#define HD44780_BIDIRECTIONAL_DBUS (1<<0)

Setting HD44780_BIDIRECTIONAL_DBUS as a capability flag returned by the capabilities method indicates that the data bus direction can be controlled and the data bus can operate in both input and output mode. LCD display will be much faster if the LCD is operated in bidirectional mode because the ‘LCD ready’ status can be read from the bus to indicate that the LCD is ready; for output-only (unidirectional) interfaces, this status cannot be read from the data bus so the high-level driver falls back to the minimum timings for each HD44780 command that are given in the datasheet.