AXE027 3.5mm wiring question

OLDmarty

Senior Member
Hi all,

Regarding the AXE027 3.5mm stereo-plug wiring, is it wired differently (from the 'normal' standard) for a reason?

If i rewired my plug so the shield outer is now Ground (0v) will it become an issue where the data pin *might* come into contact with the shield outer during insertion and damage the AXE027 internal chip-driver????

I was just about to convert my 3.5mm connectors to 'normal' wiring, where tip and ring can be TX/RX data, and shield is Ground (0v), but will i introduce a problem as mentioned above?

Thanks in advance.
Marty
 

hippy

Senior Member
The 3.5mm wiring order is as it is for historical reasons; it is wired the same as 3.5mm jacks were when digital cameras first got serial interfaces.
 

OLDmarty

Senior Member
The 3.5mm wiring order is as it is for historical reasons; it is wired the same as 3.5mm jacks were when digital cameras first got serial interfaces.
I find it odd, that it isn't wired with shield as GND(0v) like traditional Audio for the past 17 decades ;-)

There must be a logical reason why digital cameras of that era chose to NOT use the shield connection on their plug as GND, and again i *think* it comes back to something about tip & sleeve touching the shield surround during plugin and potentially damaging a driver IC on the host end.

Anyhoo
 

hippy

Senior Member
i *think* it comes back to something about tip & sleeve touching the shield surround during plugin and potentially damaging a driver IC on the host end.
Possibly but none of the combinations are perfect. A single contact isn't a circuit, and all three contacted is by default okay, which leaves six possibilities where there will be some odd electrical configuration.

Note that the camera / PICAXE wiring is at the top right. O=PICAXE Out, I=PICAXE In, o=PC Out, i=PC In -

Code:
       I    O    0V              O    I    0V
   ----V----V----V---.       ----V----V----V---.
----.----.----.      |    ----.----.----.      |
  o |  i | 0V  >     |      i |  o | 0V  >     |
----^----^----'      |    ----^----^----'      |
   ------------------'       ------------------'
Code:
       I    0V   O               O    0V   I
   ----V----V----V---.       ----V----V----V---.
----.----.----.      |    ----.----.----.      |
  o | 0V |  i  >     |      i | 0V |  o  >     |
----^----^----'      |    ----^----^----'      |
   ------------------'       ------------------'
Code:
       0V   I    O               0V   O    I
   ----V----V----V---.       ----V----V----V---.
----.----.----.      |    ----.----.----.      |
 0V |  o |  i  >     |     0V |  i |  o  >     |
----^----^----'      |    ----^----^----'      |
   ------------------'       ------------------'
I suspect the original motivation has something to do with contact switching being available on the tip contact, which will only activate when the plug is fully inserted, that being used to indicate to the CPU when the jack is inserted -

Code:
                       -.-
  Tip                  .|.
____________________   |_|
\    /  /|\         |   |
 \  /    `----------|---^---> 1 = Inserted
  \/                |
                   _|_ 0V
I believe that once that was settled upon, everything else followed from there.

It may also be that the O-I-0V top right choice was made so ring switching could be used to keep the input to the CPU UART circuit held low until the cable was inserted ( partially or fully ) without using a pull-down.
 
Last edited:
There is just a few good reasons for the way the downloadcable is connected.
First is called ESD.
The first contact is made between the external ground and a low impendance output. neutralising static electricity.
Sliding in further.the gnd of the serial cable (your PC) is connected with the input of the Picaxe and the PC input is connected with the input of the picaxe. This is no problem.
Fully inserted the output of the PC (sleeve) is connected whith the input of the picaxe.

So:
- As the sleeve is the output of the PC because it should never short with ground and never be connected with another output.
- The tip should not be the PC input because it is the first contact made and subject to ESD.

This leaves the only right combination and this is the one supplied in the Picaxe downloadcable and Picaxe documentation.

Just my 0.02 cts.

Erik
 
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