The TI interface is pulled high by resisters, never driven high.  The proper
connection would use two i/o lines.  They should be input at all times unless
you are driving the line low:

PIN = LOW
TRIS = 0 (output)
delay
TRIS = 1 (input)

-Adam

Andrew Seddon wrote:
>
> Sorry for the large post but I have no website.
>
> Well there`s good news and bad news. First off I have kind of got the
> interface routines working. I say that because it sometimes works and
> sometimes dosn`t. When I program my TI-83+ to read a byte then spew it back
> out everything works fine. However when I try to make two TI`s, a 83+ and a
> TI-82 talk through the PIC it dosn`t work!! Dammned if I know whats up..
> Below is the code that I am using if anybody is interested or can find a bug
> in it.
>
> BTW it`s in C2C but most of the custom function calls are self explanitary.
> Sorry about the lack of comment.
>
> The calc variable is used as I have two calculators connected to one PIC,
> both operating off the lower four bits of the A and B ports. The electrical
> connection is just
>
> CALC 1 - WHITE - A0 + A2
>         - RED   - A1 + A3
>
> CALC 2 - WHITE - B0 + B0
>         - RED   - B1 + B3
>
> there are no resistors, diodes anyting, is that the problem??
>
> Here`s the code. Some redundent code commented out, was used it testing.
>
> //RS232 settings
> #pragma RS232_TXPORT RC
> #pragma RS232_RXPORT RC
> #pragma RS232_TXPIN  0
> #pragma RS232_RXPIN  1
> #pragma RS232_BAUD 19200
>
> #pragma TURBO_MODE 1
> #pragma CLOCK_FREQ 50000000
>
> char tris_a=255;
> char tris_b=255;
>
> char bytetemp=0, bytetemp1=0;
> char temp4=0;
>
> void sendDECNumber( char data )
> {
>     putchar( '0' + data/100 );
>     putchar( '0' + (data%100)/10 );
>     putchar( '0' + data%10 );
> }
>
> void sendHEXNumber( char data )
> {
>     //Send high nibble
>     if( (data>>4) > 9 )
>         putchar( 'A' - 10 + (data>>4) );
>     else
>         putchar( '0' + (data>>4) );
>     //Send low nibble
>     if( (data&0x0F) > 9 )
>         putchar( 'A' - 10 + (data&0x0F) );
>     else
>         putchar( '0' + (data&0x0F) );
> }
>
> void printf( const char* text )
> {
>     char i = 0;
>     while( text[i] != 0 ) {
>         putchar( text[i++] );
>         delay_ms(10);
>         }
> }
>
> void output_white_high(char calc)
> {
>     if (calc == 1)
>         {
>             set_bit(tris_a, 0);
>             set_tris_a(tris_a);
>         }
>
>     if (calc == 2)
>         {
>             set_bit(tris_b, 0);
>             set_tris_b(tris_b);
>         }
>         delay_ms(2);
> }
>
> void output_white_low(char calc)
> {
>     if (calc == 1)
>         {
>             clear_bit(tris_a, 0);
>             set_tris_a(tris_a);
>         }
>
>     if (calc == 2)
>         {
>             clear_bit(tris_b, 0);
>             set_tris_b(tris_b);
>         }
>         delay_ms(2);
> }
>
> void output_red_high(char calc)
> {
>     if (calc == 1)
>         {
>             set_bit(tris_a, 1);
>             set_tris_a(tris_a);
>         }
>
>     if (calc == 2)
>         {
>             set_bit(tris_b, 1);
>             set_tris_b(tris_b);
>         }
>         delay_ms(2);
> }
>
> void output_red_low(char calc)
> {
>     if (calc == 1)
>         {
>             clear_bit(tris_a, 1);
>             set_tris_a(tris_a);
>         }
>
>     if (calc == 2)
>         {
>             clear_bit(tris_b, 1);
>             set_tris_b(tris_b);
>         }
>         delay_ms(2);
> }
>
> char input_white(char calc)
> {
>     if(calc == 1)
>         {
>            if(input_pin_port_a(2)) return 1;
>            else return 0;
>         }
>
>     if(calc == 2)
>         {
>             if(input_pin_port_b(2)) return 1;
>             else return 0;
>         }
>         delay_ms(2);
> }
>
> char input_red(char calc)
> {
>     if(calc == 1)
>         {
>            if(input_pin_port_a(3)) return 1;
>            else return 0;
>         }
>
>     if(calc == 2)
>         {
>             if(input_pin_port_b(3)) return 1;
>             else return 0;
>         }
>         delay_ms(2);
> }
>
> char get_byte(char calc)
> {
>     char bitcount=0, tempbit=0;
>     while(bitcount < 8)
>     {
