive data out the serial port at
the rate set by the
ATBD
command. If the serial port baud-rate is slower than
the over-the-air rate, an internal buffer in the modem will hold the data as it is
sent out the serial port.
9.3. Bit Errors
Unlike Packet Mode operation, there is no error-detection nor error-correction in
the Streaming Mode, so user data may contain bit errors
. The user’s application
must be able to handle these errors or additional bytes of noise data.
If the application that is using the M8 cannot tolerate have erroneous data when
the channel is noisy, the modem should be operated in the Packet Mode instead
of Streaming mode (In Packet Mode, data is always first checked for bit-errors,
and never outputted if it detects any errors).
The end of a transmission is detected by the receiving modem by the presence
of a special end-of-message signal send over-the-air. The end-of-message over-
air signal is a CW carrier signal sent at the end of data. Transmitting M8S
automatically put the end-of-
message signal out after the user’s data has been
transmitted. If the receiving modem does not receive this CW signal (due to noise
or interference), the receiving modem may continue to output some more data,
until it detects that the RF carrier is gone, or the received signal is actually very
noisy. This may take a byte or two of time, and during this time period, the
receiving modem may output random noise bytes.
9.4. Carrier Detect
To reduce bit errors and additional noise bytes, the user may configure the
modem to require and RF carrier Detect before receiving any data. Because the
RF carrier Detect Threshold is set above the noise-floor of the receiver, bit-errors
will be rare if RF carrier is required to receive. By default, the M8 does not
require RF carrier detect to receive. To enable it, use the
ATRF 1
command. To
disable the need for RF carrier detect, us the
ATRF 0
command (Factory
default).
When
ATRF
is
0
, the modem will be more sensitive, and be able to receive weak
signals, but there is more likely to be many bit errors when the signals are weak.
