Raveon RV-M7, Техническое руководство, Страница: 34 / 67

RV-M7 Technical Manual
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utilization is only about 50%. But, if the serial port baud rate is set much higher, say 2-8X the over-
the air rate, the channel utilization becomes near 100%.
Because the M7 can handle serial-port data rate far in excess of the over-the-air rate, the efficiency
of the M7 in Packet Mode is approximately the same as other brand modems that cannot operate in
a Packet Mode — with the added benefit or ARQ, error-free data, and addressing.
Note that many Windows applications which use the serial port, such as HyperTerminal, put large
gaps between the bytes of data they send out the serial port. If an application is not getting the
desired throughput, verify that it is not an artifact of the Operating System or the computer.
Flow Control
If large amounts of data will be sent with the M7 , 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.
You can modify this CTS threshold with the ATJF xx command. If you would like CTS negated when
there is 1or more bytes in the M7’s buffers, set
ATJF to 1 ( ATJF 1).
Packet Size
The over-the-air packet size may be set with the ATTT xx command. Once the modem receives one
full packet of data into via the serial port, it will automatically key the transmitter and send the data.
Factory default is 80 bytes. The M7 will also automatically send all of the data in its buffer when
there is a pause in the incoming data stream, regardless of the ATTT setting.
Key-On_Data
When serial data is entering the M7 ’s RS-232 port, the M7 looks for pauses in the data as indication
that it is time to send a packet of data over the air. The factory default duration of the pause it
looks for is 20mS, but the user may change this to over values using the
ATR3 xxx command, where
xxx is in milliseconds. ATR3 2 (2mS) is a good setting if you are configuring the M7 for use in a
polled SCADA system.
Busy-Channel Lock Out
If your system operation require the M7 modem to monitor-before-transmit, of if you do not want
the M7 to transmit on a channel that is busy, you can enable “Busy-Channel-Lockout”, using the
ATBC 1 command. ATBC 0 disables BCL, and thus the modem will transmit whenever it has data to
send out. The factory-default is BCL disabled. Use caution when enabling it, as a CW interferer, PC
with poor shielding, or some other source of RF can stop the modem from transmitting. The
threshold where the M7 senses RF carrier, and determines that the channel is busy is set by the
ATRA command. This is factory calibrated to an equivalent RF level of approximately -110dBm.
6.4 Addressing (Packetized Mode only)
Addressing Basics
One of the more powerful aspects of the M7 modem is its addressing scheme. Incorporating
addressing in the modem allows multiple radio systems on the same frequency to co-exist, and not