Why is there a minimum spacing?

Not really, except to the extent that: (1) I was doing my graduate work at WPI in 1976-79; (2) I was working at DEC at the time (and going to school at night); (3) My job at DEC focused on the 10 Mb/s Ethernet design and the writing of the original DIX specifications; and (4) DEC was developing the VAX product line at the same time.

Before going to the Ethernet project, I did some signal integrity work on the VAX 11/780 (code-named Star). While the VAX 750 (code-named Comet) and VAX 730 (code-named Nebula) were being designed in the same building where I worked (an abandoned and converted shopping mall in Tewksbury, MA), I didn't have much to do with them. My thesis involved the development of methodology for analyzing signal behavior on long transmission lines, which came directly from my Ethernet work.

-- Rich Seifert Networks and Communications Consulting 21885 Bear Creek Way (408) 395-5700 Los Gatos, CA 95033 (408) 228-0803 FAX

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Rich Seifert
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I used to support some VAX 11/780s. That was where I first saw ethernet. However, the first lan I worked on was part of a Collins 8500C system, which used time division multiplexing on a ring. A device would transmit individual bytes in it's time slot and the receiving device would listen to that time slot. IIRC, the high speed "TDX" loop ran at 8 Mb/s and the low speed "TDM" loop was 2 Mb/s. Each device had relays, which were used to connect it to one of two rings. There was also a loop sync box, that would emit a "chirp", whenever a device joined or left a ring. This was in the late '70s, though (IIRC), the low speed TDM loop originated in the '60s.

Reply to
James Knott

I seem to recall, that when the Intel 386 CPU was announced, it was claimed to be as powerful as the VAX 11/780. I remember the VAX systems we had, included some 16 MB memory boards, that had stacked (one chip welded on top of another) chips, to obtain that amount of memory.

Reply to
James Knott

Actually, it's due to signaling limitations. That's why the maximum distance is different for 10base5, 10base2 and 10baseT. Due to different cable characteristics, they can support the signal for different distances. Note that the velocity factor is similar for all three, so timing is not dependent on cable type. As for the collision distance, a collision has to be detected within 512 bit times. At 10 Mb/s, that's 51.2 uS (round to 50 uS for convenience). Now, at the speed of light, that signal can travel 15 Km. Since there is also the return trip, the maximum length would be 7.5 Km. Allowing for the cable velocity factor, you're down to somewhere around 6 Km, which greatly exceeds the distance for any copper segment. Of course, with full duplex connections, there are no collisions to worry about.

Reply to
James Knott

"Dougie!" said

Sure. But that's lengths of cable where one may reasonably expect small mismatches in the resistive impedance and it makes sense to try to cancel some of the reflections. I'd be interested to know the exact reasoning, since successive bits need bear no relationship to each other, so what cancels for a 1xx1 pattern will reinforce for a 1xx0 pattern.

That aside, my query was about the spacing between taps which is specified as multiples of 2.5m but also referred to as a minimum of

2.5m. Taps produce short pulse reflections, occupying about 5m of cable and it is much easier to think of a lot of short pulses being reflected than to try to use frequency domain concepts like wavelength on what is a wide-band signal.

The multiples vs minimum question keeps cropping up but from what Rich has said I infer there's no technical benefit of sticking to multiples rather than minimums, in fact random spacing is probably better. However, if you do so, you *will* meet a minimum spacing criterion automatically as well as having a simple and practical way of being absolutely sure you're doing it right.

According to Rich, who did the work for the standard, it's all about reflections. I've seen several people earnestly assuring each other that it's to do with triggering collision detection but that would appear to be an urban myth. It seems that even the minimum spacing is a really an "on average" criterion. However, to put that into a standard, you'd have to say something "a maximum of 7 taps in any

10m, a maximum of10 taps in any 20m and a maximum of 12 taps in any 30m, each maximum being taken over the entire length of the [segment]" etc. That would be far too complex and I can understand why they decided on a simple foolproof method.
Reply to
DHP

Henry said

[No it's not -oops!]

Thanks everyone for the ensuing discussion and thanks especially to you, Rich, for giving the definitive facts behind the case.

Reply to
DHP

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