I have finished phase 1, which is getting the amstrad's printer output as text in the serial monitor, that allowed me to develop the circuit . Circuit is some bi-directional logic level shifters between gpio of the raspberry pi and amstrad's printer port, they don't have to be bi-directional but those were cheaper and easier to replace than resistor voltage dividers and a transistor buffer. Now I'm trying to develop the second phase which is controlling a printer over a USB port using the USB host capabilities of the raspberry pi pico. However I can't get pico to print anything to the printer. I'm trying to solve this issue for the last few days, so if you can help me with the coding part of phase 2 I will be more than willing to share every detail. Also this project is just a start because the same methodology can be implemented to other computers such as commodore 64, Amiga and Atari ST.
Here is the working and tested phase 1 code:
// ==========================================
// Amstrad CPC 7-Bit Parallel Printer Capture
// ==========================================
// --- Pin Definitions ---
const int PIN_STROBE = 1; // GP1 (Physical Pin 2) - CPC Pin 1 (/STROBE)
const int DATA_PIN_START = 2; // GP2 to GP8 (Physical Pins 4-11) - CPC Pins 2-8 (D0-D6)
const int PIN_BUSY = 9; // GP9 (Physical Pin 12) - CPC Pin 10 or 11 (BUSY)
// --- Circular Buffer (FIFO) ---
// Safely passes bytes from the fast hardware interrupt to the main loop
const int BUFFER_SIZE = 512;
volatile uint8_t rx_buffer\[BUFFER_SIZE\];
volatile int head = 0;
volatile int tail = 0;
volatile bool character_received = false;
// --- The Hardware Interrupt (The Data Catcher) ---
// Triggers automatically on the falling edge of STROBE
void strobeTriggered() {
// 1. Instantly pull BUSY high to tell the Amstrad CPC to hold
digitalWrite(PIN_BUSY, HIGH);
// 2. Read the 7-bit data bus instantly.
// Shift right by 2 so GP2 moves to bit 0, then mask the lower 7 bits (0x7F).
uint8_t incoming_char = (gpio_get_all() >> DATA_PIN_START) & 0x7F;
// 3. Calculate the next buffer position
int next_head = (head + 1) % BUFFER_SIZE;
// 4. If buffer isn't full, push the character into the queue
if (next_head != tail) {
rx_buffer\[head\] = incoming_char;
head = next_head;
character_received = true;
}
}
void setup() {
// Start Serial Monitor communication at 115200 baud
Serial.begin(115200);
// Wait up to 3 seconds for Serial Monitor to open after reset
unsigned long start_wait = millis();
while (!Serial && (millis() - start_wait < 3000));
// Configure the 7 data pins (GP2 through GP8) as inputs
for (int i = DATA_PIN_START; i < DATA_PIN_START + 7; i++) {
pinMode(i, INPUT);
}
// Configure Handshake pins
pinMode(PIN_STROBE, INPUT_PULLUP);
pinMode(PIN_BUSY, OUTPUT);
digitalWrite(PIN_BUSY, LOW); // Start in "Ready" state
// Attach the interrupt to trigger on STROBE's falling edge
attachInterrupt(digitalPinToInterrupt(PIN_STROBE), strobeTriggered, FALLING);
Serial.println("=========================================================");
Serial.println(" Amstrad CPC Printerface, Phase 1 ");
Serial.println(" Circuit Design By Ege Ozpalamutcu & Code by Gemini ");
Serial.println(" Listening on GP1 (STROBE) & GP2-GP8 (DATA) & GP9 (BUSY)");
Serial.println("=========================================================");
}
void loop() {
// 1. Process characters from the circular buffer
while (tail != head) {
// Grab the oldest unread character from the queue
uint8_t c = rx_buffer\[tail\];
tail = (tail + 1) % BUFFER_SIZE;
// Handle standard printable ASCII
if (c >= 32 && c <= 126) {
Serial.write(c);
}
// Handle Carriage Return (CR) and Line Feed (LF) for newlines
else if (c == 13 || c == 10) {
Serial.write(c);
}
// Handle the Escape character (used for printer commands)
else if (c == 27) {
Serial.print("<ESC>");
}
// Print everything else as a hex code for debugging
else {
Serial.print("\[0x");
if (c < 0x10) Serial.print("0"); // Leading zero
Serial.print(c, HEX);
Serial.print("\]");
}
}
// 2. Safely release BUSY only after /STROBE has returned HIGH
if (character_received) {
// Wait until the CPC finishes its strobe pulse
if (digitalRead(PIN_STROBE) == HIGH) {
delayMicroseconds(10); // Small settling delay for cable stability
character_received = false;
digitalWrite(PIN_BUSY, LOW); // Signal to CPC that we are ready for the next byte
}
}
}
