25 Aug
2016
25 Aug
'16
2:05 p.m.
Tuesday night, I got a small 2 node thicknet segment going between my DEC
VXT2000 and my laptop. I took a quick demo video,
https://youtu.be/A5T2GlAN2N4
The hardware setup is as follows: I have a twisted pair (10BASET) to AUI
converter (a Lantronix LTX-C) (not to be confused with a 10BASET MAU) which
is connected to (and driving) a Cabletron coax MAU with 10BASE5 vampire
tap. This is connected to a small piece of 10BASE5 coax, which is
terminated with a 47 ohm resistor pushed into each end. This coax runs over
to an identical MAU with vampire tap, and an AUI cable connects it to my
DEC VXT2000.
Anyway, I hope to demo a larger 10BASE5 network at VCF MW, but I need
outside help since I only have two AUI capable devices (the LTX-C and my
VXT2000). So if you're going to be at VCF MW, and you've got some gear with
AUI on it, bring it along and I can hook you up. I have about 50m of cable
I think, and plenty of MAUs, but I'm short on AUI cables with both ends on
it, so bring along any you have if you'd like to connect.
Regards,
Joe Zatarski
25 Aug
25 Aug
2:22 p.m.
You will need to terminate the coax.
(terminator)---------------<tap>------. . .
----<tap>----------------(terminator).
Typical installations would use "N" connectors.
For example http://tech.mattmillman.com/projects/10base5/
The cable segments on each end should be (at least about 3 feet) before the
first and last tap.
I say at least 3 feet, since I've forgotten the recommended length and the
whole transmission line reflection discussion of what a terminated
transmission like looks like was very heated at times. A "perfect' line vs.
a real line and all that.
Anyway, you need to terminate the line or your going to have so many issues
you may not even get a packet to make it from one end of the line to the
other.
On Thu, Aug 25, 2016 at 2:05 PM, Joseph Zatarski <jzatar2 at illinois.edu>
wrote:
Tuesday night, I got a small 2 node thicknet segment
going between my DEC
VXT2000 and my laptop. I took a quick demo video,
https://youtu.be/A5T2GlAN2N4
The hardware setup is as follows: I have a twisted pair (10BASET) to AUI
converter (a Lantronix LTX-C) (not to be confused with a 10BASET MAU) which
is connected to (and driving) a Cabletron coax MAU with 10BASE5 vampire
tap. This is connected to a small piece of 10BASE5 coax, which is
terminated with a 47 ohm resistor pushed into each end. This coax runs over
to an identical MAU with vampire tap, and an AUI cable connects it to my
DEC VXT2000.
Anyway, I hope to demo a larger 10BASE5 network at VCF MW, but I need
outside help since I only have two AUI capable devices (the LTX-C and my
VXT2000). So if you're going to be at VCF MW, and you've got some gear with
AUI on it, bring it along and I can hook you up. I have about 50m of cable
I think, and plenty of MAUs, but I'm short on AUI cables with both ends on
it, so bring along any you have if you'd like to connect.
Regards,
Joe Zatarski
10:16 p.m.
You will need to terminate the coax.
(terminator)---------------<tap>------. . .
----<tap>----------------(terminator).
He said he has connected a 47 Ohm resistor at each end of the
coax. That's close enough to the correct 50Ohm terminal to
work.
Anyway, you need to terminate the line or your going
to have so many issues
you may not even get a packet to make it from one end of the line to the
other.
Correct. You won't. No matter how short the coax is. You will get collisions.
The reason is that the transmitter in a coax MAU is a current source which
effectively develops a voltage across the terminators. The receiver is a
voltage detector. A collision is sensed by the MAU if it sees more voltage
across the coax than it should. This (on a correctly terminated cable) means
that 2 transmitters are putting current into the cable/terminators at the
same time.
But if the termination resistors are too high or missing (even if only one
is missing) then a single transmitter's current will develop enough voltage
across the coax to be detected as a collision.
-tony
10:26 p.m.
On Thu, Aug 25, 2016 at 11:16 PM, tony duell <ard at p850ug1.demon.co.uk> wrote:
47 and 56 were resistors of choice back in the day for make-shift
termination on thin-net networks in a pinch.
10base5 also had rules for minimum bend radius as well as tap locations
to be at the maxima of the reflection point. For early gear, failure to
put it at a vibration node would often result in unreliable behavior, though
I can't recall if that included collisions or not.
Warner
You will need to terminate the coax.
(terminator)---------------<tap>------. . .
----<tap>----------------(terminator).
He said he has connected a 47 Ohm resistor at each end of the
coax. That's close enough to the correct 50Ohm terminal to
work. Anyway, you
need to terminate the line or your going to have so many issues
you may not even get a packet to make it from one end of the line to the
other.
Correct. You won't. No matter how short the coax is. You will get collisions.
The reason is that the transmitter in a coax MAU is a current source which
effectively develops a voltage across the terminators. The receiver is a
voltage detector. A collision is sensed by the MAU if it sees more voltage
across the coax than it should. This (on a correctly terminated cable) means
that 2 transmitters are putting current into the cable/terminators at the
same time.
But if the termination resistors are too high or missing (even if only one
is missing) then a single transmitter's current will develop enough voltage
across the coax to be detected as a collision.
10:28 p.m.
47 and 56 were resistors of choice back in the day for make-shift
termination on thin-net networks in a pinch.
Thinwire (10base2) and 10base5 are electrically very similar. There
is a difference (I forget what, but I seem to remember if you used the
common 8392 transceiver IC there is a small passive component (resistor,
perhaps diode) that's fitted for one and not the other), but it is small. You
can often get away with using BNC-N adapters to connect one to the other.
51 Ohms is a prefered value and the nearest to 50 ohms you will easily
get. 47Ohms should be fine too. (From the method used for collision
detection it is generally better for the termination to be too low rather than
too high.) Actually a 47 ohm at one end and a 56ohm at the other has almost
the correct DC resistance (25.5 ohms, as against 25 ohms for a pair of 50
ohm terminators) and is probably the best you'll get with common
parts.
-tony
26 Aug
26 Aug
3:34 a.m.
On 26/08/2016 06:26, Warner Losh wrote:
10base5 also had rules for minimum bend radius
True, because bending the cable alters the geometry and introduces
impedance discontinuities, though (to be picky) the allowable bend
radius varies between cable manufacturers because the precise cable
construction dictates the tightest bend that wouldn't upset the
impedance. What I recall from the standard is that cables must support
a bend radius of 254mm /or less/ in order to be flexible enough for
reasonably easy installation. A sort of "maximum minimum bend radius".
as well as tap locations to be at the maxima of the
reflection point.
Actually it's to /avoid/ maxima and thereby to ensure things are out of
phase, minimising adverse interference effects. The node positions are
at 2.5m intervals, a distance which is chosen so that taps and
terminators are very unlikely to be exact wavelengths apart and hence
will /not/ be at maxima, so conflicting signals will be out of phase.
IIRC correctly it's deliberately not quite 1/19th of the wavelength.
For the same reason, cable sections are supposed to be odd multiples of
the half-wavelength of the signal (23.4m, etc).
For early gear, failure to
put it at a vibration node would often result in unreliable behavior, though
I can't recall if that included collisions or not.
--
Pete
Pete Turnbull
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days inactive
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ard＠p850ug1.demon.co.uk -
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