Connecting an LED to the DASP# signal is the most popular method of indicating disk activity, as demonstrated on the AT2LP development board. However, some drives either do not support the use of DASP# as an activity indicator, or they interfere with the operation of other devices when the DASP# signal is shared. Some designers require visual feedback to the user about disk activity, so the lack of DASP# support is a stumbling block for providing such a feature. There are alternatives to the DASP# signal which will provide the same basic functionality. They include:
CS0# -- This signal is asserted any time registers are accessed or data is transferred in PIO mode.
DA0 -- This signal is asserted during all UDMA activity and any ALT-STAT register polls. From a user's point of view, this would include all relavent disk activity.
Connecting an LED directly to these signals may change the rise/fall times of the signals, so care should be taken to ensure that overall ATA signal integrity is maintained if one or both of these signals is used as a disk activity indicator.