Rx

PROTECTION

DIODE

UART

Tx

GND

SHDN = GND

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RS-232

RS-232 Receivers

The receivers convert RS-232 signals to CMOS-logic out-

put levels. The MAX3222/MAX3237/MAX3241 receivers

have inverting three-state outputs. In shutdown, the

receivers can be active or inactive (Table 1).

The complementary outputs on the MAX3237 (R1OUTB)

and the MAX3241 (R1OUTB, R2OUTB) are always active,

regardless of the state of EN or SHDN. This allows for

Ring Indicator applications without forward biasing other

devices connected to the receiver outputs. This is ideal

accommodate peripherals, such as UARTs (Figure 2).

5k

MAX3222/MAX3237/MAX3241

Shutdown Mode

Supply current falls to less than 1µA in shutdown mode

(SHDN = low). When shut down, the device’s charge

pulled to ground, and the transmitter outputs are dis-

abled (high impedance). The time required to exit shut-

a) OLDER RS-232: POWERED-DOWN UART DRAWS CURRENT FROM

ACTIVE RECEIVER OUTPUT IN SHUTDOWN.

down is typically 100µs, as shown in Figure 3. Connect

has no effect on R_OUT or R_OUTB.

MAX3222/MAX3237/MAX3241

Enable Control

The inverting receiver outputs (R_OUT) are put into a

TO

LOGIC

P

TRANSITION

DETECTOR

Rx

R1OUTB

R1OUT

MAX3237

MAX3241

R1IN

high-impedance state when EN is high. The complemen-

tary outputs R1OUTB and R2OUTB are always active,

regardless of the state of EN and SHDN (Table 1). EN

has no effect on T_OUT.

Applications Information

Capacitor Selection

The capacitor type used for C1–C4 is not critical for

proper operation; polarized or nonpolarized capacitors

can be used. The charge pump requires 0.1µF capaci-

PROTECTION

DIODE

UART

Tx

GND

T1IN

SHDN = GND

5k

T1OUT

tors for 3.3V operation. For other supply voltages, refer to

Table 2 for required capacitor values. Do not use values

lower than those listed in Table 2. Increasing the capaci-

tor values (e.g., by a factor of 2) reduces ripple on the

transmitter outputs and slightly reduces power consump-

tion. C2, C3, and C4 can be increased without changing

C1’s value. However, do not increase C1 without also

increasing the values of C2, C3, and C4, to maintain the

b) NEW MAX3237/MAX3241: EN SHUTS DOWN RECEIVER OUTPUTS (EXCEPT

FOR B OUTPUTS), SO NO CURRENT FLOWS TO UART IN SHUTDOWN. B

OUTPUTS INDICATE RECEIVER ACTIVITY DURING SHUTDOWN WITH EN HIGH.

Figure 2. Detection of RS-232 Activity when the UART and

Interface are Shut Down; Comparison of MAX3237/MAX3241

(b) with Previous Transceivers (a).

proper ratios (C1 to the other capacitors).

When using the minimum required capacitor values,

make sure the capacitor value does not degrade exces-

sively with temperature. If in doubt, use capacitors with a

higher nominal value. The capacitor’s equivalent series

resistance (ESR), which usually rises at low tempera-

tures, influences the amount of ripple on V+ and V-.