Kontron PCI-COM422, User Manual

Page: 28 / 36

page 28 PCI-COM422 User’s Guide
Baud Rate Divisor Values:
Baud Rate Divisor x1 Divisor x8 Max Diff. Cable Length*
921600 - 1 250 ft
460800 - 2 550 ft
230400 - 4 1400 ft
153600 - 6 2500 ft
115200 1 8 3000 ft
57600 2 16 4000 ft
38400 3 24 4000 ft
28800 4 32 4000 ft
19200 6 48 4000 ft
14400 8 64 4000 ft
9600 12 96 – Most Common 4000 ft
4800 24 192 4000 ft
2400 48 384 4000 ft
1200 96 768 4000 ft
* These are theoretical maximums for typical conditions and good quality cables based on the EIA 485 and EIA 422 standard for balanced
differential drivers.
In C, the code to set the chip to 9600 baud is:
outportb(BASEADDR,
0x0C);
outportb(BASEADDR +1,0);
The second initializing step is to set the Line Control Register at Base A3. This
register defines word length, stop bits, parity, and the DLAB.
Bits 0 and 1 control word length and allow word lengths from 5 to 8 bits. Bit settings are
extracted by subtracting 5 from the desired word length.
Bit 2 determines the number of stop bits. There can be either one or two stop bits. If Bit
2 is set to 0, there will be one stop bit. If Bit 2 is set to 1, there will be two stop bits.
Bits 3 through 6 control parity and break enable. They are not commonly used for
communications and should be set to zeroes.
Bit 7 is the DLAB discussed earlier. It must be set to zero after the divisor is loaded or
else there will be no communications.
The C command to set the UART for an 8-bit word, no parity, and one stop bit is:
outportb(BA3, 0x03)