
81
Bit 2
This bit specifies the number of stop bits in each transmitted or received serial character. The
following table summarizes the information.
BIT 2
0
1
1
1
1
WORD LENGTH
--
5 bits
6 bits
7 bits
8 bits
NUMBER OF
STOP BITS
1
1.5
2
2
2
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 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).
Bit 4
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.
Bit 5
Stick Parity bit. When parity is enabled it is used in conjunction with bit 4 to select Mark or Space
Parity. When LCR bits 3, 4 and 5 are 1 the Parity bit is transmitted and checked as 0 (Space Parity).
If bits 3 and 5 are 1 and bit 4 is 0, then the Parity bit is transmitted and checked as 1 (Mark Parity). If
bit 5 is 0 Stick Parity is disabled.
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.
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.
Modem Control Register (MCR)
Address Offset = 4H, DLAB = X, READ/WRITE
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.