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More On RADIO PTT – RX ENABLE Failsafe Feature
The proven design of the
RADIO PTT-RX ENABLE
failsafe feature used in the
RTR-1 is now improved for the NCC-2 and the RTR-2.
In the RTR-2, the purpose of the failsafe circuit is to allow the transceiver to listen to a
receive antenna while reducing, to nearly zero, the chances of accidentally transmitting
into the option modules and RX ANT IN. Therefore, the RTR-2 relay is wired so that
when the unit is turned off, the transceiver (
RADIO
) is connected to the transmit
antenna or amplifier (
MAIN ANT IN
). When the unit is off, you can use your station
normally. See
Figure A
on page 7.
When we turn the RTR-2 on, we still want the same connections, the
RADIO
to
MAIN ANT IN
,
when the transceiver is keyed to transmit, or when we want to listen only to the transmit antenna
(
MAIN ANT
). Okay, let’s tell the RTR-2 when it is safe to switch to receive mode. Well,
unfortunately, we can’t because transceivers don’t put out an “Okay to Receive” signal, they only
send out a “Hey, I’m transmitting signal” on the amplifier keying line (which we connect to
RADIO PTT
).
So when we want to switch the RTR-2 to the receive mode (
RX ANT
), so that the
RADIO
is
connected to the Option Modules and the RX ANT IN(
Figure B
on page 7), we have to be
absolutely certain that the transceiver keying signal, which automatically switches the
RADIO
back
to the
MAIN ANT
, is always there,
OR ELSE!
That transceiver amplifier keying cable
MUST
be
installed. But, how can the NCC-2 know that it is safe to go to the receive mode? It does it with the
RX ENABLE
signal, which is sent on the
shield
of the
RADIO PTT
keying cable to the chassis of
the transceiver, and back on the RF coaxial cable shield.
No keying cable? No receive! Only
transmit.
How can that work? Simple; the
shell
of the
RADIO PTT
connector is
NOT
at RTR-2 chassis
ground. It carries the
RX ENABLE
voltage that is looking for the chassis ground. The keying cable
MUST
be there.
This is the failsafe interlock
.
Let us say it another way: The
RX ENABLE
signal travels from the
RADIO PTT
on the shield of
the keying cable to the transceiver chassis ground. The transceiver amplifier keying signal on the
center conductor takes the RTR-2 out of the receive mode, sending the
RADIO
back to the
MAIN
ANT IN
. Of course, the amplifier keying signal is sent on to your amplifier without delay, through
a Schottky-diode for isolation, via the
ACC PTT
connector. (See the caution on the next page).
The
RADIO PTT – RX ENABLE
keying cable can be as simple as a common RCA audio patch
cable connection from certain transceivers that have an RCA keying connector that has its shell at
chassis ground.
