Electronic Circuits - interfacing with a Z80 microprocessor v8 (input from I/O device)
My seb80 custom computer now has its own page here
This is version 8 of my Z80 interfacing project. My goal in this version was to introduce an I/O input device, from which the CPU could read data via the IN instruction.

Previous versions:
  • version 1 - initial wire-up, executes NOPs, observe incrementing address
  • version 2 - experiment with and observe machine cycles M2 (memory read) and M3 (memory write)
  • version 3 - address and data buses
  • version 4 - introduce ROM and run program from it
  • version 5 - introduce RAM
  • version 6 - oscillator clock, NMI button, and INT/NMI tests
  • version 7 - I/O output device

To create a way to input from a device, I used a second 74LS138 3-to-8 decoder as well as a 74LS244, which becomes enabled only when the specific device is read. When this happens, the 74LS244 gains control of the data bus, writing to it.

The input device I built is a segment of 8 DIP switches.

TP6 - test program for I/O input


day 1
This begins part 2 of interfacing with I/O devices; in this version I want to add the necessary circuitry to be able to input from devices.
Just like for output devices, a 74LS138 3-to-8 decoder (part is SN74LS138N) will be used to enable one of up to four input devices.
Already, I/O devices at port 4, 5, 6, 7 (A2=1) are be output devices.
Thus, I/O devices at port 0, 1, 2, 3 (A2=0) will be input devices.
NOTE: for input devices that won't rely on these built-in selectors, care has to be taken that they are wired to ports whose A2=1, to not conflict with the built-in input device selectors which expect A2=0.

Input devices:

A2 A1 A0
0 0 0 Y0 input device at port 0
0 0 1 Y1 input device at port 1
0 1 0 Y2 input device at port 2
0 1 1 Y3 input device at port 3

day 2
The second 74LS138 was easy to design; it's just like the output one, except being enabled by Z80's RD line (along with IORQ).
The switches will be buffered by a 74LS244, enabled by one of the input device selector's outputs.

day 3
Wired up the input selector 74LS138.
Wrote TP6, a test program to test input via a device made up of switches:

Address Bytes Mnemonic
0000 ; TP6
0000 ; test program for I/O input
0000 ;
0000 ; the reason for these is to verify that
0000 ; the input device selector remains inactive
0000 ; during outputs
0000 3E 81 LD A, 10000001b
0002 D3 04 OUT (04h), A
0004 3E C3 LD A, 11000011b
0006 D3 05 OUT (05h), A
0008 3E E7 LD A, 11100111b
000A D3 06 OUT (06h),A
000C 3E FF LD A, 11111111b
000E D3 07 OUT (07h),A
0010 3E 00 LD A, 0
0012 D3 06 OUT (06h), A ; clear LEDs
0014 ; now read from the four input devices, to
0014 ; verify correct signals from the input
0014 ; device selector
0014 DB 00 IN A, (00h) ; reads garbage when not connected
0016 DB 01 IN A, (01h) ; reads garbage when not connected
0018 DB 02 IN A, (02h) ; reads garbage when not connected
001A DB 03 IN A, (03h) ; read switches
001C D3 06 OUT (06h), A ; display value on LEDs

Verified that the 74LS138 correctly selects an input device, as wired.
Wired up the 74LS244. It was much easier than before, with the extensive wiring diagram I've been maintaining.
Program TP6 ran successfully, demonstrating the full I/O cycle (read from DIP switches, write to LEDs).

Z80 diagrams

Here are some useful diagrams, including the Z80 pinout, timing diagrams, etc.

27C256 EPROM diagrams

Here are some useful diagrams, including pinout, timing, etc.

62256 RAM diagrams

Here are some useful diagrams, including pinout, timing, etc.