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Hermes Operator Note Page 7 of 32
3 MODEM OPERATION
3.1 Introduction
The internal modem power supply system is turned on/off with the DTR
input. The modem is ready after an initialisation period. The modem enters
receive mode using the last set of configuration parameters (RF channel,
squelch level etc).
The modem starts the transmit sequence after the first data byte is received
from the RS232 port. The incoming data is stored while the transmitter is
turned on and the preamble output. The start, stop and parity (if 8 bit data)
are stripped off the input data stream and the data stored as 8 bits only.
The transmission is ended when the input buffer is empty. After the
transmission is complete and the transmitter turned off, the modem
switches back to receive mode. Transmission has priority over reception so
the user has to check by means of the CD handshake line if the radio
channel is free before inputting data.
The timing of the power, handshaking and data transfer is shown in Figure 1
for the G-MAX radio (running at 9600 baud over-air in a 12.5kHz channel)
and for the SXn50 series of radios (running at 19,200 baud over-air in a
25kHz channel).
3.2 Handshaking Operation During Transmit and Receive
The DTE must set the RTS input for the modem to accept data. The RTS
input can be set continuously if no hardware handshake lines are available
on the DTE. The CTS line must be monitored for data packets greater than
256 bytes to ensure that input buffer overflow does not occur. If the data
input is not stopped within 10 characters from the point of buffer overflow,
the input data will be discarded.
Handshaking is also required during the code loading.
The modem starts transmitting when the data is input, regardless of the
state of the CD output. The DTE must check the CD output prior to data
transmission. When the CD output is set this indicates that a RF signal is
present at the antenna. This output is only active when the modem is not
transmitting.