Ribbon Cable

I missed the beginning of the conversation but saw a question about ribbon
cable propagation. Depending on the quality of the ribbon you can get a
couple of hundred megahertz (what we call a Teflon ribbon sometimes seen
on ISCSI3)  and even a cheap ribbon cable can be somewhat tamed.

The two things to control is impedance and crosstalk and they have a
common remedy which is t include a lot of ground wires in with the signal
wires.  The ideal situation is every other wire is a ground wire which
drastically reduces the ability of signals to capacitivly or inductively
couple.  Every other wire being being a ground also gives a controlled
inductance throughput the length and then the designer's job is to make
the transition back to the PCB not have huge changes is impedance that
will cause a reflection.  A ribbon cable in this mode will usually have an
impedance of 100-120 ohms so you need to have a receiver that absorbs the
signal by matching the impedance... if the signal gets to the end and
finds a 10K load after traveling down a 120 Ohm pipe the signal will
reject and ring.

If anyone has a question about a configuration and concerned about
propagation give me a shout, there are also a lot of (java based) tools on
the net that help calculate impedance for things like PCB traces and
cables.

Bil

-----Original Message-----
From: owner-cbm-hackers@musoftware.de
[mailto:owner-cbm-hackers@musoftware.de] On Behalf Of Jim Brain
Sent: Sunday, December 15, 2013 11:24 AM
To: cbm-hackers@musoftware.de
Subject: Re: Thanks for the Verilog help

On 12/15/2013 6:32 AM, silverdr@wfmh.org.pl wrote:
> On 2013-12-14, at 21:49, Jim Brain <brain@jbrain.com> wrote:
>
> Really? I didn't think us to be in such frequency range here so that it
would matter that much.
CMD cautioned many times about SuperCPU usage on the SX.  Mine worked, but
that was to across the board.  I'm sure some of it is no doubt hurt by the
fact that the expansion port drives through 3 circuit boards, 1 ribbon
cable, 3 connectors, and adds 8 inches of travel path to the unbuffered
expansion port signals.
>> and EasyFlash and a lot of the newer cart options (Chameleon, 1541U,
etc.) won't work when doing things that require tight timing.  If you
could turn the EF3 KERNAL replacement function off, I bet the EF portion
would work, and 1541U no doubt works as long as the function you are
requesting does not require tight timing.
> I don't have the Chameleon (tried to put my hands on one for some time)
but 1541U-II works well in the very same machine. I mean the KERNAL
replacement function, which is the most timing critical AFAIU.
Without looking at the differences, I cannot explain that.
>
>> It's the cable, pretty much.  I bet if you pulled the case, and ordered
a shielded ribbon cable (or better, created a non ribbon cable option),
the problem would disappear.
> Pardon the ignorance but what's so wrong with the ribbon at the
frequencies we deal here with? Or - generally - with the ribbon. How much
it differs from e. g. traces running parallel across the big C64 PCB? What
with the (even longer) ribbon used for the USER PORT in SX-64?
Well, I think it's probably no worse than an 8" cart expander with
parallel lines, and most such carts won't work if I attach them to 2
X-Panders in series and use the last slot on the second XPander. That's
not a completely fair test per se, but I think the cross talk of those
lines would wreak havoc no matter what tech was used.

Jim

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