LTC2873

22

REV B

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APPLICATIONS INFORMATION

Logic Loopback

Aloopbackmodeconnectsthedriverinputstothereceiver

outputs(non-inverting)providinganechoforself-test.This

applies to both RS232 and RS485 transceivers. Loopback

mode is entered when the LB pin is set to a logic-high and

the relevant receiver is enabled. The RS485 driver output

can be disabled in loopback mode if DE485/F232 is held

low, or functions normally with DE485/F232 high. The

RS232 driver output cannot be disabled when loopback

is engaged in RS232 mode, and functions normally. The

loopback signal traverses a path from the logic input cir-

cuit at DI to the logic output at RO and does not exercise

the entire driver or receiver circuit. Thus loopback, alone,

is not a sufficient test to ensure full functionality of the

LTC2873. Loopback does not affect the operation of the

termination resistors.

Robust ESD Protection

TheLTC2873featuresexceptionallyrobustESDprotection.

The transceiver interface pins (A and B) are protected to

without latchup or damage. This protection holds whether

the device is unpowered or powered in any mode of opera-

tion. To note, ±26kV is an upper limit of the tester—the

actual device protection level is higher. Every other pin on

the device is protected to ±4kV ESD (HBM) for all-around

robustness. Figure 22 shows the LTC2873 being struck

repeatedly with 26kV of ESD energy (air gap discharge)

during operation with no damage or circuit latchup.

RS485 Cable Length vs Data Rate

Many factors contribute to the maximum cable length

that can be used for RS485 or RS422 communication,

including driver transition times, receiver threshold, duty

Figure 22. LTC2873 Struck Repeatedly with 26kV of ESD Energy While Operating. No Damage or Circuit Latchup Occurs