User guide
SATELLINE-3AS NMS / NMS Epic / NMS 869 / VHF
User Guide, Version 1.0
34
5.6
Handshaking lines
When using the RS-232 serial interface, handshaking signals can be used to control data
transfer on the DATA port. For example, the radio modem can inform the DTE that the radio
channel is busy, and that it is not allowed to initiate transmission.
A common way of using handshaking signals is to monitor the CTS line and ignore the others.
Usually the terminal device is fast enough to handle the data received by the radio modem, so
the use of RTS line is not necessary.
Handshaking is not needed if the system protocol is designed to prevent collisions (data
contentions) by using poll queries, or if there is only little traffic and, if there is no harm from
occasional data contention situations (two or more radio modems trying to transmit at the same
time).
5.6.1
CTS line
CTS (Clear To Send) is a signal from the radio modem to the DTE. It indicates when the radio
modem is ready to accept more data from the DTE. The options for CTS line controls are:
1) C
Clear To Send
CTS line is set active when the radio modem is ready to accept data for transmission. CTS will
shift into inactive state during data reception, and when a pause (packet end) is detected in
transmitted data. CTS shifts back into active state either when reception ends or the radio
modem has finished data transmission. CTS will also shift into inactive state when the serial
interface data transfer speed is greater than the radio interface transfer speed, and the transmit
buffer is in danger of overflowing.
2) T
TX buffer state
CTS line will shift into inactive state only when the data buffer for the data to be transmitted is in
danger of overflowing.
5.6.2
CD line
CD (Carrier Detect) is a signal from the radio modem to the DTE. It indicates when there is
activity on the radio channel. The options for CD line controls are:
1) R
RSSI-threshold
CD is active whenever a signal exceeding the defined threshold level required for reception,
exists on the radio channel. It doesn’t make any difference, whether the signal is actual data
transmission, a signal of a radio transmitter not belonging into the system or even interference
caused e.g. by a computer or some other peripheral device. CD is also active, when the radio
modem itself is transmitting.