S
H
U
T
+2.7V
0V
RECEIVER
RS-232 INPUT
VOLTAGES
D
O
W
N
-2.7V
VCC
STATUS
0V
tSTSL
t
STSH
tONLINE
+5V
DRIVER
RS-232 OUTPUT
VOLTAGES
0V
-5V
Figure 14. AUTO ON-LINE® Timing Waveforms
The first stage, shown in Figure 21, detects an
inactive input. A logic HIGH is asserted on
RXINACT if the cable is disconnected or the
external transmitters are disabled. Otherwise,
RXINACT will be at a logic LOW. This circuit is
duplicated for each of the other receivers.
The STATUS pin outputs a logic LOW signal
if the device is shutdown. This pin goes to a
logic HIGH when the external transmitters are
enabled and the cable is connected.
When the SP3243E devices are shut down, the
charge pumps are turned off. V+ charge pump
output decays to VCC, the V- output decays to
GND. The decay time will depend on the size
of capacitors used for the charge pump. Once
in shutdown, the time required to exit the shut
down state and have valid V+ and V- levels is
typically 200µs.
ThesecondstageoftheAUTOON-LINE® circuitry,
shown in Figure 22, processes all the receiver's
RXINACT signals with an accumulated delay that
disables the device to a 1µA supply current.
The STATUS pin goes to a logic LOW when the
cable is disconnected, the external transmit-
ters are disabled, or the SHUTDOWN pin is
invoked.Thetypicalaccumulateddelayisaround
20µs.
For easy programming, the STATUS can be
used to indicate DSR or a Ring Indicator sig-
nal. Tying ONLINE and SHUTDOWN together
will bypass the AUTO ON-LINE® circuitry so this
connection acts like a shutdown input pin.
When the SP3243E drivers or internal charge
pump are disabled, the supply current is reduced
to 1µA. This can commonly occur in hand-held
or portable applications where the RS-232 cable
is disconnected or the RS-232 drivers of the con-
nected peripheral are turned off.
The AUTO ON-LINE® mode can be disabled by
the SHUTDOWN pin. If this pin is a logic LOW,
the AUTO ON-LINE® function will not operate
regardless of the logic state of the ONLINE pin.
Table 3 summarizes the logic of the AUTO ON-
LINE® operating modes. The truth table logic of
the SP3243E driver and receiver outputs can be
found in Table 2.
Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 510-668-7017 • www.exar.com
SP3243E_100_072309
12
EXAR [ EXAR CORPORATION ]
