This page describes the various hardware modules that I’ve built for the modular Z80 computer.
The heart of the system is the Z80 processor module. This is little more than a breakout board for the Z80’s DIP-40 package, although it does include
pull-up resistors for some of the signals (BUSREQ, INT, NMI, WAIT) to ensure that their default state is high (inactive), while ensuring that they can be
pulled low by external modules.
This module provides 8K of ROM, starting at address 0x0000, which is the Z80’s reset vector.
A 74LS32 is used to OR the MREQ and RD signals together, producing a signal which is only low (active) when the Z80 is attempting to read from memory
(as opposed to reading from I/O). This is wired to the OE (output-enable) of the ROM. Address lines A15, A14 and A13 are OR-ed together to produce
the ROM’s CE (chip-enable) signal, mapping it to the lower 8K of the Z80’s address space.
This module provides 32K of RAM located at the higher end of the Z80’s address space (0x8000 to 0xFFFF). Like the ROM module, the MREQ and RD signals
are used to interface to the RAM, along with the WR signal for write operations. The A15 address line is inverted and used as the RAM’s CE signal,
mapping it to the upper 32K of the address space.
This module uses a Motorola 6850 ACIA to provide a single serial port which can be connected to using an FTDI USB-to-TTL-serial adapter.
Four pins are provided for connecting to the FTDI adapter - power, ground, Tx and Rx. This provides the possibility of powering the computer from the FTDI
adapter.
The 6850 uses the system clock of 3.6864MHz for baud-rate generation, which when divided by 64 gives a baud-rate of 57600 - plenty fast for most applications.