Project Logs

Collapse

• Hardware notes

  Keith • 09/18/2025 at 23:14 • 0 comments

  the IC1 = 6809
  IC2 = 6522
  IC3,9 = 74LS244
  IC4 = 2716 OR 2732 EPROM
  IC5,6 = 2114
  IC7,8 = 74LS244
  IC10 = INS8208
  IC11 = 74S470 256 x 8-bit PROM
  IC12 = 74LS74
  IC13,14 = 74LS00
  IC15 = 74LS86

  The PROM  was the memory address decoder. These days it is harder to find than GAL16V8 which would do the same job. Stephen Crozier did this replacement for his Eurobeeb 6502 board.

  As a programmable part, you would have to work out the data by analysing the memory maps described in the manual.

  I can't find the circuit for this board. I'm confident it is just an Acorn 6809 board with a 6551 serial chip added. So most if it will be the same. If anyone has the circuit, I'd love a copy.

  Bram Prosman has heard that the Eurobeeb 6502 and 6809 boards are essentially the same, with only the CPU circuitry slightly different.

  The MAME project emulates the Eurobeeb 6809, and the source code shows the 16 possible memory maps that the address decoder can select. See https://github.com/mamedev/mame/blob/mame0280/src/mame/acorn/eurocube.cpp

  // Map 4 - FLEX, low cost (6809 only)
    m_view[4](0x0000, 0x1fff).ram();                   // M3
    m_view[4](0x2000, 0x3fff).ram();                   // M2
    m_view[4](0xe000, 0xffff).rom().region("m0", 0);   // M0
    m_view[4](0xe000, 0xe7ff).ram();                   // M1
  //m_view[4](0xe000, 0xefff).lrw8(                    // I/O Block

   Which shows

  0000-3FFF is 16K RAM 
  4000-DFFF is 40K free space (could be filled with RAM)
  E000-E7FF is 2K RAM
  E800-FFFF is the top 6K of an 8K EPROM, with a half-K hole for i/o

  EE00-EFFF is the half-K i/o device space (the commented line in the code contradicts my ROM disassembly)

  This gives us enough information to design a memory-mapping GAL.

View project log