[ih] More Topology, Packet Radio

[Noel Chiappa]

    > From: Lawrence Stewart

    > I guess I am surprised by the comments here about the subleties of the
    > 1822 distant host signaling. I dont think the Alto board had
    > optoisolaters and it did work in both local and distant host modes

The thing is that the 1822 spec _looks_ like it's symmetrical between the Host
and IMP sides, but it's not, really - not 100.000%. The IMP end of the DH
interface did have optoisolators (1822, May '78 revision, pg. 4-24: "DC
isolation is done at the IMP end of the cable ... This isolation is
accomplished by optically isolating the signals.").

So if you had i) a host 1822 interface which didn't have that (because such
wasn't required for a _host_ interface), and ii) tried to use said host 1822
interface to emulate an _IMP_, to another host, and iii) the other host's 1822
interface played fast and loose, and _depended_ on there being isolation at
the 'IMP' end...  it wouldn't work.

Now that I look at the 1822 DH stuff, it (pg. 4-26) "drives the odd-number
connector pin of each pair to +0.5 volts, and the other pin to -0.5 volt".
What the DM ITS' 1822 interface did, IIRC, was tied the - pin to the _host's
ground_, producing 1.0V signals (from its perspective) on the other pin. The
problem was that the DH interface _also_ had ground ("the cable shields
should be very solidly connected to the host's signal ground"); so when the -
output was tied to actual ground, then _on an interface which didn't DC
isolate the + and - outputs_, one no longer got 1.0V on the + pin - and it
wouldn't work.

	Noel