Serial Diagnostic Link

   As explained in more detail HERE, the SDL is a serial link between the Type 4 process board and the revised (switch on the left side) Op Panel.

The link is controlled via the two least significant bits of port E1h. Bit 0 drives the DISPLAY_STROBE line and bit 1 then controls the DISPLAY_RESET line.

See the code snippets down below for more information.


Assembler (IDA)

Register si points to the text string. The string should be 8 bytes long.

                mov     bx, si
                add     bx, 7

next_char:
                mov     cl, [bx]
                mov     ah, ch
                shl     ch, 5
                rol     cx, 3

loop:
                in      al, 0E1h
                and     al, 11111110b
                out     0E1h, al
                or      al, 00000001b
                out     0E1h, al
                loop    loop
                or      al, 00000010b
                out     0E1h, al
                and     al, 11111110b
                out     0E1h, al
                or      al, 00000001b
                out     0E1h, al
                and     al, 11111101b
                out     0E1h, al
                mov     ch, ah
                inc     ch
                dec     bx
                cmp     bx, si
                jnb     short next_char

C (Borland Turbo C)

The string can be up to 8 characters long. Shorter strings will not fill the entire display, longer strings will be truncated.

#define LED_POS_NUM 8

#define CHAR_BIT 8

unsigned short rotl16( unsigned short val, unsigned int c )
{
	const unsigned int mask = ( CHAR_BIT * sizeof( val ) - 1 );

	c &= mask;
	return ( val << c ) | ( val >> ( ( -c ) & mask ) );
}

int SDL_send( char * str )
{
	int num;
	unsigned short int n = 0;
	int val = 0;
	int i;

	if ( str == NULL )
		return 0;
	
	num = strlen( str );
	if ( num == 0 )
		return 0;

	if ( num > LED_POS_NUM )
		num = LED_POS_NUM;

	for ( i = 0; i < num; i++ )
	{
		n = ( i << 8 );
		n <<= 5;
		n |= str[ num - 1 - i ];
		n = rotl16( n, 3 );

		while ( n > 0 )
		{
			val = inportb( 0xE1 );
			val &= 0xFE;
			outportb( 0xE1, val );
			val |= 0x01;
			outportb( 0xE1, val );

			n--;
		}

		val |= 0x02;
		outportb( 0xE1, val );
		val &= 0xFE;
		outportb( 0xE1, val );
		val |= 0x01;
		outportb( 0xE1, val );
		val &= 0xFD;
		outportb( 0xE1, val );
	}
	
	return num;
}

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