23 Jan
2018
23 Jan
'18
12:19 p.m.
From: Grant Taylor
I can fairly clearly see the RG-8/U on the side of the
cable that David
is holding ... Sure, there was probably a better alternative that came
along after, with better shielding and marking bands.
You keep mixing up the 3 Mbit and 10 Mbit. _They were not the same_. (I
_really_ need to retake those photos with a ruler in them...)
The stuff with better shield, marking bands, etc is 10 Mb; it's about 1.05cm
in diagmeter. The black stuff (the stuff Dave is holding in the video) is 3Mb;
the piece I have is .95 cm.
The video, perhaps confusingly, shows a mix of 3Mb and 10Mb gear. The small
aluminium sheet transceiver (the one attached to the black cable when Dave is
doing his spiel) is the 3Mb one; the others are 10Mb.
Noel
23 Jan
23 Jan
12:27 p.m.
On Jan 23, 2018, at 3:19 PM, Noel Chiappa via cctalk
<cctalk at classiccmp.org> wrote:
The Ethernet spec says that the cable OD is in the range .365 to .415 inch, which is 9.27
to 10.54 mm. The nominal OD of RG-8/U is .405 inches, or 10.28 mm, which is within spec
for Ethernet cable.
One place where the two cable specs differ is in the velocity factor, 0.66 for RG-8/U and
0.77 for Ethernet cable. That relates to the dielectric -- solid polyethylene for RG-8/U
and foamed material (unspecified) for Ethernet. Also, Ethernet requires a solid inner
conductor (for the tap) while RG-8/U may come stranded. (Maybe only in some variants,
I'm not sure.) And there are the stripes, of course, but those have no electrical
significance. You can use a tape measure if you don't have the stripes.
paul
From: Grant Taylor
I can fairly clearly see the RG-8/U on the side
of the cable that David
is holding ... Sure, there was probably a better alternative that came
along after, with better shielding and marking bands.
You keep mixing up the 3 Mbit and 10 Mbit. _They were not the same_. (I
_really_ need to retake those photos with a ruler in them...)
The stuff with better shield, marking bands, etc is 10 Mb; it's about 1.05cm
in diagmeter. The black stuff (the stuff Dave is holding in the video) is 3Mb;
the piece I have is .95 cm. 
4:04 p.m.
On 1/23/18 12:27 PM, Paul Koning via cctalk wrote:
>> The stuff with better shield, marking bands, etc is 10 Mb; it's about 1.05cm
>> in diagmeter. The black stuff (the stuff Dave is holding in the video) is 3Mb;
>> the piece I have is .95 cm.
> The Ethernet spec says that the cable OD
is in the range .365 to .415 inch, which is 9.27 to 10.54 mm. The nominal OD of RG-8/U is
.405 inches, or 10.28 mm, which is within spec for Ethernet cable.
He's holding up the wrong stuff if it's RG-8
Experimental Ethernet is 75 ohms.
see
http://bitsavers.org/public_html/pdf/xerox/alto/ethernetEthernet_Transceive…
Drive Voltage: +3 volts (Nominal) into >> 37.5 ohms <<
There is a clue in the PARC Ethernet Blue and White about this where they mention they
used CATV technology
(where the vampire taps come from ) but they don't actually say there that they used
75 Ohm cable, terminated at both ends.
4:24 p.m.
On 1/23/18 4:04 PM, Al Kossow via cctalk wrote:
He's holding up the wrong stuff if it's RG-8
Experimental Ethernet is 75 ohms.
I just went out to storage and got the run of cable that was used with a Dorado.
I'll take some pics of it later.
It is orange, looks like RG-6, is Malco 98278 (Xerox PN 117P80660)
I measured the terminator resistance to be 75.0 ohms.
4:33 p.m.
On 01/23/2018 04:04 PM, Al Kossow via cctalk wrote:
There is a clue in the PARC Ethernet Blue and White
about this where
they mention they used CATV technology (where the vampire taps come
from ) but they don't actually say there that they used 75 Ohm cable,
terminated at both ends.
Wasn't that ChaosNet?
--Chuck
24 Jan
24 Jan
1:05 p.m.
On 2018-Jan-23, at 12:27 PM, Paul Koning via cctalk wrote:
The Ethernet spec says that the cable OD is in the
range .365 to .415 inch, which is 9.27 to 10.54 mm. The nominal OD of RG-8/U is .405
inches, or 10.28 mm, which is within spec for Ethernet cable.
