I changed my function to this:
function calibrate(data){ local value = format("\\\\x%02X", data); server.log("Calibration Value: "+data+" | "+value); server.log(vz89.write(vz89Addr, SET_WRITE + value + value)); server.log("i2c error: "+vz89.readerror()); imp.sleep(1.0); }
Log:
2016-05-14 11:23:29 UTC+2 [Status] Downloading new code; 1.94% program storage used 2016-05-14 11:23:29 UTC+2 [Device] Calibration Value: 0 | \\x00 2016-05-14 11:23:29 UTC+2 [Device] 0 2016-05-14 11:23:29 UTC+2 [Device] i2c error: 0