Re: IIGS Keyboard

[Rubywand <rubywand@swbell.net>]

Bradley P. Von Haden writes ...
> 

 ....

> 
> 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.

     Thanks for the info. (BTW, Perry D contacted me a week ago
requesting he not receive 'credit' in the FAQs for the S-video cable
idea since it was something he found and was not sure about.)

     If someone knows of a reliable source for GS keyboard cables at a
sane price, this would be a good time to post the information.

     The concern about capacitive loading may be worth considering,
although all kinds of capacitive loading can occur normally as stuff is
tacked onto the ADB chain. Still, it would be good to get more input
from users and, also, do some testing. 



Rubywand