1802 CPU for RC2014

An RCA1802 Processor Card For RC2014

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This board allows you to use an RCA1802 processor with the RC2014 bus system. It generates the various expected signals on the bus from the 1802 and provides the necessary glue. There is provision on board for the CDP1861 but this has not been tested.

The core of this board design is based on the various reference board designs. The main difference is that the bitbanged I/O pins are not really used except for the 1861 glue  (you could use them I guess) and instead the top 256 bytes of memory are used as an I/O window to 'real devices'.

A 74HC573 demultiplexes the rather strange two stage multiplexed address bus and a 74HC688 generates the RC2014 style IORQ and MREQ lines from it. A 74HC14 is used to clean up the reset line which on most RC2014 backplanes doesn't have the needed sharp edge.

The address multiplexing may need further changes. IORQ and MREQ get asserted at the point when the high order address byte is sent. For write this should be OK because the MWR cycle happens after the low byte has been sent, and thus ensures that anything qualified with \WR and \IORQ (or for memory \WR and \MREQ) won't glitch. On the read side however the processor drops MRD *before* the low address byte hits the bus. A glitched memory read is fine as by the time the device has sampled the data the correct address will have been present for some time. For I/O read with side effect however a read from $FFxx will read $FFFF momentarily. Hopefully not putting a device with side effects at I/O $FF is sufficient. However the bus cycles resulting don't look all that Z80 like but seem to work fine with everything tested except Z80 specific peripheral chips.

It should be possible to use an 1805 or 1806 with this board. That will result in VDD being fed from VCC on the 1806 and \ME being held high on the 1805


Zip Archive - 99.74 kB - 04/01/2020 at 21:15



PCB Layout (KICAD)

x-kicad-pcb - 400.13 kB - 03/30/2020 at 16:25



Schematic (KICAD)

x-kicad-schematic - 22.58 kB - 03/30/2020 at 16:25



Schematic (PDF)

Adobe Portable Document Format - 74.14 kB - 03/30/2020 at 16:24


  • 1 × CDP1802 CPU (1805 or 1806 should also work)
  • 1 × 74HC573 Electronic Components / Misc. Electronic Components
  • 1 × 1K Ohm resistor (only if using CDP1861)
  • 1 × Coaxial Socket (only if using CDP1861)
  • 1 × 74HC688 Electronic Components / Misc. Electronic Components

View all 12 components

  • Loading data from CF card

    EtchedPixels04/01/2020 at 21:28 0 comments

    The test ROM is now loading a boot block from the CF adapter and running it correctly. In theory that means we have everything needed to run ElfOS providing a paged RAM or banked memory card is used so the ROM can be banked out.

  • Initial Boot ROM

    EtchedPixels03/30/2020 at 20:47 0 comments

    A hello world boot rom is now available in the RC2014 ROM github

    Board config

    - 16C550A UART at 0xC0

    - 32K/32K RAM/ROM card with ROM in the low 32K

    - SC129 at 0x80 for debug lights

    - CPU card

    I've also pushed fixes to the 1802 emulation and rc2014 1802 emulator to fix a bug in RET/DIS emulation where the documentation is misleading, and I've pushed the 1802 bug fixes the the assembler - which for other reasons is in the 6303 C compiler on my github.

    The ROM source is probably mostly useful if you need the uart setup code to run with something more interesting like IDIOT or BASIC or perhaps you want to run it with 512K of banked RAM, a PPIDE disk interface and TMS991A video.. 8)

  • Bring Up

    EtchedPixels03/30/2020 at 16:31 0 comments

    It lives.

    Currently it's executing the first test program from ROM

        .org    0

        ldi    #0xFF
        phi    6
        ldi    #0x80
        plo    6
        ldi    #0xFF
        plo    8
        dec    7
        glo    7
        bnz    loop
        dec    8
        glo    8
        str    6
        br    loop

    which is cycling LEDs on the debug card.

    The configuration I am using at this point is a 32K/32K RAM/ROM card with ROM low, the 1802 CPU card, and an SC129 GPIO/Debug card. It's got a 16x50 UART plugged but in but I've yet to try and set that up.

View all 3 project logs

Enjoy this project?



Todd Decker wrote 05/20/2021 at 02:44 point


  Are you sure? yes | no

EtchedPixels wrote 04/01/2020 at 21:26 point

I've regenerated and uploaded Gerbers because if you are in the USA it's cheaper for you to order five from JLCPCB than for me to post you one.

  Are you sure? yes | no

stephenbuck wrote 04/30/2020 at 21:31 point

I just ordered a set of PCBs from JLCPCB and they're aboard the proverbial slow boat from China. Looking forward to getting this working with my RC-2014! Thanks for making this project available!

  Are you sure? yes | no

stephenbuck wrote 03/31/2020 at 17:19 point

I've been hoping someone would build an 1802 board for the RC-2014. Will boards be available on Tindie at some point?

  Are you sure? yes | no

EtchedPixels wrote 03/30/2020 at 20:32 point

I got mine off ebay from a UK based seller.

  Are you sure? yes | no

Dave's Dev Lab wrote 03/30/2020 at 19:17 point

did you pick up some 1802's from BGMicro? i grabbed a handful for a future project!

  Are you sure? yes | no

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