8

LT1785/LT1785A

LT1791/LT1791A

Overvoltage Protection

The LT1785/LT1791 RS485/RS422 transceivers answer

an applications need for overvoltage fault tolerance on

data networks. Industrial installations may encounter

common mode voltages between nodes far greater than

the – 7V to 12V range specified for compliance to RS485

standards. CMOS RS485 transceivers can be damaged by

voltages above their absolute maximum ratings of typi-

SWITCHI G TI E WAVEFOR S

Figure 8. Receiver Enable and Disable Times

Figure 7. Receiver Propagation Delays

Figure 6. Driver Enable and Disable Times

1.5V

2.3V

2.3V

1.5V

0.5V

0.5V

OUTPUT NORMALLY LOW

OUTPUT NORMALLY HIGH

5V

0V

DE

5V

0V

A, B

A, B

1785/91 F06

1.5V

RO

A – B

0V

0V

1.5V

OUTPUT

INPUT

1785/91 F07

1.5V

1.5V

1.5V

1.5V

0.5V

0.5V

OUTPUT NORMALLY LOW

OUTPUT NORMALLY HIGH

5V

0V

RE

5V

0V

RO

RO

1785/91 F08

APPLICATIO S I FOR ATIO

cally – 8V to 12.5V. Replacement of standard RS485

transceiver components with the LT1785 or LT1791

devices eliminates field failures due to overvoltage faults

or the use of costly external protection devices. The limited

overvoltage tolerance of CMOS RS485 transceivers makes

implementation of effective external protection networks

difficult without interfering with proper data network

performance within the – 7V to 12V region of RS485

operation.