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Z8 For RC2014

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An RC2014 processor board for the Z8 (Z86C91 or similar). This is a CPU card designed to be run with some kind of external memory board and optional I/O (there is some I/O on the Z8 itself). It includes the clock socket so you just need a backplane with reset generator.

For those not familiar with it when Intel created the 8080 Zilog took a good idea and extended it to make a great CPU. When Intel created the 8051 and friends Zilog took a really bad idea and extended it. The basic architectural concept is the same with a register space, external memory spaces and register indirections but the Zilog take on it has a regular instruction set where any register can do operations and any pair of registers is a data pointer. The end result actually being quite pleasant to program.

The Z8 interface to RC2014 is not too complicated. The bus is demultiplexed with a 74HC573. Half of  a 74HC139 turns the DS and RW signals into Z80 style \RD and \WR. The other half of it uses DM (data memory), A15 and a GPIO pin so that memory is accessed except when data request for > 0x8000 occur and the GPIO line is low in which case IORQ is lowered instead. This allows a program to programmatically map 32K of I/O space. Possibly it would be better to ignore A15 and just require the user switches mappings for external data access and gets the full 64K. I may change that for a second revision.

A 74HC00 completes the logic providing an inverter for the IRQ input, one to generate MREQ and IORQ and a third inverter for AS to latch the 74HC573 correctly. I used a 74HC00 because I have some to hand, it's also nicer to have  a spare NAND gate rather than inverters on a board  for any debugging problems.

For a crystal you need something that gives sane baud rates if you want to use internal I/O. I am currently using a 7.3728MHz standard RC2014 crystal and 19200 baud but double that would work.

It should be possible to use this board with the ROM BASIC Z8671. I've not tried it with any of the older parts. In that case you need the RAM jumpered low and ROM high, and to burn the designed baud rate setting into ROM.

x-kicad-pcb - 425.89 kB - 04/05/2020 at 19:01

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z86c91.sch

Kicad 5

x-kicad-schematic - 20.04 kB - 04/05/2020 at 19:01

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lib - 12.76 kB - 04/05/2020 at 19:01

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z86c91.pro

Kicad 5

x-kicad-project - 3.87 kB - 04/05/2020 at 19:01

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z8.zip

Gerbers

Zip Archive - 117.97 kB - 04/05/2020 at 19:00

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View all 6 files

  • 1 × Z86C91 or similar
  • 1 × 74HCT573 Electronic Components / Misc. Electronic Components
  • 3 × 1K or similar resistor
  • 1 × 74HCT00 Electronic Components / Misc. Electronic Components
  • 1 × 74HCT139 Electronic Components / Misc. Electronic Components

View all 11 components

  • First Signs Of Life

    EtchedPixels04/05/2020 at 18:59 0 comments

    The board is now booting up and printing an 'X' on the serial port as well as writing to the external debug LED card. At the moment I am running it with a 32K/32K RAM/ROM card and the debug board.

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Colin MacArthur wrote 12 hours ago point

Works GREAT with z8671 Chip...

Would you PLEASE post you z8 Romless code

THX

CM

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