This feature allows the processor to verify the
transmit and receive data paths of the Serial Port.
In the diagnostic mode, the receiver and the
transmitter interrupts are fully operational. The
MODEM Control Interrupts are also operational
but the interrupts' sources are now the lower four
bits of the MODEM Control Register instead of the
MODEM Control inputs. The interrupts are still
controlled by 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.
Bit 0
This bit controls the Data Terminal Ready (nDTR)
output. When bit 0 is set to a logic "1", the nDTR
output is forced to a logic "0". When bit 0 is a logic
"0", the nDTR output is forced to a logic "1".
Bit 1
Bits 5 through 7
These bits are permanently set to logic zero.
This bit controls the Request To Send (nRTS)
output. Bit 1 affects the nRTS output in a manner
identical to that described above for bit 0.
Bit 2
LINE STATUS REGISTER (LSR)
Address Offset = 5H, DLAB = X, READ/WRITE
Bit 0
This bit controls the Output 1 (OUT1) bit. This bit
does not have an output pin and can only be read
or written by the CPU.
Data Ready (DR). It is set to a logic "1" whenever
a complete incoming character has been received
and transferred into the Receiver Buffer Register
or the FIFO. Bit 0 is reset to a logic "0" by reading
all of the data in the Receive Buffer Register or the
FIFO.
Bit 3
Output 2 (OUT2). This bit is used to enable an
UART interrupt. When OUT2 is a logic "0", the
serial port interrupt output is forced to a high
impedance state - disabled. When OUT2 is a
logic "1", the serial port interrupt outputs are
enabled.
Bit 1
Overrun Error (OE). Bit 1 indicates that data in the
Receiver Buffer Register was not read before the
next character was transferred into the register,
thereby destroying the previous character. In
FIFO mode, an overrun error will occur only when
the FIFO is full and the next character has been
completely received in the shift register, the
character in the shift register is overwritten but not
transferred to the FIFO. The OE indicator is set to
a logic "1" immediately upon detection of an
overrun condition, and reset whenever the Line
Status Register is read.
Bit 4
This bit provides the loopback feature for
diagnostic testing of the Serial Port. When bit 4 is
set to logic "1", the following occur:
1. The TXD is set to the Marking State(logic "1").
2. The receiver Serial Input (RXD) is
disconnected.
3. The output of the Transmitter Shift Register is
"looped back" into the Receiver Shift Register
input.
Bit 2
Parity Error (PE). Bit 2 indicates that the received
data character does not have the correct even or
odd parity, as selected by the even parity select
bit. The PE is set to a logic "1" upon detection of a
parity error and is reset to a logic "0" whenever the
Line Status Register is read. In the FIFO mode
this error is associated with the particular character
in the FIFO it applies to. This error is indicated
when the associated character is at the top of the
FIFO.
4. All MODEM Control inputs (nCTS, nDSR, nRI
and nDCD) are disconnected.
5. The four MODEM Control outputs (nDTR,
nRTS, OUT1 and OUT2) are internally
connected to the four MODEM Control inputs
(nDSR, nCTS, RI, DCD).
6. The Modem Control output pins are forced
inactive high.
7. Data that is transmitted is immediately
received.
Bit 3
74
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