Hello peter02,
I’m not sure what Bascom AVR will do if the byte to be sent is defined as an integer.
Try changing the “Dim Ds1621_i2c_address As Integer” To “Dim Ds1621_i2c_address As Byte” and “Dim Ds1621_i2c_r_address As Integer” To “Dim DS1621_i2c_r_address As Byte”.

What is the length of your cable to the outside DS1621?

Greetings,
Drifter2006