In the standard TNC2 clone when the DCD (a.k.a Receive) LED comes on the TNC cannot key the PTT line on the packet radio. The TNC can [must] still receive packets. This is the basis of the time-sharing concept of a packet radio channel.
The DCD in a 1200b TNC will come on with any noise on the radio channel - therefore the radio must be used with squelch on. 9.6kb packet does not use the same DCD scheme. The receive audio is typically derived directly from the discriminator prior to amplification/conditioning for voice audio or squelch. The DCD will come on only when a legit 9.6kb packet signal is on the channel - that is built into the 9.6kb modem. The radio volume or squelch controls have no bearing on 9.6kb packet. Early on a 'mod' was developed -"State Machine DCD" - that provided 'information coherence' for 1200b signals - thus allowing the receiver to be run open squelch - the DCD would only light with a legit 1200b signal present. Later this was included standard in many TNCs - typically MFJs - or provided as an optional modification. All this did not solve the problem of shared voice/packet channels with voice signals present. In the TNC2 Clone specifications pin five of the standard 5-pin DIN TNC to radio connector is for (external) 'squelch'. Originally this was intended for use on a shared voice/packet channel to provide DCD to the TNC when voice stations (i.e. repeater) were being received. It was typically driven by a COR in the packet radio receiver, signifying a voice station was on the channel. This way the DCD would come on with a packet station or with a voice station on channel and inhibit packet transmissions accordingly. Kantronics uses a DB9 connector for 1200b or DB15 for 9.6kb. Both contain provisions or external squelch input. Kantronics also has the CD command [internal or software] for 1200b which allows the TNC to be used on receivers with open squelch. In CD = software the DCD will only come on when there is a legitimate 1200b packet signal being received. It should be possible to run two TNCs with one radio - using the combinations of DCD control - so that only one TNC can transmit at a time - and neither will 'step on' legit packet or voice transmissions on the channel. It may be necessary to tweak the timing parameters in each TNC . Other considerations such as receiver audio impedance/levels may need to be taken into account. The Kantronics KPC-9612 dual port 9.6kb/1200b TNC has provisions for using 'one radio' if both baud rates are used on the same packet channel. NOTE: That KPC-9612 on the shelf collecting dust in the closet is still the same TNC manufactured today. We use many of them here on the Florida Layered Packet Network - but with two radios - 2M 1200b and UHF 9.6kb. http://www.fadca.org/map/index.html bud N0IA
