W076/M452 replacement cards

From: vrs <vrs_at_msn.com>
Date: Thu Nov 27 23:52:18 2003

> > > > Is it that easy to mod the TTY? Where do you get the relay?
> > >
> > > The instructions for doing this are in at least one DEC printest,
similar
> > > instructions are in the Intel MCS8i manual, and doubtless elsewhere.
> >
> > I found them. I'd still need relay part number 4915. There are also
some
> > things called "thyractors" called for.
>
> IIRC, a thyractor is some kind of overvoltage clamping device. You could
> probably replace it with a pair of zener diodes in inverse series, or
> with a VDR.

I Googled for thyractors, which seem to be chokes combined with thyristors.
Then I googled for thyristors, which are roughly as you describe. None of
which helped me understand the TTY mods.

> >
> > > No idea where you'd get the relay these days. I wonder if you could
> > > design a solid-state switching circuit that would work, with an
> > > opto-isolator on the input
> >
> > Maybe, if I had specs for the relay. If I had the specs, I might even
be
> > able to tell if I have a prayer of delivering sufficient drive current,
>
> I have no idea what the official spec is, but the DEC cards didn't supply
> that much current (I would guess at less than 100mA, but that is a guess).

Curiouser...my "Figure 5-7 Teletype Connections" shows a resistor in series
with the relay in the 4915 reader control. Also, my inspection of one of
my M706 modules shows the resistor is 1/2W. That implies that at 150 ohms
(if I applied my IIR rule correctly), there can't be more than 57ma through
it. I'd love to know the value of that resistor in series with the relay!

> > though I'd still prefer to test on real hardware before having a bunch
of
> > boards manufactured.
>
> Agreed.... Pity my ASR33 is almost impossible to get to, or I'd measure
> the relay coil current for you (mine has an official DEC modification PCB
> in it).
>
> -tony
>
Received on Thu Nov 27 2003 - 23:52:18 GMT