Apollo stuff

[Paul Koning]

>>>>> "Jochen" == Jochen Kunz <jkunz at unixag-kl.fh-kl.de> writes:

 Jochen> On Wed, 20 Jul 2005 19:26:52 -0400 "Bob Shannon"
 Jochen> <bshannon at tiac.net> wrote:

 >> Actually several other pins are active.  The token ring card can
 >> tell when no cable is installed, so there are at least 2 cable
 >> detect pins.
 Jochen> Most likely it determines this by measuring the impedance of
 Jochen> the two coax connections. But I may be wrong here.

 Jochen> At least I know from some DECstation and VAXstation
 Jochen> framebuffers that they report an error at Power On Self Test
 Jochen> when no monitor is connected.  As these framebuffers use 3W3
 Jochen> they have no extra pins for cable detection. They measure
 Jochen> somehow DC impedance, AC waveform or the like to detect an
 Jochen> open wire.  --

Either would work.  DC impedance is easy -- for a classic
transmission line configuration terminated in the characteristic
impedance at both ends, you'd see a DC impedance of R/2 (37.5 ohms in
this case) if it's hooked up, but R (75 ohms) if there is no
connection. 

A peak voltage sensor would work too -- peak voltage when correctly
terminated is the line driver output peak voltage, but with an open
circuit it's double that.

	paul