Signal reflections from the stub cause low level communication problems
for an ide device connected in the middle.
It's a historical thing.
When the earth cooled and IBM XTs strode the planet, it was considered
preferable to carry data bits in parallel for greater speed. That's
why DOS-era Lap Link users preferred using the parallel printer port
to serial ports, even the latter were designed for those sort of tasks
The original 40-pin IDE cable was fine at a time when data flowed over
the cable at a relatively slow rate, under processor control. It was
still fine when hardware took over data transmission from the
processor, in the original UDMA mode at PCI's baseline 32MHz.
Then it was discovered you could transfer data on both edges of a
clock pulse - so although the PCI was still clocking at 32MHz, it
would be possible to transfer data from controller to hard drive at
66MHz (UDMA66). This later became UDMA100 and finally UDMA133, which
maxed out the speed of the PCI.
However, data flowing at that rate starts to create capacitance
between the parallel conductors in the cable, so the 80-pin cable was
born - it was the same cable pinouts, except now there were ground
lines between each active signal to help screen them.
At the same time it was noticed that there were signal reflection
issues, and designing to minimize these may have required adjustments
according to where on the cable the device was attached. So these new
80-pin cables have each plug marked for a particulat purpose; the
controller at the "long" end, then slave in the middle, and the master
at the end. This used to be required in using Cable Select jumpering,
but now it's required for Master and Slave for these other reasons.
Since then, ways have been found to send data very fast, one bit at a
time, in serial. So we have USB, Firewire and S-ATA that are all
serial transmission systems, and these outperform what used to be the
faster parallel methods. If one attempted the same speed on parallel
cables, the interference would make it impossible to sustain over any
sort of realistic cable length.
The other reason why serial is preferred today, is to reduce the pin
count of the various chips, and the interconnects on the circuit
boards between these,
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