21 Jul
2005
21 Jul
'05
7:49 p.m.
>>>> "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