Interconnecting classic computers
ard at p850ug1.demon.co.uk
Tue May 6 17:03:50 CDT 2008
> Eric Smith wrote:
> > Fred wrote:
> >> If we are going to discuss the WORST systems, surely we can't leave out
> >> the TRS80 casstte port system that RS came up with for classrooms!
> > Well, don't leave us in suspense. In what way was it done badly?
> > Was it unreliable? Did it fail to meet its design objectives?
> Our school system had one, back in 1981.
> There was a panel that was a mux/demux to 24-ish diskless model 3s. The
I thought the standard one was for 16 slaves to one host, but there's no
reason for that limit.
I seem to rememebr the same hardware could be used with Model 1s, Model 3s
and Cocos, but that the host and slaves had to e the same type of machine.
> cassette in/out from each of these came to the mux/demux box, through a
> selector switch, to the teachers model 3 w/dual floppy system.
> At the start of a lesson, the teacher would tell the students, OK, type
> "CLOAD<RETURN>" now.
> The teacher would load the lesson program into memory, then type "CSAVE"
> to broadcast it to all the listening machines. I believe it was
> bidirectional. To upload, the teacher would select one of the 24
> machines to listen to and the CLOAD/CSAVE was swapped.
>From what I;ve read, anything CSAVEd on the host was broadcast to all the
slaves (obviosuly only those CLOADing would actually receive it), the
selector swithc was used to let the host receive something from a slave
(one machine at a time).
> OK, so it depends on the notoriously unreliable TRS-80 cassette
> interface, and on the cooperation of 24 clueless sixteen year olds.
Muc of the unreliability of that tape interface was due to people using
cheap tapes and the fact that there was a design fault in the CTR80
recorder that Radio Shack shipped with many TRS-80s. Basically that
machine would put glitches on the tape if stopped by the Remote socket in
_play_ mode (!).
There was a fix for this -- solder a 10uF capacitor across the
(DC-supplied) erase head. I read about it in the manaul for some
commerical program, Tandy UK knew nothing about it, so I just did it and
found all my cassette problems went away.
Anyway, this doesn't apply to the 'network' unit, which presulaly was
just a few op-amps to conver the record and replay signal levels. Does
anyone have a schemaitc of that unit?
> What could possibly go wrong?
> Truthfully, it was probably more reliable than CSAVE/CLOAD with a
> cassette since there was no gain/bias/speed variation to contend with,
> and it was probably about the cheapest solution to the problem.
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