1. INTRODUCTION

ST485 is a RS-485 based interface designed for multipoint differential transmission on a single twisted

pair cable. It allows half duplex bi-directional transmission, long cable length and high data rate. Typical

applications include LANs, industrial (PLC devices), automotive and computer interfaces. The system

evolution in the data communication field leads to the development of faster devices with lower data bit

errors and ST485 meets all these requirements.

2. FUNCTIONAL DESCRIPTION

Figure 1 shows the internal structure of the ST485. The driver and receiver line pins are connected

together; in fact the bi-directional communication can only be half- duplex. The control pins RE and DE

are used to enable respectively the receiver and driver and, in many applications, are connected

together. The driver has a TTL input while the output is differential. The receiver differential input is

internally connected to the driver output.

Figure 1: ST485 Internal Structure

The Vcc supply voltage is 5V, typical DC consumption is less than 0.5mA without load and Data rate is

greater than 10Mbps.

3. FEATURES

The most important parameters in a RS-485 device are:

- The receiver input threshold. It is ±200mV. This feature ensures an improvement of the noise immunity;

- The receiver input resistance. It should be high enough to allow the connection of many transceivers

D

R

8

6

5

7

RO

3

4

2

1

__

RE

DE

DI

Vcc

B

A

GND

June 2004 (rev. 3)

1/7

AN1348

APPLICATION NOTE

ST485: AN RS-485 BASED INTERFACE WITH

LOWER DATA BIT ERRORS

A. Randazzo