Parallel vs Serial speed

[markw at mohawksoft.com (markw at mohawksoft.com)]

> From: Matthew Gillen <me at mattgillen.net>
> Subject: Re: Parallel vs Serial speed
> To: David Kramer <david at thekramers.net>
> Cc: discuss at blu.org
> Message-ID: <45C93F2B.2030805 at mattgillen.net>
> Content-Type: text/plain; charset=ISO-8859-1
>
> David Kramer wrote:
>> markw at mohawksoft.com wrote:
>>
>>> In the above examples, we see that the drives perform similarly, but
>>> that
>>> the SATA interface gives an advantage in transfer speeds, but not the
>>> 100mb/s vs 300mb/s advertised difference.
>>
>> Here's something I don't understand.  My gut tells me that a parallel
>> interface, which sends all the bits at once, should be several times
>> faster than a serial interface, which must send one bit at a time.  So
>> how is SATA faster than IDE?
>
> Same reason parallel ports (old printer ports) went the way of the dodo in
> favor of serial ports:  because it to get 8 pins (or whatever) to sync up
> you have to run at a much slower rate (more than a factor of 8).

I'm sorry but I have to correct this statement. The IBM printer port and
the old IDE interface cables are TTL level signals. 0~5 volts LS logic.
The reason why they are slow is that the current needed to drive a fast ~4
volt swing is high based on the inductance of any real length of cable.

RS-232 suffered similar problems, but it had more noise immunity because
it used +- voltage for 1 and 0 over the serial line.

IDE, IBM parallel port, and RS-232 all suffered from "voltage" based
signal transmission, all the faster interfaces now use current.

High speed drivers are expensive, they use a lot of current, and shielding
between adjacent lines is important.

Parallel over the same interface is ALWAYS faster and ALWAYS [n-1] * more
expensive than serial, where [n] is the number of bits.

It has nothing to do with synchronization, if anything, serial is a harder
interface because it requires circuitry or software to convert it to
parallel data from serial data.

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