Company Confidential
35
Raveon Technologies Corp.
out 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 M8 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 M8. 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
M8
modem will hold the data as it is
sent out the serial port.
6.3. 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. 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 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 noise. This may take a byte or
two of time, and during this time period, the receiving modem may output random
noise bytes.
6.4. Carrier Detect
To reduce bit errors and additional noise bytes, the user may configure the M8 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.
6.5. Serial Data Flow Control
If large amounts of data will be sent with the
M8
, 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