This is a
design circuit for electrical isolation that based control by opto isolator. When the SDA (Serial DAta) lines on both
the left and right lines are 1, the circuit is quiescent and optoisolators IC1
and IC2 are not actuated. When the SDA line at the left becomes 0, current
flows through the LED in IC1 via R2. The SDA line at the right is then pulled
low via D2 and IC1. This is the figure of the circuit;
In
this operation of the circuit, opto isolator IC2 does not transfer this 0 to
the left, because the polarity of the LED in IC2 is the wrong way around for
this level. This arrangement prevents the circuit holding itself in the 0 state
for ever. As is seen, the circuit is symmetrical. So, when the SDA line at the
right is 0, this is transferred to the left. The lower part of the diagram,
intended for the SCL (Serial CLock) line, is identical to the upper part. Resistors
R1, R4, R5, and R8, are the usual 3.3 kΩ pull-up resistors that are obligatory
in each I2C line. If these resistors are already present elsewhere in the
system, they may be omitted here. The current drawn by the circuit is slightly
larger than usual since the pull-up resistors are shunted by the LEDs in the
optoisolators and their series resistors. Nevertheless, it remains within the
norms laid down in the I2C specification.
