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PCI-COM422/4 Manual

Page 5-4

Manual MPCICOM422-4.E1b

Transmission

RS422 transmission is simple to implement. The AUTO feature of the PCI-COM422/4 card

automatically enables the transmitter when data is ready to send so no software enabling is required.

The following software example is for non-AUTO operation.

To transmit a string of data, the transmitter must first check Bit 5 of the Line Status Register at Base

Address +5. That bit is the transmitter-holding-register-empty flag. If it is high, the transmitter has

sent the data. The process of checking the bit until it goes high followed by a write is repeated until

no data remains.

The following C code fragment demonstrates this process:

outportb(BASEADDR +4, inportb(BASEADDR +4)|0x02);

/*Set RTS bit without altering states of other bits*/

while(data[i]); /*While there is data to send*/

{

while(!(inportb(BASEADDR +5)&0x20)); /*Wait until transmitter is empty*/

outportb(BASEADDR,data[i]);

i++;

}

outportb(BASEADDR +4, inportb(BASEADDR +4)&0xFD);

/*Reset RTS bit without altering states of other bits*/

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