1.1.4 ‘Reset/Presence’ Signal

A ‘Reset’ and ‘Presence’ signal is shown in the figure below. Note that the time scale is different from the first waveforms. The host pulls the bus low for at least eight time slots, or 480 μs, and then releases it. This long low period is called the ‘Reset’ signal. If there is a target present, it will pull the bus low within 60 μs after it is released by the host and hold it low for at least 60 μs. This response is called a ‘Presence’ signal. If no presence signal is issued on the bus, the host must assume that no device is present on the bus, and further communication is not possible.