>     while ((input_red(calc) == 1) && (input_white(calc) == 1)){nop();}
>     //putchar('1');
>     if (input_red(calc)==0)
>                 {
>                  output_white_low(calc);
>                  while(input_red(calc)==0);
>                  //putchar('2');
>                  bitcount=bitcount + 1;
>                  output_white_high(calc);
>                  delay_ms(5);
>                 }
>
>     if (input_red(calc)==1)
>                 {
>                  output_red_low(calc);
>                  while(input_white(calc)==0);
>                  //putchar('3');
>                  if (bitcount == 0) set_bit(tempbit, 0);
>                  if (bitcount == 1) set_bit(tempbit, 1);
>                  if (bitcount == 2) set_bit(tempbit, 2);
>                  if (bitcount == 3) set_bit(tempbit, 3);
>                  if (bitcount == 4) set_bit(tempbit, 4);
>                  if (bitcount == 5) set_bit(tempbit, 5);
>                  if (bitcount == 6) set_bit(tempbit, 6);
>                  if (bitcount == 7) set_bit(tempbit, 7);
>
>                  bitcount=bitcount+1;
>                  output_red_high(calc);
>                  delay_ms(5);
>                 }
>     }
>
>     return tempbit;
> }
>
> void transmit_high(char calc)
> {
>     output_white_low(calc);
>
>     while(input_red(calc)==1){nop();}
>
>     output_white_high(calc);
>
>     while(input_red(calc)==0){nop();}
>
> }
>
> void transmit_low(char calc)
> {
>     output_red_low(calc);
>
>     while(input_white(calc)==1){nop();}
>
>     output_red_high(calc);
>
>     while(input_white(calc)==0){nop();}
>
> }
>
> void send_byte(char byte, char calc)
> {
>
>     bytetemp=byte;
>
>     while((input_red(calc)==0) || (input_white(calc)==0)){nop();}
>     delay_ms(2);
>
>     asm{
>     bank _bytetemp
>     mov w, _bytetemp
>     and w, #%00000001
>     bank _bytetemp1
>     mov _bytetemp1, w
>     }
>     if(bytetemp1==0) transmit_low(calc);
>     else transmit_high(calc);
>
>     asm{
>     bank _bytetemp
>     mov w, _bytetemp
>     and w, #%00000010
>     bank _bytetemp1
>     mov _bytetemp1, w
>     }
>     if(bytetemp1==0) transmit_low(calc);
>     else transmit_high(calc);
>
>     asm{
>     bank _bytetemp
>     mov w, _bytetemp
>     and w, #%00000100
>     bank _bytetemp1
>     mov _bytetemp1, w
>     }
>     if(bytetemp1==0) transmit_low(calc);
>     else transmit_high(calc);
>
>     asm{
>     bank _bytetemp
>     mov w, _bytetemp
>     and w, #%00001000
>     bank _bytetemp1
>     mov _bytetemp1, w
>     }
>     if(bytetemp1==0) transmit_low(calc);
>     else transmit_high(calc);
>
>     asm{
>     bank _bytetemp
>     mov w, _bytetemp
>     and w, #%00010000
>     bank _bytetemp1
>     mov _bytetemp1, w
>     }
>     if(bytetemp1==0) transmit_low(calc);
>     else transmit_high(calc);
>
>     asm{
>     bank _bytetemp
>     mov w, _bytetemp
>     and w, #%00100000
>     bank _bytetemp1
>     mov _bytetemp1, w
>     }
>     if(bytetemp1==0) transmit_low(calc);
>     else transmit_high(calc);
>
>     asm{
>     bank _bytetemp
>     mov w, _bytetemp
>     and w, #%01000000
>     bank _bytetemp1
>     mov _bytetemp1, w
>     }
>     if(bytetemp1==0) transmit_low(calc);
>     else transmit_high(calc);
>
>     asm{
>     bank _bytetemp
>     mov w, _bytetemp
>     and w, #%10000000
>     bank _bytetemp1
>     mov _bytetemp1, w
>     }
>     if(bytetemp1==0) transmit_low(calc);
>     else transmit_high(calc);
> }
>
> main()
> {
>     set_tris_a(255);
>     set_tris_b(255);
>     set_tris_c(11111110b);
>
>     output_port_a(0);
>     output_port_b(0);
>     output_port_c(0);
>
>    printf("\n\r START \n\r");
>
>    //while(1){
>    //temp4=get_byte(1);
>    //sendHEXNumber(temp4);
>    //printf("\n\r");
>    //}
>
>    while(1){
>    if(input_red(1)==0) {temp4=get_byte(1); send_byte(temp4, 2);
> sendHEXNumber(temp4);}
>    if(input_white(1)==0) {temp4=get_byte(1); send_byte(temp4, 2);
> sendHEXNumber(temp4);}
>    if(input_red(2)==0) {temp4=get_byte(2); send_byte(temp4, 1);
> sendHEXNumber(temp4);}
>    if(input_white(2)==0) {temp4=get_byte(2); send_byte(temp4,
> 1);sendHEXNumber(temp4);}
>             }
>
> }
>
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