User's Manual

and send it 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
FireLine 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
FireLine. 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 FireLine modem will
hold the data as it is sent out the serial port.
4.9. 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 FireLine 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 FireLines
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.
4.10. Carrier Detect
To reduce, and virtually eliminate bit errors and additional noise bytes, the
user may configure the FireLine 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 FireLine 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 bit errors when the signals are
weak.