5902 Bell 202 Modem Hardware Manual
October 19, 2007
21
The modem de-asserts the DCD output and turns off the CD LED when the soft carrier turn-off tone
is received. RTS will not be recognized until 1 to 10 ms after the tone ends.
4.5.6
Clear-to-Send Hold Time
The CSH switch controls the time CTS is held asserted after RTS is de-asserted. There are two
options.
CTS HOLD TIME
(+/
– 1 ms)
1 ms
8 ms
CSH switch
open
closed
The modem holds the CTS signal asserted and transmits carrier (marks) after RTS is de-asserted by
the DTE. (De-asserting RTS signals the end of a transmission.) This allows the DTE to transmit data
with gaps between characters, without causing the modem to go through RTS/CTS delay timing on
each gap. Any gap longer than the CTS hold time signals the end of transmission.
The soft carrier turn-off tone (if any) is generated after the conclusion of the CTS hold time.
4.5.7
Radio Carrier Detect
The RCD switch selects the signals used to detect carrier. There are two options.
The RCD switch also affects the length of the RTS/CTS delay and the Carrier Detect delay. Refer to
section
4.5.1
-
RTS/CTS Delay
and
4.5.3
-
Carrier Detect Delay
sections for details.
CARRIER
SOURCE
AUDIO ONLY
AUDIO AND RCD
INPUT
RCD switch
Open
closed
The modem hardware detects audio energy within specific frequency bands as a received carrier. If
the audio input is obtained from an unsquelched radio receiver, the resulting noise causes false data
reception. The radio carrier detect option configures the modem to monitor the RCD (radio
frequency carrier detect) input. If RCD is used, the modem detects carrier only when audio carrier
tones are present and the RCD input is asserted.
If the RCD switch closed, the RCD input must be connected to the radio’s carrier detect output.
Otherwise the modem will never detect a received audio signal.
The RCD input signal is ignored in full duplex operation.
Refer to section
4.4
-
Connecting to a Radio
for details on wiring the RCD signal to a radio.
NOTE:
A true RCD signal will always prevent transmission in half duplex mode, even if AUDIO
ONLY is selected as the carrier detect source. Therefore, the RCD signal can be externally
controlled to inhibit transmission.
If the RCD input signal is true, the modem will not close the KEY output or transmit carrier. This
prevents the modem from transmitting over another radio transmission.
