ST AN1348 APPLICATION NOTE
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
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.
and DE
Figure 1: ST485 Internal Structure
RO
1
__
2
RE
DE
3
DI
4
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
R
D
8
Vcc
7
B
A
6
5
GND
June 2004 (rev. 3)
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AN1348 - APPLICATION NOTE
together. ST485 has a Rin greater than 40kOhm allowing the connection of more than 32 units as well;
- The common mode voltage VCM for the receiver, defined as the algebraic mean of the two localground-referenced voltage: VCM=(VA+VB)/2. This parameter is from -7V to +12V for RS-485 standard.
VCM enables different ground-referenced devices to work correctly and allowing ground difference up to
±7V. For an example see figure 2;
- The differential driver output voltage. It depends on the driver output current and, obviously, on the used
RLOAD. It must be greater than 1.5V with 27Ohm RLOAD;
- The driver short-circuit protection; it should happen that more drivers transmit at the same time giving a
conflict. ST485 limits the short circuit current up to 70mA providing a protection for the whole line.
Figure 2: Common Mode Voltage Capability
ST485 #1 ST485 #2
GND
GND 2
Vcc
8
B
7
A
6
5
R
D
RO
1
__
RE
2
DE
3
DI
4
RO
__
RE
DE
DI
1
2
3
4
R
D
Vcc
8
B
7
A
6
GND
5
GND 1
Difference in ground potential
(max ±7V)
4. DIFFERENTIAL LINE ADVANTAGES
Why differential line transmission?
- When a signal is transmitted on a single twisted cable, many factors contribute to increase disturbs on
the line, like spike and inducted currents. With a differential transmission the same inducted disturb is
present on both input A and B, so the difference is null. Figure 3 shows the signal ground-referenced of
the ST485 inputs A and B with a noise signal inducted; CH1= receiver input A, CH2= receiver input B,
CH4= receiver output. The common mode disturb is not present on the receiver output.
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AN1348 - APPLICATION NOTE
Figure 3: Common mode Noise Signal Inducted on a ST485 Differential Line
- In a single ended transmission the distance between logic symbol ‘1’and ‘0’ is lower respect to the
differential one. Table 1 and figure 4 show that, with Vcc=5V, the voltage distance between ‘1’and ‘0’logic
symbols is 10V without load. Obviously the termination (120Ohm for a twisted cable or 54Ohm for a
shield cable) reduces this voltage distance.
Figure 4: Single Ended and Differential Signals
5V
A
Ground-referenced
signals
0V
5V
0V
RO
__
RE
DE
DI
1
2
3
4
R
D
8
Vcc
5V
7
B
A
6
5
GND
B
0V
A - B
5V
Differential signals
0V
-5V
Table 1: Single Ended and Differential Signals
Logic Level Single Ended Differential Referenced to ground
0 Vout=0V Vout: A=0V; B=5V Vdiff = 0V - 5V = -5V
1 Vout=5V Vout: A=5V; B=0V Vdiff = 5V - 0V = +5V
Differential (V
A-VB
)
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