AN1348 - APPLICATION NOTE

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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 local-

ground-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

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.

GND

D

R

8

6

5

7

RO

3

4

2

1

__

RE

DE

DI

Vcc

B

A

D

R

1

3

4

2

RO

6

5

7

8

__

RE

DE

DI

Vcc

B

A

Difference in ground potential

(max ±7V)

GND 1

GND 2

GND

ST485 #1

ST485 #2