One place where the two cable specs differ is in the velocity factor, 0.66 for RG-8/U and
0.77 for Ethernet cable. That relates to the dielectric -- solid polyethylene for RG-8/U
and foamed material (unspecified) for Ethernet. Also, Ethernet requires a solid inner
conductor (for the tap) while RG-8/U may come stranded. (Maybe only in some variants,
I'm not sure.) And there are the stripes, of course, but those have no electrical
significance. You can use a tape measure if you don't have the stripes.
I was attempting some calculations to see if I could derive the 2.5M transceiver spacing
and was wondering what the velocity factor for the cable was, as it should affect the
transceiver spacing in theory.
I haven't seen any pictures during this thread of the transceivers we used with the
10MB yellow hose - heavy gauge metal boxes about 3" * 4" * 1" with N
connectors. I remember piling up the 3 ft diameter loops of yellow coax in the machine
room where you had a bunch of machines together and didn't need the cable lengths
between them, until the DEC DELNI (ethernet AUI hub) cleaned up that mess.
2:11 p.m.
On Jan 24, 2018, at 4:05 PM, Brent Hilpert via cctalk
<cctalk at classiccmp.org> wrote:
On 2018-Jan-23, at 12:27 PM, Paul Koning via cctalk wrote:
The velocity factor is specified as 0.77. The Manchester encoding of 10 Mb Ethernet means
the dominant frequency is 10 MHz, which in the coax would have a wavelength of 23.1 meters
(0.77 * c / 10e6). So the 2.5 meter spacing is 0.1082 wavelengths, i.e., a number chosen
NOT be be a round value. If you look at the integer multiples of that value you'll
find hardly any that are close to an integer; the first one I see is 37x which is 4.004.
This ensures that there is very little in the way of coincident reflections.
There are lots of other spacings that would work, for example 2.0 meters is also a decent
choice -- all that is required is "pick a spacing such that nearly all the tap points
are not round multiples of 1/2 wavelength of 10 MHz apart". 2.5 seems a fine choice
given the max cable length (500 meters) and station count (100); there is no real benefit
in picking a smaller value.
If you're using actual RG-8/U (VF = 0.66) then the answers come out different, of
course.
paul
The Ethernet spec says that the cable OD is in
the range .365 to .415 inch, which is 9.27 to 10.54 mm. The nominal OD of RG-8/U is .405
inches, or 10.28 mm, which is within spec for Ethernet cable.
One place where the two cable specs differ is in the velocity factor, 0.66 for RG-8/U and
0.77 for Ethernet cable. That relates to the dielectric -- solid polyethylene for RG-8/U
and foamed material (unspecified) for Ethernet. Also, Ethernet requires a solid inner
conductor (for the tap) while RG-8/U may come stranded. (Maybe only in some variants,
I'm not sure.) And there are the stripes, of course, but those have no electrical
significance. You can use a tape measure if you don't have the stripes.
I was attempting some calculations to see if I could derive the 2.5M transceiver spacing
and was wondering what the velocity factor for the cable was, as it should affect the
transceiver spacing in theory. 
2:18 p.m.
On Wed, Jan 24, 2018 at 1:05 PM, Brent Hilpert via cctalk
<cctalk at classiccmp.org> wrote:
I haven't seen any pictures during this thread of the transceivers we used with the
10MB yellow hose - heavy gauge metal boxes about 3" * 4" * 1" with N
connectors.
I have a 3COM 3C100 transceiver that must have come along with some
old Sun servers I acquired a few years back. It's a heavy black box
about 4" x 5" x 1" with two N connectors and the 15-pin AUI connector.
I opened it up just now to look inside to see why it is so heavy. The
whole thing is potted inside with a solid black compound so there is
nothing to see.
2:26 p.m.
On 1/24/18 1:05 PM, Brent Hilpert via cctalk wrote:
I haven't seen any pictures during this thread of
the transceivers we used with the 10MB yellow hose - heavy gauge metal boxes about 3"
* 4" * 1" with N connectors. I remember piling up the 3 ft diameter loops of
yellow coax in the machine room where you had a bunch of machines together and didn't
need the cable lengths between them, until the DEC DELNI (ethernet AUI hub) cleaned up
that mess.
likely 3Com
http://www.piercefuller.com/library/10126.html
ones with vampire taps
http://www.computerhistory.org/collections/catalog/102625376
is the dec H4000
Cabletron ST-500
http://www.computerhistory.org/collections/catalog/102642433
http://www.computerhistory.org/collections/catalog/102677160
also was availble with N connectors
http://www.computerhistory.org/collections/catalog/102682788
dec H4005
2307
days inactive
2308
days old
8 comments
6 participants
participants (6)
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aek＠bitsavers.org -
bhilpert＠shaw.ca -
cclist＠sydex.com -
glen.slick＠gmail.com -
jnc＠mercury.lcs.mit.edu -
paulkoning＠comcast.net