Z80 / Z84C Swap (Doh!)

drlegendre . drlegendre at gmail.com
Sun Aug 16 21:48:43 CDT 2015

Howdy Eric,

Always a pleasure to hear from you!

The Zilog P/N of the chip is "Z84C0008PEC", and it's further marked "Z80
CPU" with a datecode of "8904" - April of 1989?

In the interim, I've pulled that chip from the Osborne and replaced it with
the 'proper' Z80 from a Timex / Sinclair 1000; I did not record its
markings, other than to note that it was produced in late 82, if I recall.
The system now behaves better, but issues remain as follows:

Started up +sans-KB+, the machine made a slight 'chirp' and a clean display
came up with a message to the effect of "Insert disk in drive A and press
RETURN". I attached the KB and restarted the machine. This time, the
display came up with the same message, but there was something going on
with the video.. it was quite flicker-y, with (scanline?) artifacts
cruising around the screen.

I placed a disk in Drive A and upon pressing RETURN the display immediately
stabilized and went into a loop printing "BOOT ERROR" on successive lines,
eventually scrolling the original boot message off the top of the display.
It did not seem to even attempt seeking the disk - and it behaves the same
way, whether or not a disk is in Drive A. Tried this routine several times,
with minor alterations, and always the same - straight to the BOOT ERROR
scroll and no other activity in the drive.

Drive LED was on in all cases.

So it seems there's more to this, than just a missing CPU. Hopefully my
ham-fisted use of the Z84C hasn't caused any other issues! Oh, and FWIW,
the machine seems to have the 1.4 BIOS and the double-density daughtercard
on the mainboard.

On Sun, Aug 16, 2015 at 9:29 PM, Eric Smith <spacewar at gmail.com> wrote:

> On Sun, Aug 16, 2015 at 6:20 PM, drlegendre . <drlegendre at gmail.com>
> wrote:
> > What I did learn is that Z-80 were made in CMOS versions, and the Z84C is
> > one.
> >
> > So what did I most likely do, here? Hose the CPU for sure? Collateral
> > damage on the board? Both / Neither?
> Actually it's surprising that the CMOS Z80 didn't work. The CMOS parts
> are intended to be a drop-in replacement for the NMOS; unless they
> have an "L" suffix, they are 5V, with TTL-compatible input thresholds.
> I've replaced NMOS Z80 parts with CMOS equivalents in a number of
> systems without difficulty.

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