HDD header hardcore

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What follows are some details excerpted from the ATA-6 spec. This stuff is probably only of interest to the real hardware hackers, and you don’t need to know it to do most mods that use the HDD header signal. The HDD header hints article covers the practical issues of using the hard drive header in mod projects. this article picks up where that one left off, and provides some technical detail taken chapter and verse from the relevant specs.

  • The latest ATA published spec at time of writing is ATA-6.
  • The correct designation for the hard drive activity signal as used in the specs is DASP-. (sec. 5.2.3)
  • DASP- is carried on pin 39 of the 40-pin connector. (table A3, Annex A)
  • The functional signal description for DASP- is as follows: (sec. 5.2.3)

    During the reset protocol, DASP- shall be asserted by Device 1 to indicate that the device is present. At all other times, DASP- may be asserted by the selected active device.

  • The DASP- driver type is open collector with a 10kohm pullup on the drive. (table 5, sec. 4.2.1)
  • There is a requirement that the host not drive DASP- (note 9, table 5, sec. 4.2.1)

    The host shall not drive DASP-. If the host connects to DASP- for any purpose, the host shall ensure that the signal level detected on the interface for DASP- shall maintain VoH and VoL compatibility, given the IoH and IoL requirements of the DASP- device drivers.

Update: ever-helpful cpemma has provided this outstanding schematic which depicts how the motherboard ORs the signals from the drives:

HDD header circuit diagram
this is just lovely

And along with that, this excellent description:

The transistor switch Q1 with 10k pull-up resistor R1 (as specified) is fitted in each drive linked to its pin 39. Between each motherboard IDE connector and the LED header is a diode so activity on any drive triggers the one led, current-limited by the resistor R2 (330R on a couple of my old boards, 220R on the latest so YMMV).

One consequence is that with the diode dropping about 0.6V and the saturated transistor switch around 0.2V from the original 5V, a few higher-voltage blues (Radio S*ack’s?) may not perform well. ;)

Digging through the ATA specs isn’t fun. I sure hope this little cheat sheet helps somebody.

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