NeXT '040 Cubes - test-drb@ccmp.vtda.org

21 Mar 2001

On Mar 21, 21:22, Tony Duell wrote:
...
  >
 > On Mar 21, 10:07, MTPro(a)aol.com wrote:
 >
 > > << First, put a 50-ohm terminator on that connector, so the NeXT
thinks
...
  > it is connected to a live (but very small!)
network. >>
 > >
 > > Is this something I can pick up at say Radio Shack? Does it looks kind
of
...
   like a little
metal cap that fits on the coaxial connector? 
 Wait a second. Standard ethernet has a 50 Ohm terminator at each end of
 the cable. so the DC resistance between the core and the shield is 25 ohms.

Yes, but the standard method of terminating a single (isolated) point is to
use ONE terminator (maybe not for Vaxstations :-)).

...
  According to all the notes I have on how ethernet
transceivers really
 work, the transmitter is a current source, which develops a voltage
 across this impedance. The receiver is a voltage sensor. If 2
 transmitters send at the same time you get a greater-than-normal voltage
 on the cable, which is what is detected as a collision. 
...
  But of course, _1_ transmitter will develop twice the
voltage across 50
 ohms as across 25 ohms. Which means having 1 terminator on the network
 (however short -- even just on the network connector) will generate
 'collisions' all the time 
I've not looked that closely at the internal circuitry of a transceiver,
but I can assure you that that's NOT what happens with any transceiver I've
tried.  Having no terminator at all does indeed make many transceivers
believe there are constant collisions, but I've always found one to be
enough to prevent that, providing it's directly on the transceiver.  The
official method for SGIs is to put a microtransceiver with a single
terminator on the AUI connector.

...
  You should use a non-inductive resistor here. Some
cheap ready-made
 terminators are anything but... 
Yes, and it should also be around 1/2 - 1 watt.  Many terminators use 1/4
watt and get away with it, but the nominal voltage is 10V (and hence from
P=V^2/R, and allowing a 50% duty cycle, you want 1W).

...
  > types: those with a characteristic impedance of
50 ohms, used for test
 > equipment, thin Ethernet, etc; and those with a characteristic impedance of
...
   75 ohms,
mostly used for TV and video signals). 
 IIRC, the central pin is of slightly different diameters (IIRC the 75 ohm
 one is thinner). If you put the thin-pinned type of plug onto the other
 type of socket, it won't always make good contact. If you do the reverse
 you can damage the socket contact.

 And of course putting mixed connectors together will cause a (small)
 mismatch which can be a bad thing. 
Yes, they are different, and I *think* that's the right way round :-)

-- 

Pete						Peter Turnbull
						Network Manager
						Dept. of Computer Science
						University of York