Re: IIGS Keyboard

[bpvh@primenet.com (Bradley P. Von Haden)]

Rubywand <rubywand@swbell.net> wrote:

>Jack Somers writes ...

>I recently picked up a very inexpensive IIGS that I would like to
>revive.  I needed a keyboard and mouse for it, which I just
>received in mail.  I found the keyboard without a cable.  It is a
>mac keyboard actually, which I was told would work with the IIGS. 
>I am not sure what cable it needs to be able to be used with the
>IIGS.  Is it what is called "ADB"?  And will this work with my
>IIGS???
>
>     When this question came up several months ago, I was digging
>through catalogs looking for connectors, etc. to make the cable
>and felt pretty good about finding the stuff. Then, this
>information was posted by Perry Dueck:
>
> >>
>     You could try going to your nearest video/audio electronics
>store and purchasing an 'S-VHS video cable' (used with "super
>video input/output" TV monitors and super video vcr's).  Those
>cables are wired and pinned out exactly the same as a GS ADB
>cable.
> <<

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.


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