Chapter 5 Input/Output Interfaces
Compaq Deskpro 4000N and 4000S Personal Computers
First Edition – September 1997
5-20
Line Status Register, I/O Port 3FDh/2FDh (Read Only)
This register contains the status of the current data transfer. Bits <2..0> are cleared when read.
Bit Function
7 Parity Error, Framing Error, or Break Cond. Exists (if set)
6 TX Holding Reg. and Transmitter Shift Reg. Are Empty (if set)
5 TX Holding Reg. Is Empty (if set)
4 Break Interrupt Has Occurred (if set)
3 Framing Error Has Occurred (if set
2 Parity Error Has Occurred (if set)
1 Overrun Error Has Occurred (if set)
0 Data Register Ready To Be Read (if set)
Modem Status Register, I/O Port 3FEh/2FEh (Read Only)
This register contains the status of the modem signal lines. A set bit indicates that the associated
signal is active.
Bit Function
7 DCD- Active
6 RI- Active
5 DSR Active
4 CTS Active
3 DCD- Changed Since Last Read
2 RI- Changed From Low to High Since Last Read
1 DSR- Has Changed State Since Last Read
0 CTS- Has Changed State Since Last Read
Scratch Pad Register, I/O Port 3FFh/2FFh
This register is not used in this system.
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