Re: IIGS Keyboard-- using SVideo cable-- follow-up

[Rubywand <rubywand@swbell.net>]

Bradley P. Von Haden wrote:

 ....

> 
> While the connectors on ADB cables and S-video cables (no such thing
> as S-VHS cable) are the same, the wiring inside is not.  Here is an
> old post from Derek Taubert from my archives:
> 
>     From: derek@slab.isdn.uiuc.edu (Derek Taubert)
>     Newsgroups: comp.sys.apple2
>     Subject: Re: Desktop case: phase 1
>     Date: 28 Jun 1996 00:12:15 GMT
> 
>     S-Video has two signals which both need individual 75 ohm
>     transmission lines. The other 2 pins are ground, and connected to
>     the shielding around the two signals.  So in an S-Video cable you
>     have two coax cables, while an ADB cable has 4 individual wires.
>     The shielding of the 75 ohm lines will most likely be isolated
>     from each other, so you do actually have 4 separate current paths
>     (so it MIGHT work), but the crosstalk and impedance is likely to
>     mess up the ADB digital signal.
> 
>     Just buy the right cable for the job. I'm sure you can find a mac
>     place that sells exactly what you're looking for.
> 
>     --
>     Derek Taubert - http://slab.isdn.uiuc.edu/~derek
>     taubert@uiuc.edu - NeXTMail welcome
>     * Check out TCP/IP for the Apple IIgs!
>     * http://www.winternet.com/~taubert/gstcp.html
> 
> Furthermore, according to Harold Hislop, the problem with using
> S-video cables in place of ADB cables is that the S-video cable are
> more heavily shielded -- leading to "too much capacitance between
> Ground and the ADBData line".  This can lead to blowing out the ADB
> chip.


     Okay; after the earlier post, I extracted the SVideo cable from our
main TV and did some measurements.

     First, it turns out that our SVideo cable (a Radio Shack model with
gold pins) is about the same total length (roughly 6 inches shorter) as
the standard IIgs Keyboard ADB cable.


     o  o    "top pins"

   o      o  "bottom pins"
     ___
    |___|


     Resistance checks on the four lines showed 1-2 Ohms (plug to plug)
on all four pins of the std ADB cable and a connection between the outer
shells of the plugs (for the shield).

     The SVideo cable shows 1-2 Ohms on the top two pins and less than
an Ohm for the lines connected via the bottom two pins. There is no
connection between the outer shells from end to end. None of the lines
was connected to any other line.

     It appears that, as claimed, the four lines of the standard ADB
cable run together inside a shield whereas the SVideo cable's four lines
consist of two wires each with its own shield. Also, the low R for the
SVideo cable's bottom pins indicate that these are the shields.

     To check out the above, a search led to a description of the pinout
of the standard SVideo cable:


Charles E. Fox (1996/11/13) on the rec.video newsgroup posted ....

>>
        Svideo connector (from Panasonic C-1331YC monitor manual)

        Pin 1 ground    Pin 2 ground    Pin 3  Y signal Pin 4 C signal
<<

     Pin 1 & 2 are the bottom two pins, which confirms the conclusion
from the measurements.

     According to the IIgs Hardware Reference, the top pins carry +5V
and Ground. One of the bottom pins is Data and the other is labeled
"Reserved".

     So, when using an SVideo cable as a substitute for the ADB cable,
the Data line is going to be a shield and +5 (or Ground-- probably +5
assuming Left-Right pairing) will be inside the shield. This is kind of
a quirky-looking setup. On the other hand, there is no obvious reason
why it should hurt anything.

     A check of the IIgs schematic indicates that the drive for the Data
line is a separate transistor-- i.e. not the ADB Microcontroller IC. If
something is going to blow due to an overload, it should be the
transistor (which would be fairly easy to replace).

     Anyway, there seems to be no obvious reason why an SVideo cable
should not work as a substitute for the standard GS KB cable. So, I
swapped in the SVideo cable for the ADB Keyboard cable on our IIgs and
powered-up the computer. Nothing blew and the mouse and keyboard worked
fine.

     One downside of using the SVideo cable is that it is not nice and
coiled like the standard GS Keyboard cable; another may be increased
TV/Radio interference. Also, the color (black) is not a good match.

     If you can find a standard ADB cable for your GS keyboard, that is
the way to go. Otherwise, an SVideo cable seems to be an okay
substitute.
 


Rubywand