if (PrinterDataRegFull) PrinterTimer++; // if transmitting, increment timer
switch(PrinterTimer){
case 0:
// Serial transmitter is idle. Check if we have a byte to send
if(PrinterQueueHead != PrinterQueueTail){ // if non-equal, there are bytes in the buffer
// Get next byte and place it in the transmit data register
PrinterDataRegister = PrinterQueue[PrinterQueueTail]; // fetch next byte to transmit
PrinterQueueTail = (PrinterQueueTail + 1) % PRINTER_QUEUE_SIZE; // increment tail pointer
PrinterDataRegFull = 1; // signal that the transmit register is occupied
// Send start bit
PRINTER_TXD = 0; // set output to SPACE (TTL logic 0, RS232 +ve)
}
break;
case 5: // Start sending bit 0 at t=100 uSec (ideal = 104 uSec. Error = -4.0% of one bit)
case 10: // Start sending bit 1 at t=200 uSec (ideal = 208 uSec. Error = -8.0% of one bit)
case 16: // Start sending bit 2 at t=320 uSec (ideal = 313 uSec. Error = +7.2% of one bit)
case 21: // Start sending bit 3 at t=420 uSec (ideal = 417 uSec. Error = +3.2% of one bit)
case 26: // Start sending bit 4 at t=520 uSec (ideal = 521 uSec. Error = -0.8% of one bit)
case 31: // Start sending bit 5 at t=620 uSec (ideal = 625 uSec. Error = -4.8% of one bit)
case 36: // Start sending bit 6 at t=720 uSec (ideal = 729 uSec. Error = -8.8% of one bit)
case 42: // Start sending bit 7 at t=840 uSec (ideal = 833 uSec. Error = +6.4% of one bit)
PRINTER_TXD = (PrinterDataRegister & 1) ? 1 : 0; // start sending next databit (starting with D0)
PrinterDataRegister >>= 1; // rotate the transmit data register
break;
case 47: // Start sending 2 stop bits at t=940 uSec (ideal = 938 uSec. Error = +2.4% of one bit)
PRINTER_TXD = 1; // set output to MARK (TTL logic 1, RS232 -ve)
break;
case (57 - 1): // This will give next start bit at t=1140 uSec (ideal = 1146 uSec. Error = -5.6% of one bit)
// At next interrupt, we can start sending next byte
PrinterDataRegFull = 0; // signal that the transmit register is empty
PrinterTimer = 0; // clear the transmit timer
break;
}