After a long Internet search and testing I have now understood the part and it works well. Great for use with MCP23017 for a GPIO expansion and eventual connection of 5V relay board and Raspberry 3.3V GPIO. "Bread plate" of course "breadboard" for testing the circuit. On the LV side 3.3V is applied and the opposite side 5V. For bidirectional transfer - for example I2C (SDA, SCL) weren both external connections used because they support both directions. The Rx ports are only for signals in one direction (5V -> 3.3V) - ie for example can be very good for the interrupt signals of the MCP to a GPIO pin of Raspi be used. Have tested and works great. This gives you more inputs / outputs (16) available. Assuming a 1 Channel Converter so you just have no back channel way for the interrupt. When this is all MCP GPIOs used as raw also ok. Part is a direct plug and testable! I'm very satisfied about my tests. Only you do not get and therefore description may recognize only professionals the function immediately.
