2.5V, 1µA, 460kbps,

RS-232-Compatible Transceivers

10

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When shut down, the device’s charge pumps turn off,

outputs are high impedance, and READY is driven low.

The time required to exit shutdown is typically 30µs

(Figure 2).

By connecting FORCEON to INVALID, the MAX3318/

MAX3319 shut down when no valid receiver level is

detected and wake up when a valid receiver level is

detected.

A system with AutoShutdown Plus may need time to

wake up. Figure 5 shows a circuit that forces the trans-

mitters on for 100ms, allowing enough time for the other

system to realize that the MAX3318/MAX3319 is awake.

If the other system outputs valid RS-232 signal transi-

tions within that time, the RS-232 ports on both systems

remain enabled.

Connecting to the PC

(MAX3318/MAX3319)

If direct software control is desired, use INVALID to

indicate DTR or ring indicator (RI) signal. This can be

used to connect a hand-held device to a PC. One

example is using the hot sync function on a personal

digital assistant (PDA). The transmitter and receiver

signals (T_OUT and R_IN) are used for communication,

while INVALID causes a change of state on RI. The

change of state on RI will trigger an interrupt on the PC

and allow communication to begin between the device

and the PC. This eliminates the need for the PC to poll

constantly the receiver or transmitter lines to determine

if the device is connected.

Applications Information

RS-232-Compatible Operation

The MAX3316–MAX3319 do not meet EIA-232 require-

ments for transmitter output voltage levels. EIA-232 com-

pliance specifies transmitter output voltage swings of

±5V when loaded with 3k

Ω and 2500pF.

The receiver inputs are fully EIA-232 compliant.

The MAX3316–MAX3319 will function properly with most

modern RS-232 interfaces. This allows RS-232-compati-

ble communication in low-voltage systems without the

added expense of a voltage tripler or switched-mode

power supply.

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-

tors. Increasing the capacitor values (e.g., by a factor of

2) reduces ripple on the transmitter outputs and slightly

reduces power consumption. C2, C3, and C4 can be

increased without changing C1’s value. However, do

not increase C1 without also increasing the values of

+0.3V

-0.3V

INVALID

R_IN

INVALID ASSERTED IF ALL RECEIVER INPUTS ARE BETWEEN +0.3V AND -0.3V FOR

AT LEAST 30

µs.

30µs

TIMER

R

Figure 3a. INVALID Functional Diagram, INVALID Low

+2.7V

-2.7V

INVALID

R_IN

INVALID DEASSERTED IF ANY RECEIVER INPUT HAS BEEN BETWEEN +2.7V AND -2.7V

FOR LESS THAN 30

µs.

30µs

TIMER

R

Figure 3b. INVALID Functional Diagram, INVALID High

-2.7V

-0.3V

+2.7V

+0.3V

0

INDETERMINATE

INVALID HIGH

INVALID LOW

INVALID HIGH

INDETERMINATE

Figure 4a. Receiver Positive/Negative Thresholds for INVALID