AN1702

Network isolation

7/11

Figure 4.

RS-485 terminal node with optocouplers

The choice of proper optocouplers should be based on rise and fall times, which are related

to the signaling rate. The RS-485 requirements for the signal quality are that the rise (or fall)

time should not exceed 30% of the bit time (reciprocal of the signaling rate). For a signaling

rate of 250 kbit/s, the maximum rise or fall time of the optocouplers is:

Equation 1

Therefore, an optocoupler with rise/fall times faster than 1.2 msec is required. The biasing

resistors should be selected taking into account the current characteristics of the

optocouplers, and the threshold voltages of the transceiver input pins, in order to ensure a

reliable driving of the transceiver.

R

DE

RE

RO

A

B

RECEIVED

DATA

SEND/

RECEIVE

CONTROL

SENT

DATA

ST485EB

R

DE

RE

RO

A

B

RECEIVED

DATA

SEND/

RECEIVE

CONTROL

SENT

DATA

ST485E

0.3

1

250 kbps

-------------------------

⎝⎠

⎛⎞ 1.2 msec

=