AN1348 Datasheet, PDF (1/7 Pages) STMicroelectronics – ST485 is a RS-485 based interface designed for multipoint differential transmission

AN1348
APPLICATION NOTE
ST485: AN RS-485 BASED INTERFACE WITH
LOWER DATA BIT ERRORS
A. Randazzo
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
RO ₁
R
__
RE 2
DE 3
DI 4
D
8 Vcc
7B
6A
5 GND
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
June 2004 (rev. 3)
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