Compaq Deskpro 4000N Technical Reference Manual page 108

Chapter 5 Input/Output Interfaces
Baud Rate Divisor Latch Register, I/O Port 3F8h, 3F9h/2F8, 2F9h
When bit <7> of the Line Control register is set (1), a write to this pair of locations loads the
decimal value used to divide the 1.8462-MHz clock to create the desired baud rate for serial
transmission. The possible baud rates are shown as follows:
Baud Rate
50
75
110
134.5
150
300
600
1200
1800
2000
Interrupt Enable Register, I/O Port 3F9h/2F9h
Bits <3..0> of this register are used for enabling interrupt sources. A set bit enables interrupt
generation by the associated source.
Bit Function
7..4 Reserved
3 Modem Status Interrupt Enable (if set) (CTS, DSR, RI, CD)
2 Receiver Line Status Interrupt Enable (if set) (Overrun error,
parity error, framing error, break)
1 Transmitter Holding Register Empty Interrupt Enable (if set)
0 Baud Rate Divisor Interrupt Enable (if set)
Interrupt ID Register, I/O Port 3FAh/2FAh (Read Only)
This read-only register indicates the serial controller as the source of the interrupt (bit <0>) as
well as the reason (bits <3..1>) for the interrupt. Reading this register clears the interrupt and
sets bit <0>.
Bit Function
7,6 FIFO Enable/Disable
0 = Disabled
1 = Enabled
5,4 Reserved
3..1 Interrupt Source:
000 = Modem status (lowest priority)
001 = Transmitter holding reg. Empty
010 = Received data available
011 = Receiver line status reg.
100,101 = Reserved
110 = Character time-out (highest priority)
111 = Reserved
0 Interrupt Pending (if cleared)
5-18 Compaq Deskpro 4000N and 4000S Personal Computers
Decimal Divisor Baud Rate
2304
1536
1047
857
768
384
192
96
64
58
Divisor = 1846200 / (Desired baud rate X 16)
First Edition – September 1997
Decimal Divisor
2400 48
3600 32
4800 24
7200 16
9600 12
19200 6
38400 3
57600 2
115200 1