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20-90 VHF & UHF TRANSCIEVER
Product Manual
Sea Air and Land Communications Ltd, 10 Vanadium Place, Addington, Christchurch 8024, New Zealand
September 2021
Channel Busy Detection
The 20-90 provides a channel busy function to prevent transmission when the channel is in use, or
when there is interference on the channel. An output may be configured to provide this busy
indication to an external high power transmitter or other channel activity indicator.
TX/BUSY Indicator LED
A rapidly flashing red LED indicates that the 20-90 is transmitting.
A solid red LED indicates channel activity, and may inhibit the transmitter.
Receiving a message: The TX/BUSY LED is on (solid) whenever a message is being received.
Normally the 20-90 transmitter will be inhibited when a message is
being received. This can be disabled in cases where unwanted
weak signals may block the transmitter but would not interfere with
local reception. The RSSI threshold may be used instead to detect
local signals or interference.
RSSI threshold:
Other signals or interference on the channel can be detected by the
received signal strength (RSSI). When enabled, the TX/Busy LED will
be on whenever the received signal strength is above the RSSI
threshold level. The transmitter will be inhibited until the channel is
clear again.
Busy Hold time:
This is the delay between the channel becoming clear and the
transmitter being free to transmit. This delay reduces the flickering of
the busy state with rapidly fluctuating interference. The delay may
be increased as required. Maximum delay is 5 seconds.
The Busy Hold time also controls the delay between receiving a
message and any response from the 20-90, including
acknowledgements, special function replies, or store and forward.
Using a different delay for each transmitter can help control channel
access when multiple transmitters are waiting to transmit.
Busy Output:
An output on the 20-90 may be used to provide a channel busy
signal. The CLOSED state (low) represents a busy channel. This output
mirrors the internal transmit inhibit function. The output will be closed
if the signal strength is above the threshold when the RSSI threshold is
enabled, or a messages is being received and “Receive State” is
enabled.