RS-232 Transmitters

The transmitters are inverting level translators that

convert CMOS-logic levels to 5.0V EIA/TIA-232 levels.

The MAX3224/MAX3226/MAX3244 guarantee a

250kbps data rate (1Mbps for the MAX3225/MAX3227/

MAX3245) with worst-case loads of 3kΩ in parallel with

1000pF, providing compatibility with PC-to-PC com-

munication software (such as LapLink™). 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 ±12V. The transmitter inputs

do not have pull-up resistors. Connect unused inputs to

1µA Supply Current, 1Mbps, 3.0V to 5.5V,

RS-232 Transceivers with AutoShutdown Plus

8

_______________________________________________________________________________________

MAX3244

MAX3245

I/O

CHIP

WITH

UART

CPU

RS-232

POWER-

MANAGEMENT

UNIT OR

KEYBOARD

CONTROLLER

FORCEOFF

FORCEON

INVALID

Figure 1. Interface Under Control of PMU

MAX3244

MAX3245

T1OUT

R2OUTB

Tx

5k

Ω

UART

T1IN

THREE-STATED

LOGIC

TRANSITION

DETECTOR

R2IN

PROTECTION

DIODE

R2OUT

FORCEOFF = GND

TO

μP

Rx

PREVIOUS

RS-232

Tx

UART

SHDN = GND

GND

Rx

5k

Ω

RECEIVER OUTPUT IN SHUTDOWN.

EXTERNAL DEVICES AND R2OUT IS THREE STATED, ELIMINATING A CURRENT

PATH THROUGH THE UART’S PROTECTION DIODE.

GND

PROTECTION

DIODE

I

I

Figure 2. The MAX3244/MAX3245 detect RS-232 activity when

the UART and interface are shut down.