Apollo stuff

[Bob Shannon]

There are etches going to the other pins.

----- Original Message ----- 
From: "Paul Koning" <pkoning at equallogic.com>
To: <cctalk at classiccmp.org>
Sent: Thursday, July 21, 2005 2:49 PM
Subject: Re: Apollo stuff


>>>>>> "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
> 
>