Page "System Management Bus" Paragraph 23

In I²C, a slave receiver is allowed to not acknowledge the slave address, if for example it's unable to receive because it ’ s performing some real time task.

SMBus requires devices to acknowledge their own address always, as a mechanism to detect a removable device ’ s presence on the bus ( battery, docking station, etc.

) I²C specifies that a slave device, although it may acknowledge its own address, may decide, some time later in the transfer, that it cannot receive any more data bytes.

I²C specifies that the device may