7.7 KiB



This repository contains firmware and gerber files for the Apple-1 Serial Interface expansion card.


This project aims to provide an alternative to the Apple Cassette Interface for the Apple-1 computer with use of the serial communication (9600/8-N-1). The expansion card is based around inexpensive and easily available ATMega328P MCU which operates as half-duplex UART. The best way to connect the device to a regular modern PC is to use FTDI FT232RL USB dongle via exposed pin headers (5V TTL) or traditionally via standard DB9 RS232 connector.

The board tries to mimic the original ACI behaviour via the modified ACI loader program $C100 located on the on-board ROM. The usual way to communicate with the device is to hook it up to a PC and use a terminal emulator program to send and receive binary files representing the Apple-1 memory contents. An alternative is to use custom software/scripts like the scripts/ python script.

In order to learn how to use the loader program, please refer to the original ACI documentation or the instructions provided here.

Some Apple-1 program binaries ready to be downloaded are available here.

Memory map

address function
$C0FC-$C0FF these addresses contain the 'A1SI' string which might be used to determine whether the A1SI expansion card is present.
$C100-$C7FF address space reserved for on-board ROM.
$C100 the original ACI loader program adapted to the A1SI.
$C300 "teletype on apple-1" test program - the data from remote terminal appears on Apple-1 screen.
$C400 "teletype on remote" test program - the data from Apple-1 appears on remote terminal.
$C500 0-255 repeating counter - all possible bytes are repeatedly sent to remote terminal.
$C600 serialmonitor - the well known Woz Monitor variation, but now on the remote terminal.

entries for programmers:

address function
$C000 ready flag - contains 0x01 if the device is ready to write/read, 0x00 otherwise.
$C080 read address - the location for data read from the interface, first read of this address switches the device to read mode.
$C081 write address - the location for data to be written via the interface. First write operation to this address switches the device to write mode. If you want to immediately write bytes after switching to write mode you must wait for the ready flag at $C000 to be set.
$C23A software reset routine - resets the MCU.
$C1EC block read routine - reads data from the interface and writes to the specified address range.
$C213 block write routine - reads the data from the specified address range and write via the interface.
$0034 lower byte of the end address for block read/write routines.
$0035 upper byte of the end address for block read/write routines.
$0036 lower byte of the beginning address for block read/write routines.
$0037 upper byte of the beginning address for block read/write routines.


The contents of this repository are as following:

  • gerber/ - gerber files needed to manufacture the PCB
  • firmware/ - contains source code for the ATMega328 MCU firmware
  • mapping/ - contains EQN and JED files for GAL20V8 based address decoder for the board
  • src/ - contains the 6502 assembly sources for the A1SI onboard ROM routines
  • scripts/ - contains python based file transfer utility


There are two .jed files for the GAL20V8 based address decoder:

  1. address_decoder-C000.jed - defines the standard mapping where the device mimics the original ACI loader program at $C100.
  2. address_decoder-C300.jed - defines an alternative mapping to run Apple-1 Serial Interface together with the original ACI within the same system. All program and hardware entries are shifted by offset $300, so the loader program can be accessed at $C400.
  3. address_decoder-C600.jed - defines an alternative mapping to run Apple-1 Serial Interface together with both the original and Improved ACI 2.0 (by Uncle Bernie) within the same system. All program and hardware entries are shifted by offset $600, so the loader program can be accessed at $C700.

Please be sure to program the ROM with the bin image corresponding to the selected mapping.

You can use the convenient make targets to burn ROM and GAL, if you have the minipro utility installed:

make burn-C000-gal
make burn-C000-rom

make burn-C300-gal
make burn-C300-rom

make burn-C600-gal
make burn-C600-rom


You need the following to successfully build the firmware:

  • xa cross assembler.
  • avr-gcc toolchain.
  • avr-libc set of libraries.
  • AVRDUDE AVR programmer software or equivalent to program the ATMega328P MCU.
  • Software capable of translating the .eqn files into .jed's. I used for this purpose the DOS based EQN2JED from OPALjr PLD Development Package. This is only required if you wish to make some changes to the GAL based address decoder. The default .jed file should be fine for most users.
  • EEPROM programmer to write ROM and GAL. I used TL866 programmer for this purpose. It can also program the ATMega328P MCU.
  • You can use the minipro utility to write ROM, GAL and MCU.

How to build?

To build the firmware just type:


And to clean the build:

make clean

Applesoft BASIC support

You can also LOAD and SAVE your Applesoft BASIC programs thanks to the branch of the applesoft-lite project which has been modified to be compatible with the A1SI expansion card:


You can use the A1SI expansion card to test the reliability of the Apple-1 Cassette Interface.

How to install hardware?

Solder the "C" memory segment line near 74154 demultiplexer to the "T" signal line. Afterwards just put the board in right orientation (as marked on the PCB) in the Apple-1 expansion slot. Or you can use the port expander if the on-board slot is already occupied:


Gerber files can be found in the gerber/ directory. The previous version of the board without DB9 connector can be found in the gerber/noDB9 directory. The KiCad project files with board design and schematics can be found here:

Bill of materials

name part quantity
C1, C2 Ceramic 22pF 2
C3..C9 Ceramic 100nF 7
C10 Electrolytic 22uF 1
C11..C14 Electrolytic 10uF 4
R1, R2 0.68K 0.25W 2
R3..R9 1K 0.25W 7
XTAL1 16Mhz crystal HC49 low profile 1
ATmega328P1 ATMega328P microcontroller DIP28 1
7402 7402 quad 2 input NOR gates DIP14 1
74S257mux1,2 74S257 quad multiplexers DIP16 2
8T97buf1,2 8T97N hex tristate buffers DIP16 2
20V8B1 GAL20V8 SPLD (35ns or less) DIP24 1
AT28C64B1 AT28C64 ROM or similar DIP-28 1
MAX232 Maxim MAX232 RS-232 driver/receiver DIP16 1
LED1 LM358N double op amp DIP8 1
TX_LED1, RX_LED1 LED phi 3mm 2
L7805 LM7805 regulator (optional) 1
SERIAL1 1x4 2.54mm pin header 1
RS232 DB9 horizontal female header (mounting holes offset 7.48mm) 1
ROM_PWR1 1x3 2.54mm pin header (optional - make solder blob to "5V" instead) 1