The Master initiates communication by issuing a Reset pulse by holding the 1-Wire line low for >480µsec. After the bus Master has released the line, it goes into receive mode and waits for a period of 15 to 60µsec. During this time the 1-Wire line returns to the logic high idle state of the supply voltage through an external pull up resistor or an active/integrated line driver. After the 1-Wire line is returned to the high state following the reset pulse, all 1-Wire Slave devices on the bus acknowledge the Master by pulling the line low for a period of 60 to 240µsec. This is termed as the Presence pulse. By the end of the 240µsec Presence pulse low time, all Slave devices will have released the line and wait for the return to idle high status. At the end of the recovery time slot, the 1-Wire line idles at high again. At this time all Slave devices on the 1-Wire line are synchronized to a known state. In a multi Slave environment all devices will pull the line low simultaneously. The Master can only determine that at least one Slave device exists on the line from a presence pulse detection. If not known in advance, to further resolve the number of devices requires a device detection sequence called search ROM.