AN1348 - APPLICATION NOTE

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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

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

A

B

A - B

5V

0V

D

R

8

6

5

7

RO

3

4

2

1

__

RE

DE

DI

Vcc

B

A

GND

Ground-referenced

signals

Differential signals

5V

5V

5V

0V

0V

0V

-5V