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Hermes Operator Note Page 8 of 32
3.3 Data Transfer Modes
In FEC mode every byte of the data, including the "end-of-file" string is
scrambled within the modem to ensure that the error correction mechanism
operates over non-consecutive bits in the data stream. The frame sync
pattern is not scrambled.
In non-FEC (High Speed) mode the data is scrambled to minimise problems
that could occur with many consecutive 1's or 0's in the input data stream.
3.4 RS232 Data Handling
The over-air data rate is faster than the input data rate because only the 8
data bits of the input character of 11 bits are transmitted. To ensure that no
data is lost at the output, and for the modem to transmit the data in one
continuous packet it is essential that the input and output data rates are set
to at least 2 x the over-air-baud rate. The default over-air baud rate for this
modem working in a 12.5kHz channel is 9600 baud. For this set the input
and output RS232 data rates to 19,200. For a 25kHz channel, with an over-
air baud rate of 19,200, set the data rates to 38,400 baud.
Some applications running under the Windows™ operating system output
the data with gaps between single or groups of characters. In some cases
this can reduce the character rate below the over-air data rate. When this
occurs, the system will assume that the input data has finished and will turn
the transmitter off when the input data buffer is empty.
If more characters are input to the modem while the transmitter is being
turned off, the transmit process will re-start. This will appear as bursts of
RF close together. At the receive end, the timing of the data will change as
there will be gaps in the received data stream.
This mode of operation ensures that small data packets are transmitted in
as short a time as possible to reduce the on-air time to a minimum.
3.5 Receive-to-Transmit Turnround
After the last character of a packet of data is output from the receiver, the
receive system is still processing the “end-of-packet” data. This takes ~5ms
when in non-FEC mode and ~10ms in FEC mode. During this period the
processor does not accept any input data. The figures apply for default
settings: 19,200 baud rate and 4800 symbol/sec over-air baud rate