5902 Bell 202 Modem Hardware Manual
October 19, 2007
19
4.5.2
Carrier Detect Delay
The CD0 and CD1 switches control the carrier detect delay. There are four options. The delays are
longer when the RCD option is enabled, as radios usually require longer delays.
DELAY (RCD open)
DELAY (RCD closed)
19 ms
33 ms
30 ms
67 ms
51 ms
133 ms
92 ms
265 ms
CD0 switch
Open
closed
Open
closed
CD1 switch
Open
open
Closed
closed
The modem will not receive data until carrier is detected. The carrier detect period ensures that a
sustained carrier is present, before the receiver is enabled.
The carrier detect delay prevents noise from being received at the beginning of each reception. The
carrier must be present for this period, before it is detected as carrier. If the carrier drops out during
the detect period, it will not be detected as carrier.
NOTE:
The carrier detect delay switch settings must be less than the RTS/CTS delay of the
transmitting modem, plus any propagation delays such as radio transmitter and radio
receiver attack times.
4.5.3
Carrier Loss Delay
The CL0 and CL1 switches control the carrier loss delay. There are four options. The duplex (FDX)
switch affects the selection of one option (see section
4.5.8
-
Duplex
).
CL DELAY
(+/
– 1.5 ms)
6 ms
10 ms
23 ms
39 ms FDX
Constant Carrier HDX
CL0 switch
Open
closed
open
closed
CL1 switch
Open
open
closed
closed
Carrier can be lost in the middle of a transmission, especially with radio communication. The carrier
loss delay allows the carrier to disappear for brief intervals, without the loss causing the modem to
test for the carrier detect delay when the carrier reappears.
The carrier loss delay also determines how long the modem waits after reception terminates, before
it allows transmission in half duplex mode. Therefore, the carrier loss delay is also a transmission
turnaround delay for half duplex mode.
4.5.3.1
Use With Constant Carrier Reception
In constant carrier, half duplex (HDX) mode a carrier signal is continuously received on the receive
pair, Rx + and Rx -. Selecting constant carrier reception mode allows the modem to transmit data on
the transmit pair, Tx/Rx + and Tx/Rx -, even though received carrier is detected. Close switches
CL0 and CL1 when connected to a constant carrier source in a four wire, half duplex system.
4.5.3.2
Use With Full Duplex
The carrier loss delay always functions in full duplex mode (see section
4.5.8
-
Duplex
). The constant
carrier option is not available, since the modem can transmit at any time when in full duplex.
