~ National ~ Semiconductor - Al Kossow's Bitsavers

= r---------------------------------------------------------------.
It)
=
(,)
CJ)
z
7.0 Pin Descriptions (Continued)
Power Down (PO): active low, Pin 1? When active it disables
all internal clocks, shuts off the oscillator, clears RxE,
TxE, and break control bits in the command register. All
other registers retain their data. Unlike software power
down, PO also disables the internal ALE, CE, RD, WR, address
and data paths for minimum power consumption.
Registers cannot be accessed in hardware power down;
they may be in software power down.
Receiver Data (RxD): Pin 21. This accepts serial data input
from the communications link (peripheral device, modem, or
data set). Serial data is received least significant bit (LSB)
first. "Mark" is high (1), "space" is low (0).
Data Carrier Detect (DCD): active low, Pin 23. Can be used
as a modem or general purpose input. When this modem
input is low it indicates that the data carrier has been detected
by the modem or data set. The DCD signal is a modem
control function input whose complement value can be tested
by the CPU by reading bit 5 (DCD) of the modem status
register. Bit 1 (,lOCO) of the modem status register indicated
whether the DCD input has changed state since the previous
reading of the modem status register. DCD can also
be programmed to become an auto enable for the receiver.
NOTE: Whenever the DeD bit of the modem status register changes state,
an interrupt is generated if the