Programmable DTE/DCE,

+5V RS-232 Transceiver

8

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which improves noise margins. The polarity of each

receiver’s input threshold is shown in Tables 1a and 1b.

In normal operating mode, receiver inputs are internally

connected to ground with 5k

Ω resistors. So uncon-

nected receivers with positive input thresholds have

high outputs, and those with negative input thresholds

have low outputs.

When shut down, all receivers have positive thresholds.

This allows the receiver inputs to respond to

TTL-/CMOS-logic levels, as well as RS-232 levels. The

guaranteed 0.8V input threshold ensures that receivers

shorted to ground will have a logic 1 output. Also, the

300k

Ω input resistance to ground ensures that a receiv-

er with its input left open will also have a logic 1 output.

The receiver’s 0.5V of hysteresis provides clean output

transitions, even with slow rise-time and fall-time sig-

nals with moderate amounts of noise and ringing. The

receivers have no hysteresis in shutdown mode.

HI-Z Control

The receiver inputs are terminated with 5k

Ω resistors, to

comply with the requirements of EIA/TIA-232E.

However, these internal resistors can be disconnected

by taking the HI-Z control pin to a logic high. This

makes all of the MAX214’s receiver inputs high imped-

ance, and facilitates the transmission of RS-232 data

from a single transmitter to multiple receivers. In this

case, all but one of the receiving ICs should be put into

the high input-impedance state.

Shutdown Control

In shutdown mode, the charge pumps are turned off,

the transmitter outputs are disabled. This reduces sup-

ply current typically to 4µA.

The time required to exit

shutdown is about 250µs, as shown in Figure 3.

Receivers

Receiver outputs never go into a high-impedance state;

they are always active, even in shutdown mode (see

Table 2). These awake-in-shutdown receivers are use-

ful for monitoring external activity (for example, on RI),

while maintaining minimal power consumption.

Receivers in shutdown mode are slower (20kbps) than

when not shut down (116kbps), and lack the hysteresis

present in normal operation.

Drivers

The driver outputs are high impedance in shutdown

mode, even when back-driven with voltages up

to ±15V.

__________Applications Information

Capacitor Selection

The type of capacitor (C1 to C4) used is not critical for

proper operation. The MAX214 requires 1µF capaci-

tors, although in all cases capacitors of up to 10µF

can be used without harm. Ceramic dielectrics are

suggested for the 1µF capacitors.

Table 2. Control Pin Configurations

CONTROL INPUTS

SHUTDOWN

HI-Z

DTE/DCE

RTA, RTB, RTC, RDTC

RRE

0

0

0

Receive Mode/5k

Ω

Receive Mode/5k

Ω

0

0

1

Transmit Mode

Receive Mode/5k

Ω

0

1

0

Receive Mode/HI-Z

Receive Mode/HI-Z

0

1

1

Transmit Mode

Receive Mode/HI-Z

1

0

0

Slow Receive/HI-Z

Slow Receive/HI-Z

1

0

1

Disabled/HI-Z

Slow Receive/HI-Z

1

1

0

Slow Receive/HI-Z

Slow Receive/HI-Z

1

1

1

Disabled/HI-Z

Slow Receive/HI-Z

RS-232 PINS

TRA, TRB, TRC

Transmit Mode

Receive Mode/5k

Ω

Transmit Mode

Receive Mode/HI-Z

Disabled/HI-Z

Slow Receive/HI-Z

Disabled/HI-Z

Slow Receive/HI-Z