3MODEM OPERATION
3.1Introduction
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
3.2Handshaking 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.
Hermes Operator Note | Page 7 of 32 |
