MAX3212
Table 2 summarizes the MAX3212 operating modes.
FORCEON and FORCEOFF override the automatic cir-
cuitry and force the transceiver into its normal operat-
ing state or into its low-power standby state. When
neither control is asserted, the IC selects between
these states automatically based on receiver input lev-
els. Figure 4 depicts valid and invalid RS-232 receiver
levels. The MAX3212 shuts down after sensing invalid
RS-232 levels for greater than 30 µs, ensuring the
AutoShutdown mode is not enabled for slow-moving
signals (1V/µs).
A mouse or another system with AutoShutdown may
need a period of time to wake up. Figure 5 shows a cir-
cuit that forces the transmitters on for 100ms after start-
up, allowing enough time for the other system to realize
that the MAX3212 system is awake. If the other system
outputs valid RS-232 signals within that time, the RS-
232 ports on both systems remain enabled.
Transition Detector
The MAX3212 also has an on-board transition detector
that monitors activity on the receiver inputs. In systems
with a sleep mode, the transition detector output (TRAN)
can be used to wake up the system when activity at the
receiver inputs is detected. Before putting the system to
sleep, set TRAN low by cycling EN high and then low.
TRAN remains low as long as no activity is detected on
the receiver inputs. When any receiver is toggled, TRAN
latches high. Connect TRAN to a microprocessor inter -
rupt, or if the system wakes up periodically TRAN can
be polled. Transition detection is useful when valid RS-
232 levels are present at the receiver inputs but no data
is being sent. For example, if a printer is attached to the
serial port but is not in use, the microprocessor senses
this and forces the MAX3212 off.
+2.7V to +3.6V-Powered, 1µA Supply Current,
3-Driver/5-Receiver, True RS-232 Transceiver
8 _______________________________________________________________________________________
INVALID IS AN INTERNALLY GENERATED SIGNAL
THAT IS USED BY THE AUTOSHUTDOWN LOGIC
AND APPEARS AS AN OUTPUT OF THE DEVICE.
POWER DOWN IS ONLY AN INTERNAL SIGNAL.
IT CONTROLS THE OPERATIONAL STATUS OF
THE TRANSMITTERS AND THE POWER SUPPLIES.
FORCEOFF
0
0
0
0
1
1
1
1
FORCEON
0
0
1
1
0
0
1
1
INVALID
0
1
0
1
0
1
0
1
POWER DOWN
0
0
0
0
0
1
1
1
POWER DOWN
FORCEOFF
FORCEON
INVALID
+
-
+
-
RxRIN_
EN
ROUT_
30µs
DELAY
•
•
•
•
•
•
+0.3V to +3.0V
-0.3V to -3.0V
Figure 3. AutoShutdown Logic
