鈥?/div>
POWER-
MANAGEMENT
UNIT OR
KEYBOARD
CONTROLLER
FORCEOFF
FORCEON
INVALID
PROTECTION
DIODE
V
CC
MAX3244E
MAX3245E
V
CC
Rx
UART
Tx
GND
SHDN = GND
I
PREVIOUS
RS-232
5k
CPU
I/O
CHIP
WITH
UART
RS-232
a)
OLDER RS-232: POWERED-DOWN UART DRAWS CURRENT FROM ACTIVE
RECEIVER OUTPUT IN SHUTDOWN.
V
CC
TO
碌P
LOGIC
TRANSITION
DETECTOR
Figure 1. Interface Under Control of PMU
PROTECTION
DIODE
I
MAX3244E
MAX3245E
R2OUTB
munication software (such as LapLink鈩?. Transmitters
can be paralleled to drive multiple receivers. Figure 1
shows a complete system connection.
When
FORCEOFF
is driven to ground or when the Auto-
Shutdown Plus circuitry senses that all receiver and
transmitter inputs are inactive for more than 30sec, the
transmitters are disabled and the outputs go into a high-
impedance state. When powered off or shut down, the
outputs can be driven to 鹵12V. The transmitter inputs
do not have pull-up resistors. Connect unused inputs to
GND or V
CC
.
V
CC
Rx
UART
R2OUT
THREE-STATED
5k
Tx
GND
T1IN
FORCEOFF = GND
T1OUT
R2IN
RS-232 Receivers
The receivers convert RS-232 signals to CMOS-logic
output levels. The MAX3224E鈥揗AX3227E feature
inverting outputs that always remain active (Table 1).
The MAX3244E/MAX3245E have inverting three-state
b)
NEW MAX3244E/MAX3245E: IN SHUTDOWN, R2OUTB IS USED TO MONITOR
EXTERNAL DEVICES AND R2OUT IS THREE STATED, ELIMINATING A CURRENT
PATH THROUGH THE UART'S PROTECTION DIODE.
Figure 2. The MAX3244E/MAX3245E detect RS-232 activity
when the UART and interface are shut down.
8
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