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PCI-COM422/4 Manual
Page 5-2
Manual MPCICOM422-4.E1b
On the PCI-COM422/4 card, the UART clock frequency is 1.8432 MHz. On the next page is a table
for the popular divisor frequencies:
Baud Rate Divisor Max Diff. Cable Length *
115200 1 2200 ft
57600 2 4000 ft
38400 3 4000 ft
28800 4 4000 ft
19200 6 4000 ft
14400 8 4000 ft
9600 12 4000 ft
4800 24 4000 ft
2400 48 4000 ft
1200 96 4000 ft
Table 5-1: Baud Rate Divisor Values
*These are theoretical maximums based on typical conditions and good quality cables based on the
EIA 485 and EIA 422 standard for balanced differential drivers. RS-232 communication lines have
a maximum length of 50 feet, regardless of speed.
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 Address +3. 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.