Intel 820E manual Device-Side Cable Detection, Device-Side IDE Cable Detection

Intel® 820E Chipset

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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

		IDE drive	IDE drive
		5 V	5 V
		10 kΩ	10 kΩ
		40-conductor
ICH2		cable	PDIAG#
	PDIAG#/	PDIAG#
	CBLID#
	0.047 µF
		IDE drive	IDE drive
		5 V	5 V
		10 kΩ	10 kΩ
		80-conductor
		IDE cable
ICH2	PDIAG#/	PDIAG#	PDIAG#
	CBLID#
	0.047 µF	Open
			IDE_dev_cable_det

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.

82 Design Guide