Building a better "old" computer - test-drb@ccmp.vtda.org

25 Jun 2000

...
 of "better" that you use.  I've recently
been fiddling with techniques for
...
 taking what was, back in 1982, considered a pretty good
implementation of
...
 the general case of Z80 application, e.g. the Ferguson
Big Board, which used
...
 the standard Z80, and NOT the Z80-A which ran at 4 MHz
instead of the
standard Z80's 2.5 MHz.  I've got a number of these boards so I can test
the

In 1982 that was a low end example.  DECs VT180 was a 4 serial port,
4mhz,
no wait states With DD floppy design.  It was ment to go in a VT100 so
video
was not needed.  My NS* S100 crate in 1978 was running at 4mhz even.

In the z80 world there were those that used Z80 peripherals and live with
the limits they imposed and those that went with other parts.  What was
the
limits?  Price, they were not cheap and they were SLOW.  By 1982 a Z80
not running at at least 4mhz was considered slow and by 1983 that would
be 6mhz.  Parts existed to do that.

...
 in place of potentially better chips because they did
make the design dirt
...
 simple and the supported some features that other
devices didn't support,
...
 e.g. Z80 mode-2 interrupts. 

Mode 2 was supportable without Z80 parts, easy and cheap to do.

...
 the Ferguson BigBoard, i.e four parallel ports four
serial ports, local
video and using a parallel keyboard rather than a terminal, using a single
...
 device, i.e. an FPGA or CPLD (take your pick)and one
memory IC.  When you're
...
 done, you 'll have a CPU that operates at about 25
MHz, a double/single
density FDC, the parallel and serial capabilities and other features of the
...
 Ferguson board.  I don't know whether the result
will be better. 

Try a Z180 part at 33mhz, SCC or other all on one chip like the SMC92667
and static ram on a 3x4" board.  Takes very little glue to do that.

The question goes mroe to price and creative engineering.

Allison