Re: Ribbon Cable

On 2013-12-16, at 00:36, Bil Herd <bherd@mercury-cg.com> 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.

That’s why I got somewhat surprised. We are far from hundreds of MHz here.

> 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.

That’s what they did in the 50 wire (8 bit) SCSI and 40 wire (16 bit...) PATA. Then there was still the 80-wire cable with even more ground lines to go into 66MiB/s...

If the SX-64 problems with the new carts (or carts problems with SX-64) indeed stem from the ribbon cable - Replacing the original ribbon with one having twice the number of wires and grounding every other one should immensely improve the situation.

>  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.

Just one more question: at some point (shortly before SATA became prevalent), instead of flat ribbons, thick round cables for PATA connection became fashionable. If the whole thing was about intertwining signal lines with GND - wouldn’t rolling the whole cable into a round sleeve just ruin the whole idea? How is that?


>  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.

Yes, regular SWR stuff. Like with every antenna feeder, but.. still what frequencies we talk about in an SX-64? The assumed wavelength is in meters range there. I tend to think that it would have to be really badly off charts to cause problems on digital levels. Maybe - as suggested here - it is more of PWR problem. I guess the PSU supplies enough but am not sure about the path (length, capacity).. This is easier to check though.

> 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.

Is there something you yourself use and could recommend?

-- 
SD!
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