Version
3.3
19
2) Flow control
The radio modem transmits data to the terminal device only when the RTS-line is active. An
inactive state of the RTS-line will force the radio modem to buffer the received data. This option
is used when the terminal device is too slow to handle data received from the radio modem.
3) Reception control
The RTS-line controls the reception process of the radio modem. An active RTS-line enables
reception. An inactive RTS-line will interrupt the reception process immediately, even if the radio
modem is in the middle of receiving a data packet. This option is used to force the radio modem
into a WAIT State for an immediate channel change.
6.2
Pause length
The radio modem recognises a pause on the serial line (a pause is defined as a time with no
status changes in the TD-line). The pause detection is used as criteria for:
- End of radio transmission - when the transmit buffer is empty and a pause is detected, the
modem stops the transmission and will change the radio to the receiving mode
- SL-command recognition - for an SL-command to be valid, a pause must be detected before
the actual “SL“ prefix of the SL-command.
- User address recognition - in order to detecting the message, a pause must precede it in
transmission.
Traditionally, in asynchronous data communication, pauses have been used to separate serial
messages from each other. However, the use of non-real-time operating systems (frequently
used on PC-type hardware) has changed this tradition by adding random pauses in the
asynchronous data stream. Such systems cannot serve the hardware UART properly when
performing other tasks (other applications or tasks of the operating system itself).
The pauses described above are typically up to 100 ms. When such a pause appears in the
middle of a user message, the radio modem transmits the message as two separate radio
transmissions. This will generate problems in at least two ways:
1)
The inter-character delay will be increased by at least the time of the modem transfer delay
2)
The probability of collisions on the radio path will increase. This will be especially harmful for
repeater chains
The default value for the pause length is 3 bytes.
6.2.1
Data buffering in the radio data modem
A synchronisation signal is transmitted at the beginning of each radio transmission and this
signal is detected by another radio modem, which then turns into receive mode. During the
transmission of the synchronisation signal the radio modem buffers the data to be transmitted
into its memory. Transmission ends when a pause is detected in the data flow sent by the
terminal device, and after all the buffered data has been transmitted. When the data speed of
the serial port is the same or slower than the speed of the radio interface, the internal transmit
buffer memory can not overflow. However, when the serial interface speed exceeds the speed of
the radio interface, data will eventually fill the transmit buffer memory. After the terminal device
