Hardware Reference
In-Depth Information
Many lower-cost board and cable manufacturers leave out the keying. Cheaper motherboards
often don't have pin 20 removed on their ATA connectors; consequently, they don't supply a
cable with pin 20 blocked. If they don't use a shrouded connector with a notch and a
corresponding protrusion on the cable connector, no keying exists and the cables can be
inserted backward. Fortunately, the only consequence of this in most cases is that the device
won't work until the cable is attached with the correct orientation.
Note that some systems do not display video until the ATA drives respond to a spin-up command,
which they can't receive if the cable is connected backward. So, if you connect an unkeyed ATA
drive to your computer, turn on the computer, and it seems as if the system is locked up (you don't see
anything on the screen), check the ATA cable. (See
Figure 7.6
for examples of unkeyed and keyed
ATA cables.)
In rare situations in which you are mixing and matching items, you might encounter a cable with pin
20 blocked (as it should be) and a board with pin 20 still present. In that case, you can break off pin
20 from the board—or for the more squeamish, remove the block from the cable or replace the cable
with one without the blocked pin. Some cables have the block permanently installed as part of the
connector housing, in which case you must break off pin 20 on the board or device end or use a
different cable.
The simple rule of thumb is that pin 1 should be oriented toward the power connector on the device,
which normally corresponds to the stripe on the cable.
PATA I/O Cable
A 40-conductor ribbon cable is specified to carry signals between the bus adapter circuits and the
drive (controller). To maximize signal integrity and eliminate potential timing and noise problems,
the cable should not be longer than 18 inches (0.46 meters), although testing shows that you can
reliably use 80-conductor cables up to 27 inches (0.69 meters) in length.
Note that ATA drives supporting the higher-speed transfer modes, such as PIO Mode 4 or any of the
Ultra-DMA (UDMA) modes, are especially susceptible to cable integrity problems. If the cable is
too long, you can experience data corruption and other errors that can be maddening. This is
manifested in problems reading from or writing to the drive. In addition, any drive using UDMA
Mode 5 (66MBps transfer rate), Mode 6 (100MBps transfer rate), or Mode 7 (133MBps transfer
rate) must use a special, higher-quality 80-conductor cable. I also recommend this type of cable if
your drive is running at UDMA Mode 2 (33MBps) or slower because it can't hurt and can only help. I
always keep a high-quality 80-conductor ATA cable in my toolbox for testing drives where I suspect
cable integrity or cable length problems.
Figure 7.5
shows the typical ATA cable layout and
dimensions.
