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Manual 00650-521-1
The C command to set the UART for an 8-bit word, no parity, and one stop bit is:
outportb(BA3, 0x03)
The final initialization step is to flush the receiver buffers. You do this with two reads from the
receiver buffer at Base A0. When done, the UART is ready to use.
Reception
Reception can be handled in two ways: polling and interrupt-driven. When polling, reception is
accomplished by constantly reading the Line Status Register at Base A5. Bit 0 of this
register is set high whenever data are ready to be read from the chip. A simple polling loop must
continuously check this bit and read in data as it becomes available. The following code fragment
implements a polling loop and uses a value of 13, (ASCII Carriage Return) as an
end-of-transmission marker:
do
{
while (!(inportb(BA5) & 1)); /*Wait until data ready*/
data[i++]= inportb(BASEADDR);
}while (data[i]!=13); /*Reads the line until null character rec’d*/
Interrupt-driven communications should be used whenever possible and is required for high data
rates. Writing an interrupt-driven receiver is not much more complex than writing a polled
receiver but care should be taken when installing or removing your interrupt handler to avoid
writing the wrong interrupt, disabling the wrong interrupt, or turning interrupts off for too long a
period.
The handler would first read the Interrupt Identification Register at Base A2. If the
interrupt is for Received Data Available, the handler then reads the data. If no interrupt is pending,
control exits the routine. A sample handler, written in C, is as follows:
readback = inportb(BA2);
if (readback & 4) /*Readback will be set to 4 if data are available*/
data[i++]=inportb(BASEADDR);
outportb(0x20,0x20); /*Write EOI to 8259 Interrupt Controller*/
return;
Transmission
RS485 transmission is simple to implement. The AUTO feature in RS485 mode automatically
enables the transmitter when data are ready to send, so no software enabling is required. The
following software example is for non-AUTO operation in RS422 mode. First the RTS line should
be set high by writing a 1 to Bit 1 of the Modem Control Register at Base A4. The RTS
line is used to toggle the transceiver from receive mode to transmit mode and vice versa..
