11-27
ASYNCHRONOUS SERIAL I/O UNIT
11.3.8 Interrupt Enable Register (IER
n
)
Use IERn to connect the SIOn status signals to the interrupt control unit. All f our st atus signal scan be connected to the interrupt control unit.

Figure 11-17. Interrupt Enable Register (IER

n
)
Interrupt Enable
IER0, IER1
(read/write) Expanded Ad dr:
ISA Addr:
Reset State:
IER0 IER1
F4F9H F8F9H
03F9H 02F9H
00H 00H
7 0
———— MSRLSTBERBF
Bit
Number Bit
Mnemonic Function
7–4 — Reserved; for compatibility with future devices, write zeros to these bits.
3 MS Modem Status Interrupt Enable:
0 = Modem input signal changes do not cause interrupts.
1 = Connects the modem status signal to the interrupt control un it’s
SIOINT
n
output. A change on o ne or mo re of the modem input
signals activates the modem status signal.
2 RLS Receive r Line Status Interrupt Enable:
0 = LSR error conditions do not cause interrup ts.
1 = Connects the receiver line status signal to the interrupt control unit’s
SIOINT
n
output. Sources for this interrupt include overrun error,
parity error, framing error, and break interrupt.
1 TBE Transmit Buffe r Empty Interrupt Enable:
0 = Transmit Buffer Empty signal does not cause interrupts.
1 = Connects the transmit buffe r empty signal to the interrup t control
unit’s SIOINT
n
output.
0 RBF Receive Buffer Full Interrupt Enable:
0 = Receive buffer full sig nal does not cause interrupts.
1 = Connects the recei ve buffer full signal to the interru pt control unit’s
SIOINT
n
output.
NOTE: The interrupt enable register is multiplexed with the divisor latch high register. You must clear
bit 7 (DLAB) of the serial line control register (LCR
n
) before you can access the interrupt
control register.