When the bus is idle, both the SDA and SCL lines are pulled high by the pull-up resistors or active pull-up devices. The controller generates a start condition followed by a valid serial byte containing the high-speed (HS) controller code 0000 1XXXb. This transmission is made in either standard mode or fast mode (up to 400 kHz). The OPT3004 does not acknowledge the HS controller code but does recognize the code and switches its internal filters to support a 2.6-MHz operation.

The controller then generates a repeated start condition (a repeated start condition has the same timing as the start condition). After this repeated start condition, the protocol is the same as F/S mode, except that transmission speeds up to 2.6 MHz are allowed. Instead of using a stop condition, use repeated start conditions to secure the bus in HS mode. A stop condition ends the HS mode and switches all internal filters of the OPT3004 to support the F/S mode.