Re: Ribbon Cable

I thought the main reason for multiple ground wires is current return. If you have a balanced flow (spatially), the wires are more resistant to cross-talk. Taken to an extreme, you get differential signaling.

Can you explain what you mean by "controlled inductance throughout the length"?

Thanks,
Nate

On Dec 15, 2013, at 3:36 PM, Bil Herd wrote:

> 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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