Campbell Scientific RF300-Series DRL VHF/UHF Radio Transceivers - Manual (Page 21)

SECTION 3. RADIOTELEMETRY NETWORK COMPONENTS
3-3
3.1.6 RF95A MODEM COMMUNICATION
PROTOCOL
Comprehension of this section is not
necessary for routine operation of the
RF95A Modem. The PC208W Datalogger
Support Software accounts for the
necessary communication protocol.
There must be an RF95A Modem at both the
calling (or computer) end of the transmission
link, and at the answer (or datalogger) end of
the transmission link. The modem at the calling
end is the Start Of Link (SOL) modem, and the
modem at the answer end is the End Of Link
(EOL) modem.
RF95A Modems must also be used at repeater
stations. These RF95A Modems are termed
Middle Of Link (MOL) modems.
The RF95A Modem has five general modes of
operation:
•
Wait Mode
•
Local Command Mode
•
Repeater Mode
•
End of Link Mode
•
Transparent Mode.
The RF95A is in the Wait Mode of operation
when it is waiting to enter one of the four other
modes of operation. The Wait Mode is entered
1) after the power-up sequence is completed, 2)
following the "T" command when in the Local
Command Mode, and 3) when the system is
reset by the Time-out Timer. The Time-out
Timer is a 60-second timer which is set every
time a valid transmission block is received on
the RF link. The datalogger, being in
Telecommunications Mode, will override the
Time-out Timer.
The Local Command Mode is used to set up
and shut down an RF link. The Local
Command Mode is entered when the
datalogger goes into Telecommunications Mode
after being in the Wait Mode. In this mode the
RF95A responds to command characters
received on the Serial I/O port.
The RF95A is in the Transparent Mode after
the RF link has been established. In the
Transparent Mode, any data received on the
Serial I/O port are organized into data blocks for
transfer through the RF link.
The Repeater Mode is entered by MOL RF95A
Modems. The function of the Repeater Mode is
to receive and then transmit data blocks. The
signature of each data block is checked before
being sent to the next RF station. The block is
discarded if the signature of the data block is
incorrect. The RF95A enters the Repeater
Mode when it receives a valid setup block that
sets the RF95A as a repeater.
The End Of Link Mode is entered when the
RF95A receives a valid setup block that sets the
RF95A as the EOL modem. Upon entering the
EOL Mode, the RF95A brings the Serial I/O
Ring line high which raises the datalogger ME
line, thus causing the RF95A to enter the
Transparent Mode. The Ring line is reset after
the ME line comes high.
3.1.7 RF95A Modem and the RF Link
The RF link is the communication path which is
opened between the Start Of Link modem and
the End Of Link modem, along with any Middle
Of Link modems. Any RF link must first be
established, then maintained, and finally shut
down.
When collecting data, PC208W establishes,
maintains, and shuts down the RF link as
discussed below.
3.1.7.1 Establishing the RF Link
The SOL RF95A is first brought into the Local
Command Mode of operation. In the RF95A-
ME State, this is done when the ME line is high
on the Serial I/O port and the SOL modem is in
the Wait Mode of operation. After the ME line is
brought high, the baud rate of the SOL modem
is set by repetitively pressing [ENTER]. The
SOL modem can operate at 300, 1200, or 9600
baud. When the baud rate is set, the SOL
modem will respond by sending a carriage
return line feed (CR-LF) and an exclamation
point (!). In the RF95A-SDC State, the Local
Command Mode is entered after addressing.
Some explanation is contained in Appendix E
"RF95A States."
In the Local Command Mode, the SOL modem
responds to command characters received from
the terminal or computer. The command