Baud Rate Divisor x1 Divisor x4
Max Diff. Cable Length*
460800 -
1
550
ft
230400 -
2
1400
ft
153600 -
3
2500
ft
115200 1
4
3000
ft
57600 2
8
4000
ft
38400 3
12
4000
ft
28800 4
16
4000
ft
19200 6
24
4000
ft
14400 8
32
4000
ft
9600
12
48 – Most Common 4000 ft
4800 24 96
4000
ft
2400 48 192
4000
ft
1200 96 384
4000
ft
* Recommended maximum distances for differentially driven data cables (RS422 or RS485) are for typical conditions.
Table 5-1:
Baud Rate Divisors
In C, the code to set the chip to 9600 baud is:
outportb(BASEADDR, 0x0C);
outportb(BA1,0);
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)
The third step of the initialization sequence is to set the Modem Control Register at Base
A4. Bit 1 is the Request to Send (RTS) control bit. This bit should be left low until
transmission time. (Note: When operating in the automatic RS485 mode, the state of this bit is
not significant.) Bits 2 and 3 are user-designated outputs. Bit 2 may be ignored on this card. Bit
3 is used to enable interrupts and should be set high if an interrupt-driven receiver is to be
used.
