AN1690

Fail-safe circuit comparisons with ST485EB

11/15

Figure 10.

Equivalent test circuit for a fully loaded network

This test was performed on the ST485EB driver. The resistor values were:

●

Ra = Rc = 500

Ω

●

Rb = 60

Ω

With the common mode voltage Vcm varied from -7 to +7 V, the device meets the 1.5 V

minimum differential voltage (Vod).

8

Fail-safe circuit comparisons with ST485EB

The following measurements were performed with two ST485EB devices connected in

point-to-point configuration across a twisted pair cable of 1 m length. Table 1 summarizes

the DC characteristics with different termination circuits.

Note:

Vcc=5 V

Vcm= -7 to

+7V

Vcc2

Vcc1

ST485

Driver

Rb

Ra

Rc

375

375

32 unit loads

Vod

Vcm= -7 to

+7V

Vcc2

Vcc1

ST485

Driver

Rb

Ra

Rc

375

375

32 unit loads

Vod

EB

Table 1.

DC fail-safe characteristics

Schematic

Ra=Rc (

Ω)Rb (Ω)

C (nF)

Vid (mV)

Fail-safe

current (mA)

Receiver output state

No termination

-

-

-

1430

-

Fixed high (internal fail-

safe)

DC termination

-

120

-

1.45

-

Undefined

Fail-safe DC

termination

500

120

-

280

4.72

Fixed high

Fail-safe AC

termination

22 k

Ω

120

100

4040

0.0316

Fixed high