There are three Busy-to-Ack timing variations in general usage. These variations differ in whether the Ack pulse is issued before or after the Busy signal is deasserted, or if Busy is deasserted while the Ack pulse is active. As in the previous clause, the common names for these timing variations arise from the appearance of the wave forms. The common names of the three variants are:
In particular, it is important to recall that the Busy signal can be asserted under a variety of conditions, not just during the data transfer cycle. Host systems that rely on Ack to determine the end of the data transfer cycle should ensure that Busy has been cleared before strobing the next byte. This is particularly important when connected to printers that use the Ack-in-Busy and Ack-while-Busy timing variants. These variants allow the Busy signal to be prolonged indefinitely, even after the Ack pulse has been initiated.
From the IEEE-1284STD - 1994