RE: Receiver Output Enable and DE: Driver Output Enable pins

Question

Hello I am planning to use LT1785 UART-RS484 transceiver. I will use that chip in order to get RS485 data and convert it to UART signal. So I have read the datasheet but I couldn't be sure. It says

A logic low on RE enables normal operation of the receiver output RO. A logic high level at RE places the receiver output pin RO into a high impedance state.

I shorted the RE/DE pins because I will only use this chip to receive RS-485 signal and convert it to UART. So my questions

1. Can I directly connect shorted RE/DE pin to the ground?
2. In the absolute maximum ratings Vcc is given 18V. Does that mean I can supply this chip with any voltage up to 18 V? In case of 5V supply voltage, will this chip work?

I also added the image that shows connection of the pins.

Answer 1

1. If you only want to receive, then yes, shorting both DE and RE to ground is the way to go.

2. According to the datasheet:

   Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime

   In other words, you can connect VCC to up to 18V for a short period of time, but it is not guaranteed to keep working. You should never treat an Absolute Maximums section as a license to operate at those conditions - they should be treated as worst case.

   In terms of recommended operating conditions:

   VCC: Positive Supply Input. For RS422 or RS485 operation, 4.75V ≤ VCC ≤ 5.25V. Higher VCC input voltages increase output drive swing. VCC should be decoupled with a 0.1μF low ESR capacitor directly at Pin 8 (VCC)

   So it is recommended to run the chip at 5V +/- 5% (*).

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(*) Though I'm not sure why LT decided to put that specification in the pin descriptions section rather than the recommended operating conditions table.