[ih] More Topology, Packet Radio

[Lawrence Stewart]

> On 2021, Aug 31, at 6:01 PM, Noel Chiappa <jnc at mercury.lcs.mit.edu> wrote:
> 
>> 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

I understand.  There are some legible schematics of the Alto-1822 at the CHM although the translation from .press to .pdf is spotty.  It looks like it used the TI 75115 differential receiver, which ran off 0 and +5 but would tolerate a +/- 15 v common mode.  There was a 2v reference available connected to the minus inputs for the local host case. No  optoisolators but it would handle a fair amount of offset.  The transmit was 75114 differential drivers. 

Without a negative supply, the transmit signals would be differential, but not centered around ground, so that part wouldn’t look like an IMP but would work with most differential receivers or misapplied single ended receivers.

I found both the 1975 and the 1978 versions of the BBN-1822 report and both say “Ground Isolation is provided by the IMP” but the circuits in Appendix D do not have this. They must be elsewhere in the hardware.  The 1975 version says the signals are transformer coupled and the 1978 version says optically isolated.

Pretty sensible stuff. I wonder if the Packet Radios actually had isolation or transmit signals centered around ground.

-L