R

D

DE

RE

B

A

GND

VCC

13

THVD1500

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SLLSEY4A – JULY 2017 – REVISED NOVEMBER 2018

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8 Detailed Description

8.1 Overview

The THVD1500 is a low-power, half-duplex RS-485 transceiver suitable for data transmission up to 500 kbps.

8.2 Functional Block Diagrams

8.3 Feature Description

Internal ESD protection circuits protect the transceiver against Electrostatic Discharges (ESD) according to IEC

61000-4-2 of up to ±8 kV (Contact Discharge), ±10 kV (Air Gap Discharge) and against electrical fast transients

(EFT) according to IEC 61000-4-4 of up to ±2 kV.

The THVD1500 provides internal biasing of the receiver input thresholds in combination with large input-

threshold hysteresis. With a positive input threshold of VIT+ = –50 mV and an input hysteresis of VHYS = 50 mV,

the receiver output remains logic high under a bus-idle or bus-short conditions without the need for external

failsafe biasing resistors. Device operation is specified over a wide temperature range from –40°C to 125°C.

8.4 Device Functional Modes

When the driver enable pin, DE, is logic high, the differential outputs A and B follow the logic states at data input

D. A logic high at D causes A to turn high and B to turn low. In this case, the differential output voltage defined

as VOD = VA – VB is positive. When D is low, the output states reverse, B turns high, A becomes low, and VOD is

negative.

When DE is low, both outputs turn high-impedance. In this condition the logic state at D is irrelevant. The DE pin

has an internal pull-down resistor to ground, thus when left open the driver is disabled (high-impedance) by

default. The D pin has an internal pull-up resistor to VCC, thus, when left open while the driver is enabled, output

A turns high and B turns low.

Table 1. Driver Function Table

INPUT

ENABLE

OUTPUTS

FUNCTION

D

DE

A

B

H

H

H

L

Actively drive bus high

L

H

L

H

Actively drive bus low

X

L

Z

Z

Driver disabled

X

OPEN

Z

Z

Driver disabled by default

OPEN

H

H

L

Actively drive bus high by default

When the receiver enable pin, RE, is logic low, the receiver is enabled. When the differential input voltage

defined as VID = VA – VB is positive and higher than the positive input threshold, VIT+, the receiver output, R, turns

high. When VID is negative and lower than the negative input threshold, VIT-, the receiver output, R, turns low. If

VID is between VIT+ and VIT- the output is indeterminate.

When RE is logic high or left open, the receiver output is high-impedance and the magnitude and polarity of VID

are irrelevant. Internal biasing of the receiver inputs causes the output to go failsafe-high when the transceiver is

disconnected from the bus (open-circuit), the bus lines are shorted (short-circuit), or the bus is not actively driven

(idle bus).