Z80 system

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For several years now I have wanted to build a Z80 system from
scratch. I have read much of Steve Ciarcia's book Build Your Own Z80
Computer. I have recently done quite a bit of designing, and these are
the specs of what I have come up with:

Power is supplied by an old but good ATX power supply. The case will
be wood. The components will be wire-wrapped.

Z80A 4MHz CPU - MK3880N-4

8k x 8 EEPROM (28C64) - $0000-1FFF
32k x 8 RAM (62256) - $8000-FFFF
Simple decoding logic - if A15 is 0, then ROM; if A15 is 1, then RAM

3 to 8 decoder (74LS138) - 8 /CS lines
1st 8255 PIO:
2 ports - Port A goes to Strobed Hex Keyboard, controlled by a
             Port B goes to 20x4 LCD, controlled by HD44780
2nd 8255 PIO:
2 ports - Port A: ADC (8 bit, ADC0804)
             Port B: DAC (8 bit, DAC0801)
Z80CTC - clock for SIO/UART
Z80SIO - Port A: RS-232 9-pin port (MAX232CPE)
              Port B: Cassette interface - adapted from one described
in Ciarcia's book

The machine will be operated by a simple, switch-matrix keyboard, with
which the user can enter Z80 machine code and commands; the LCD will
show a simple shell (>). The following commands can be entered, by

D [address] [address] - dump data from address to address
or - D [register] - dump register contents
P [byte 1] ... [byte N] [address] - push entered data to a spot in
memory starting with [address]
Sc [address] [address] - save data from [address] to [address] on
Rc [address] - read cassette data and place in memory starting at
E [address] - execute program from [address]; when complete, return to
? - help

What do you think? Is this feasible? How could it be improved?

Re: Z80 system

In article <944e0534-7118-4187-a1a3-
4f37398e34aa@r28g2000vbp.googlegroups.com>, Xatapu says...

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LJ electronics made a similar 6502 based system in the 80's called Emma
and Emma 2 however the display was several 7 segment LEDs. It was used
in electronics courses in the UK to teach MC programming and MP basics.


I only please one person per day. Today is not your day. Tomorrow isn't
looking good either. - Scott Adams

Re: Z80 system

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Hi!  I am also a big fan of Steve Ciarcia's BYOZ80C book.  It has been
an inspiration for my own home brew Z80 CP/M computer.

Your project sounds similar to the N8VEM project.  You may be
interested in joining and using it to implement your own ideas and
software.  It is designed specifically for easy building and for
builders to make their own home brew peripherals.

If you would like to build your own low cost home brew Z80 CP/M
computer you can make your own with the N8VEM project.  Builders of
all skills and experience levels welcome!

I have plenty of Single Board Computer, ECB backplane, ECB bus
monitor, and ECB prototyping board PCBs available.  You get your own
parts and build it yourself.

All information regarding design, hardware, and software is freely and
publicly available.  Here is more information:

http://n8vem-sbc.pbwiki.com /


If you send me an email I will send you a personal invitation to
join.  There are numerous builder project including information on a
hex keypad with LED display.

Here is a link from my friend Rolf who is also an N8VEM builder of his
system and the mini terminal


Thanks and have a nice day!

Andrew Lynch

Re: Z80 system

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My friend Rolf sent me a better link for the CCS85 miniterminal.
Thanks Rolf!


He even built an adapter board to use it with the N8VEM on the ECB

Excellent!  Thanks and have a nice day!

Andrew Lynch

Re: Z80 system

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I would strongly advise against an ATX power supply.  You'll probably
need to attach a dummy load to the 12V rail (and possibly even the
5V rail, since your machine is unlikely to draw much current) in
order to get it to work.  Added to that, they're big and noisy.
Either buy a dedicated plug-in regulated 5V PSU or stick in a 7805
regulator and associated caps.  The latter will work anywhere
between about 7.5 and 30V meaning you are pretty much guaranteed
to have a suitable power supply lying around from some dead or
unneeded piece of equipment.

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What, you mean apart from "By not using a Z80"?  I've seen many of
these amateur projects in the past and they always include a keyboard
and display.  Together these are the single most complex part of
your proposed system and you are likely to end up with limited
capabilities (small screen, restrictive, non-QWERTY keyboard).
Added to that, it isn't cheap.  Those 4x20 dispalys you are thinking
of don't cost much, but a keyboard full of keyswitches certainly

Work smarter, not harder.  Add an extra serial port and run the
system with a PC running a PC running a terminal emulation package.
If you want something that works independently of your PC buy a
dumb terminal to steer it - I got a very nice Wyse 160 on ebay for
under 20 including postage a couple of years back.  Something like
that would give you a full sized display and a proper keyboard for
much less effort and less cost that you would spend on components.

Also, consider storage.  Nowadays it is relatively easy to add a
few megabytes or more of solid state storage with very little effort
and next to no cost.  Most books dealing with the Z80 will not
advocate doing this, however: at the time they were written solid
state storage was prohibitively expensive.  That isn't true now.

Andrew Smallshaw

Re: Z80 system

put finger to keyboard and composed:

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This was my first computer:

It was a Z80 machine supplied in kit form. One of the option EPROMs
included an EDASM (editor/assembler).

- Franc Zabkar
Please remove one 'i' from my address when replying by email.

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