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disable ptt function on rear DATA jack in the presence of isolation transformers?

 

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
 

开云体育

I don’t see a 10uf capacitor in the FSK out lines?

De Spence G5STO

Sent while portable! please excuse typos.

On 5 Jan 2025, at 14:19, Rob MW0CQU via groups.io <mw0cqu@...> 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
 

开云体育

Sorry skip that…… I just couldn't decipher that on a mobile phone …. So small. There should also be a 2k resistor if the equipment uses and external PTT and does not reply on the high side signal I don’t see that in your drawing.

? If you want to disable PTT then use the TX inhibit lines. If you see the attached image of the Tuner socket then you can use a resistor I believe to tie this TX inhibit to a pull up but you need to ask Andrew Lenton this as ?I believe as it was he that told me about this. I often work remote and when my PC starts up it sometimes pulls RTS or DTR (which one I’m not sure) ?high and puts the radio in TX. I?wanted this to stop as the radio could be out of band listening to something like AM broadcast etc and certainly didn’t want TX there so came up with the idea of inhibiting the TX with a remote relay and resistor. I went with a different solution I the end as my tuner was always inline unless using the CAT connection.

? Sorry for the mixup but I’ve been reading another thread and couldn’t quite get my head around something.

? Hope this helps as the TX inhibit can easily be sequenced to enable and disable.


De Spence G5STO


image0.jpegimage1.png
Sent while portable! please excuse typos.

On 5 Jan 2025, at 16:40, Spencer Tomlinson <unavailable.equal@...> wrote:

?I don’t see a 10uf capacitor in the FSK out lines?

De Spence G5STO

Sent while portable! please excuse typos.

On 5 Jan 2025, at 14:19, Rob MW0CQU via groups.io <mw0cqu@...> 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
 

开云体育

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

FT847 audio interface.jpg

 

开云体育

Oops, forgot the 2.2k in the upper diagram!? See the revised circuit, attached.

Greg? KO6TH


Greg D via groups.io wrote:

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


FT847 audio interface.jpg

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