Raveon RV-M7, Техническое руководство, Страница: 44 / 67

RV-M7 Technical Manual
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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 M7 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 M7. 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 M7 modem will hold the data as it is sent out the serial port.
7.3 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 M7 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 M7s 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.
7.4 Carrier Detect
To reduce bit errors and additional noise bytes, the user may configure the M7 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
M7 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.
7.5 Serial Data Flow Control
If large amounts of data will be sent with the M7 , 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 M7 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 asserted and it is “Clear to Send”, the buffers are less
than 80% full.