Hi Rob,

I've not found the need to have an isolation transformer, but yes, DC blocking is needed to keep the rig from going into transmit.? If you don't have a transformer the blocking is also needed by the TNC, as some (sound cards especially) don't like having a DC bias pushed into them.?

Attached is the circuit I've used.? Component values aren't critical, but if the capacitor is too small you won't get good audio through, and if it's too large the rig will go into transmit for a short time while it charges up.? This is the same circuit as was often used on older HTs.

Note that the PTT signal is assumed to be some sort of "open collector" drive.? If it's being driven by a modem line, you'll need to add that via an NPN transistor buffer.? 2N2222, 2.2k, 1N4001 for a bit of protection.? See the upper circuit in the diagram.

Hope this helps,

Greg? KO6TH

Rob MW0CQU via groups.io wrote:

Afternoon all ,?

Checking my understanding of a perceived challenge here with you all:
before buying the bits and prototyping a cable...?

It seems that plugging the 847's rear 3.5mm data jack into anything with audio isolation transformers
at the far end (thereby effectively dc grounding the tip of the 3.5mm plug) forces the rig into
constant TX irrespective of any remote PTT settings (correct?)?

And therefore, interfaces like the SCU-17 which feature such isolation, require something like a 10nF
blocking cap (depicted as C-BLOCK --||-- below) like (hope this scales) :

RIG END? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? PC INTERFACE END

? ? ? ? ? ?|----------------------------------------|
TIP |RING|EARTH|------------------|EARTH|RING|TIP|
?|----------------------||-----------------------------|
? ? ? ? ? ? ? ? ? ? ? ? ? ? ?C-BLOCK

inline to stop this constant PTT (correct?)?

is there any other way (I cannot see one in the menu?) to disable that data port PTT without rig surgery ?

many thanks and 73 de

'CQU