Projects: ASCII keyboard emulator for Apple I Replica

Last week I built an Apple I Replica from a Briel Computers kit. While I loved the completed machine, I suffered a bit from the fact that backspace doesn’t work (you have to type the assembly *exactly* correct), and of course it loses the RAM contents when shut down.

I wanted to make an interface to the PC so I could use a modern editor and simply copy-paste to the Apple I. As a quick and dirty solution to the problem, I used an Arduino, wired it to the ASCII keyboard port of the Apple I and wrote a small sketch that listens on the serial port and sets the data pins accordingly.

Photos on Flickr.

Here’s the Arduino sketch, real simple:

/*
   ASCII keyboard
   Lionel Lemarie
   2010-02

   Listens on the serial port for characters from a PC.
   Outputs the codes like an ASCII keyboard.
   Compatible with Apple I Replica.
 */

// The order of the pins is chosen for minimal wire crossing
// when connected to an ASCII keyboard socket.
int ASCII0 = 7;
int ASCII1 = 8;
int ASCII2 = 3;
int ASCII3 = 5;
int ASCII4 = 4;
int ASCII5 = 6;
int ASCII6 = 2;
int STROBE = 10;
int NRESET = 9; //reset active low

// LED will blink when a character is emitted
int LED = 13;
unsigned long previousMillis = 0;
unsigned long interval = 200;

void setup()
{
	analogReference(EXTERNAL); // Is this needed? 

	// Initialize the digital pins as output
	pinMode(ASCII0, OUTPUT);    
	pinMode(ASCII1, OUTPUT);    
	pinMode(ASCII2, OUTPUT);    
	pinMode(ASCII3, OUTPUT);    
	pinMode(ASCII4, OUTPUT);    
	pinMode(ASCII5, OUTPUT);    
	pinMode(ASCII6, OUTPUT);    
	pinMode(STROBE, OUTPUT);    
	pinMode(NRESET, OUTPUT);    

	pinMode(LED, OUTPUT);

	digitalWrite(STROBE, LOW);   // set the STROBE pin to inactive
	digitalWrite(NRESET, HIGH);  // set the RESET  pin to inactive

	Serial.begin(9600);
}

void loop()                    
{
	unsigned long currentMillis = millis();

	if (Serial.available() > 0)
	{
		int iInput = Serial.read();
		iInput &= 127;

		if (iInput)
		{
			if (iInput>='a' && iInput<='z')
				iInput = iInput - 'a' + 'A';

			int D6 = LOW;
			int D5 = LOW;
			int D4 = LOW;
			int D3 = LOW;
			int D2 = LOW;
			int D1 = LOW;
			int D0 = LOW;

			if ( iInput & 64 ) D6 = HIGH;
			if ( iInput & 32 ) D5 = HIGH;
			if ( iInput & 16 ) D4 = HIGH;
			if ( iInput &  8 ) D3 = HIGH;
			if ( iInput &  4 ) D2 = HIGH;
			if ( iInput &  2 ) D1 = HIGH;
			if ( iInput &  1 ) D0 = HIGH;

			digitalWrite(LED, HIGH);
			previousMillis = currentMillis;

			// Output an A (100 0001)
			digitalWrite(ASCII6, D6);
			digitalWrite(ASCII5, D5);
			digitalWrite(ASCII4, D4);
			digitalWrite(ASCII3, D3);
			digitalWrite(ASCII2, D2);
			digitalWrite(ASCII1, D1);
			digitalWrite(ASCII0, D0);
			digitalWrite(STROBE, HIGH);
			delay(40);  // strobe for 40ms
			digitalWrite(STROBE, LOW); 
		}
	}

	if ( (previousMillis>0) && (currentMillis-previousMillis>interval) )
	{
		digitalWrite(LED, LOW);
		previousMillis = 0;
	}
}

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