Have you seen this valuable reference about IDE/ATA/DMA ?

    I found this terrific reference this evening.  It
answers many of my questions.  It seems to be a MUST READ
for anyone working with computer hardware configurations.

 http://www.pcguide.com/ref/hdd/if/ide/confCable-c.html

     And here is the basic 40-pin connector scheme.  The
Ultra DMA ribbon cable uses 80 wires, with every other
wire being a ground wire to bleed off stray signals that
might interfere with high speed transmission:

     Pin Signal                    Pin Signal

 1 Drive Reset                 2 Ground
 3 Data Line Bit 7             4 Data Line Bit 8
 5 Data Line Bit 6             6 Data Line Bit 9
 7 Data Line Bit 5             8 Data Line Bit 10
 9 Data Line Bit 4            10 Data Line Bit 11
11 Data Line Bit 3            12 Data Line Bit 12
13 Data Line Bit 2            14 Data Line Bit 13
15 Data Line Bit 1            16 Data Line Bit 14
17 Data Line Bit 0            18 Data Line Bit 15
19 Ground                     20 Keypin (unused)
21 DMA request                22 Ground
23 Write Data                 24 Ground
25 Read Data                  26 Ground
27 I/O channel ready          28 Cable Select/Spindle Sync
29 DMA acknowledge            30 Ground
31 Interrupt Request          32 Host 16 Bit I/O
33 Host Address Bit 1         34 Passed Diagnostics
35 Host Address Bit 0         36 Host Address Bus Bit 2
37 Host Chip Select 0         38 Host Chip Select 1
39 Drive Active/Drive 1 present
                              40 Ground