Hello, for several years, I have been working with UNO R3 cards linked together by an I2C bus. Each of these cards are used alternately as a "Master" component and a "Slave" component. In my sketch, the Wire library is declared "slave" by the Wire.begin(address) instruction and everything works without a hitch. Recently, I purchased a UNO R4 card and it too will be integrated into my I2C network. On the I2C bus, a DS1307 real-time clock is installed which works perfectly with UNO R3 or MEGA cards. However, on the R4, access to the DS1307 only works if the card is declared as a master component exclusively (Wire.begin()). I tried using the Wire.end() function, but it had no effect. Does anyone have an idea to resolve this problem? THANKS