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Intel
®
820E Chipset
R
82 Design Guide
2.12.3. Device-Side Cable Detection
For platforms that must implement device-side detection only (e.g., NLX platforms), a 0.047 µF
capacitor is required on the motherboard, as shown in the following figure. This capacitor should not be
populated when implementing the recommended combination host-side/device-side cable detection
mechanism described previously.
Figure 47. Device-Side IDE Cable Detection
80-conductor
IDE cable
IDE drive
5 V
ICH2
Open
IDE drive
5 V
40-conductor
cable
IDE drive
5 V
PDIAG#
ICH2
IDE drive
5 V
PDIAG#
PDIAG#PDIAG#
PDIAG#/
CBLID#
PDIAG#/
CBLID#
10 k
Ω
10 k
Ω
10 k
Ω
10 k
Ω
IDE_dev_cable_det
0.047 µF
0.047 µF
This mechanism creates a resistor-capacitor (RC) time constant. The ATA mode 3, 4, or 5 drive will
drive PDIAG#/CBLID# low and then release it (pulled up through a 10 kΩ resistor). The drive will
sample the signal after releasing it. In an 80-conductor cable, PDIAG#/CBLID# is not connected through
to the host, so the capacitor has no effect. In a 40-conductor cable, the signal is connected to the host, so
the signal will rise more slowly as the capacitor charges. The drive can detect the difference in rise times
and will report the cable type to the BIOS when it sends the IDENTIFY_DEVICE packet during the
system boot, as described in the ATA/66 specification.