LINE CONTROL REGISTER (LCR)
Address Offset = 3H, DLAB = 0, READ/WRITE
checked (receive data) between the last data word bit and
the first stop bit of the serial data. (The parity bit is used to
generate an even or odd number of 1s when the data
word bits and the parity bit are summed).
This register contains the format information of the serial
line. The bit definitions are:
Bit 4
Bits 0 and 1
Even Parity Select bit. When bit 3 is a logic "1" and bit 4
is a logic "0", an odd number of logic "1"'s is transmitted
or checked in the data word bits and the parity bit. When
bit 3 is a logic "1" and bit 4 is a logic "1" an even number
of bits is transmitted and checked.
These two bits specify the number of bits in each
transmitted or received serial character. The encoding of
bits 0 and 1 is as follows:
BIT 1 BIT 0 WORD LENGTH
Bit 5
0
0
1
1
0
1
0
1
5 Bits
6 Bits
7 Bits
8 Bits
Stick Parity bit. When bit 3 is a logic "1" and bit 5 is a
logic "1", the parity bit is transmitted and then detected by
the receiver in the opposite state indicated by bit 4.
Bit 6
Set Break Control bit. When bit 6 is a logic "1", the
transmit data output (TXD) is forced to the Spacing or
logic "0" state and remains there (until reset by a low level
bit 6) regardless of other transmitter activity. This feature
enables the Serial Port to alert a terminal in a
communications system.
The Start, Stop and Parity bits are not included in the
word length.
Bit 2
This bit specifies the number of stop bits in each
transmitted or received serial character. The following
table summarizes the information.
Bit 7
Divisor Latch Access bit (DLAB). It must be set high
(logic "1") to access the Divisor Latches of the Baud Rate
Generator during read or write operations. It must be set
low (logic "0") to access the Receiver Buffer Register, the
Transmitter Holding Register, or the Interrupt Enable
Register.
NUMBER OF
STOP BITS
BIT 2 WORD LENGTH
0
1
1
1
1
--
1
1.5
2
5 bits
6 bits
7 bits
8 bits
MODEM CONTROL REGISTER (MCR)
Address Offset = 4H, DLAB = X, READ/WRITE
2
2
This 8 bit register controls the interface with the MODEM
or data set (or device emulating a MODEM). The
contents of the MODEM control register are described
below.
Note: The receiver will ignore all stop bits beyond the
first, regardless of the number used in transmitting.
Bit 3
Parity Enable bit. When bit 3 is a logic "1", a parity bit
is generated (transmit data) or
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