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SubjectRe: Triton DMA
On Sun, 30 Nov 1997, Rogier Wolff wrote:

> Doug Ledford wrote:
> I'm not sure that you understand what termination is.
> If you have a cable, and hoist one end of it from 0V to 5V in a few
> ns, the other end will take a while before it notices that you did
> that (speed of light at least). Worst case the other end will have a
> completely different wave form, and you will have trouble deducing what
> the other end did to generate this. However with a small trick, you
> can make the wave form travel intact along the wire (the "transmission
> line effect"): you have to make the conductor be surrounded by a
> cylindrical grounded plane. Coax. This effect is reasonably strong: A
> grounded wire on both sides of the conductor in a flat cable is already
> a reasonable approximation of the coax leading to the transmission
> line effect.
> The trouble starts when such a conductor abruptly ends. At that point,
> the travelling wave form simply bounces back and starts propagating
> back to where it came from. However with current performance
> requirements, the source will be wanting to send the next bit along
> the wire by then. This will lead to data corruption. To prevent this
> the transmission line will have to be lead to believe that there is no
> abrupt ending to the cable. This turns out to be relatively easy: a
> simple resistor to a DC level will do. With SCSI termination, they
> have one more trick: The DC level has been chosen such that without
> any drivers the signal level will be somewhere around the switch point
> of the then-common TTL chips. This requires someone to actively pull
> it down to get a reliable "0", or to actively pull it up to make a
> reliable "1". This distributes the burden of transmitting data over
> both the pull-down and the pull-up output transistor, instead of only
> requiring just one. (You also get faster circuits if you use both of
> them.)
> Coming back to what you said, "open" versus "closed and grounded" does
> not make sense to me. If there would be "current driven" drivers
> around, that might make sense, but a SCSI driver simply needs to drive
> the signal level to 0V or 5V. Without termination however you get
> interference from the bounce-back of the travelling wave front.
> Roger.

I wish to compliment you on your concise, correct, and easy to understand
description of transmission lines as they relate to the SCSI bus. Very
few Software Engineers have this debth of understanding and very few
with such an understanding, would be able to explain it as well as you
have done. In my previous life, I was an RF engineer.

Dick Johnson

Richard B. Johnson
Project Engineer
Analogic Corporation
Penguin : Linux version 2.1.65 on an i586 machine (66.15 BogoMips).
Warning : It's hard to remain at the trailing edge of technology.

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