8 Aug
2004
8 Aug
'04
8:42 a.m.
On 4 Aug 2004 at 5:18, Tony Duell wrote:
Neat...
Talking of HP using 555 timers, they used one to drive the power-on light
in the HP86 (and I assuem the HP97). The reset input goes to an output on
the memory cotnroller chip (which is asserted if there's a self-test
error), the LED (and series resistor of course) goes between the output
of the 555 and the +6V rail.
So the LED is dark if the machine is off, on steadily if the machine is
on and no error (555 is held reset, so the output is low all the time),
and blinks (controlled by the 555) if there's an error.
IIRC the battery level circuitry in the 9114A and 9114B disk drives does
something similar, with one of the LEDs driven by a 555. In this case the
timing capacitor of the 555 is switched by the battery voltage detector
so the LED either flashses too fast to see (and appears to be on all the
time) or blinks visibly.
Anyway, there's a little extra circuitry
(transistor, a few passives)
to provided a simple amplitude envelope. Not just a sudden start/stop
beep, but one that tails away.
No real benefit from having this, but that _was_ HP for you.
Indeed, the same thing was done in the HP 64000 workstations. They used a
555 timer for the tone generator, a one-shot for the gate, but then added a transistor and an R-C circuit to provide a fast
attack, slow decay
envelope, resulting in a characteristic "Bink!" sound.
That's what the 9830 circuit does too. I can't easily reproduce the
schematic in ascii-art, and I don't have a scanner, but if anyone is
seriously interested I will see what can be done.
-tony