I2C signal line isolation circuit

I2C and SMBUS bus master-slave mode is different from 4-wire SPI QSPI and Microwire data interface standards. This
bus mode only needs 2 lines for data transmission and sends and receives data on the same line. Figure 1 can realize an isolated
2-wire interface. The isolated 5V power supply can be obtained from the main power supply through a small transformer and MAX253 transformer driver (not shown in Figure 1).
The transformer and optocoupler are selected according to the data rate and isolation voltage. Figure 1 uses 6N138. For supplier
information, please refer to the MAX253 and MAX845 manuals.

Assuming that the master end is uP or uC, the current injected into the SDA end determines the minimum conduction current of the optocoupler to be within 3mA.
Even so, the 300% current transfer ratio CTR of the optocoupler is sufficient to ensure the normal operation of the circuit. The slave end should use
devices compatible with I2C, such as the 8-bit D/A converter MAX517 or MAX127 data acquisition system.
When there is no signal transmission, the master end SDA and SCL are high, and the isolation end outputs SDA iso and SCL iso
are also high. The typical start state of I2C is that SDA jumps from high level to low level while SCL remains high. When SDA is
low, the current passes through R2 and the input end of the optocoupler, making the optocoupler output a typical value of about 0.4V. The optocoupler output plus the conduction voltage drop of the
Schottky diode D2, the pull-up resistors R1, R4 and R6 are used for compatibility with the I2C bus.
After the master end addresses in this way, the corresponding slave end responds with a low-level recognition bit. The bidirectional SDA allows data
bidirectional transmission. SCL only transmits data from the master end to the slave end. During the high level of SCL, the transition of SDA from low level to high level
generates a stop bit to terminate the data transmission.