[HPCC-Conf] Too Quiet?

[Eric Smith]

Tony wrote:
> As an aside I have a slight objection to calling this microcode. I guess
>  it _is_ the microcode of a ficticious processor that runs user
> bytecodes  as its instruction set (or whatever), but it still seems a
> little odd to  me. Anyway....

I call it microcode because HP called it microcode.  If it were up to me,
I'd simply call it machine code.

> Hmmm... However breif the 'overdrive' pulse, I'd still worry about the
> output driver on the ISA line... Should be OK, but I'd prefer to cut the
> track (which can be repaired to give the same electrical performance, and
> as I _use_ my machines rather than put them in a museum, I don't mind
> about minor mods like cut-n-jumpered tracks.

The probability of failure of a PMOS output due to external overdrive for
less that 10 microseconds is negligible.  Certainly much lower than
the probability of damaging the chip by desoldering it.  The primary
failure mechanism for overdrive is damage caused through heating.  In
10 microseconds not enough heating will occur to cause even a locallized
temperature excursion beyond the rated specs.

> Maybe if I had anything rare I'd worry more.

That's exactly the issue.  I'm the co-owner of an HP-95C.  My partner
is not willing to let me desolder a chip or cut a trace, but I've
convinced him that if I demonstrate the overdrive technique successfully
on a few HP-97 units without any resulting damage, he's willing to let
me do it to the 95C.

One more comment on this technique.  Since it involves corrupting the
normal microprogram flow, it will be necessary to disconnect the printer
of the Topcat to avoid the chance of printhead damage.  People have
damaged the printhead of the 97 when experimenting with NNNs.