Wood and Douglas 1335, Operating Instructions Manual

Page: 28 / 91

28
1892 1335 - Orion Radio Modem Operating Instructions - v2.9 / Aug 2009
Protocol Selection
TS is a basic half-duplex link for asynchronous serial data. It has no error correction.
TU also provides a half-duplex asynchronous serial data link, but it has Feed-forward Error
Correction (FEC) so that many errors can be corrected.
TP provides a half-duplex asynchronous serial data packet link with both FEC and requests
for repeat transmission of corrupt packets to provide an error-free link.
v This protocol cannot be used in conjunction with a repeater.
TR caters for non-standard data formats by sampling the input and recreating it at the other
end. The data is scrambled so that long periods of 0 or 1 can be transmitted. Any
format of data can be sent, but no error correction can be provided.
v The RTS line must be active as long as data is to be transmitted.
v TR should not be used if the data is formatted, for example RS232, because it is
much less efficient than other modes.
The parameter which controls protocol
(ATM1)
can only be changed in modes U and R.
Symbol Rate
The symbol rate (ATS163) ) can be selected, depending on the channel width available from
the radio module, which can be read back using ATFC? . The radio modem uses a 4-level
FSK system, and so you can use19200bps (9600 symbols/s) or 9600bps (4800 symbols/s)
within a 25kHz channel and 4800bps (2400 symbols/s) within a 12.5kHz channel.
Hold-off Before Transmitting
To avoid contention, the unit can listen on channel for other transmissions before transmitting
itself, and wait if it hears another signal. In some circumstances – where there is significant
interference, for example – this is not satisfactory, and the unit can be made to transmit at any
time
(ATS180) .