IBM BDM-610000049 - Manual (Page 77)

BDM-610000049 Rev G Chapter 4: Using the cpuModule 67
IDE Controller Configuration
The CPU’s onboard EIDE connector (CN10) supports several different drive speed modes, which are BIOS
configurable. Supported drive modes will depend on whether a 40-conductor or 80-conductor cable is
connecting the EIDE device. The modes and cable detection schemes described below may be set in the BIOS
Setup. Similarly, the ATA/IDE Disk Chip socket (U16) is BIOS configurable.
Cable Modes
There are two types of cables that may be used for connecting drives to the EIDE connector: 40 conductor cables
or 80 conductor cables. Depending on the cable used, different drive speeds are supported. A 40 conductor cable
can be used for speeds up to UDMA Mode 2 (Ultra ATA/33).
In order to use drive speeds faster than UDMA Mode 2 (Ultra ATA/33), an 80 conductor cable is required. The
BIOS can be configured to detect the presence of an 80 conductor cable. The 80 conductor cable adds a ground
wire between each signal, and uses standard 40 pin connectors.
Cable Detection
Every time the cpuModule is powered on or a hardware reset is issued, the BIOS will automatically detect the
presence of a 80 conductor cable connecting a device to CN10. The user selectable cable detection modes are
described below.
Device and Host Mode
For this method, there is a capacitor on the CBLID pin at the CPU, and a pull-up at the hard drive. The CPU sends
a command to the hard drive to drive the CBLID pin low, and then release it. The CPU then waits a certain
amount of time, and instructs the hard drive to read the status of the CBLID pin. If an 80 conductor cable is
attached, the CBLID signal is not connected between the CPU and the hard drive, and the hard drive will read
the signal as a logic high. If a 40 conductor cable is attached, the CBLID pin is connected between the CPU and
the hard drive, the capacitor delays the signal from going high, and the hard drive reads it as a logic low.
Host Determination of Cable Type
For this method of detection, the CPU reads the CPBLID pin, which determines if a 40-conductor or 80-
conductor cable is connected between the CPU and device. An 80-conductor cable has this signal grounded at
the CPU end, and not connected to the hard drive. A 40-conductor cable connects the CBLID signal to the hard
drive, where it is pulled to a logic high.
Device Detect
For device detect mode, the CPU issues a command to the device, which tells the CPU the fastest drive speed
mode it can use. The CPU then sets the transfer mode to the fastest speed supported by the device.
WARNING When this cable detection method is enabled, the highest transfer speed supported by the
device will be used regardless of whether a 40-conductor or 80-conductor cable is used. If the device speed
does not match the cable, data corruption and unexpected behaviors may occur. This mode should not
be selected unless the user knows the cable type and the modes supported by the connected EIDE device.