Parallel vs Serial speed

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

> markw at mohawksoft.com wrote:
>> 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.
>
> I'm sure you're correct about all the voltage analysis, except that
> "harder"
> from the above must not equate to "cheaper", since you don't see any
> computers
> with Universal Parallel Bus ports...

By cheaper I meant that the old IDE (and MFM before it) and IBM PC port
got good bang for the buck being parallel TTL. When more speed was needed,
Nyquist sort of got in the way, and they needed to create more noise free
and faster transmissions.

>
> /Something/ about parallel interfaces obviously makes them more expensive
> than
> serial for the same speed, and I didn't see anything in your post that
> explained /why/ "High speed drivers are expensive" compared to serial
> ones.

The old IBM PC Parallel port and IDE and MFM interface was originally done
with cheap LS TTL logic. It was dirt cheap to make, ribbon cable and
pressed connectors was all you needed. A serial interface, while saving
wire, would have required a UART or software which would have been
expensive or slow.

RS-232 was more exp[ensive because you needed RS232 bus transceivers to
translate the RS-232 signal voltages back to TTL were it would be fed into
a UART, and then clocked on the computer bus.

RS-232 was a point to point connection, and not suitable for any sort of
bus. (Not 100% true as people did fool around with multi-drop RS-232, but
that is a different conversation.)

As electrical transfer speeds bumped into electrical limits, new
electrical methods were made.

Once you leave the relative safety of prevailing logic voltage levels i.e.
TTL, you start adding more expense.

I guess there is confusion over the point I was making, high speed serial
is cheaper than high speed parallel, which is why we have USB, firewire,
and SATA.

Well, it is more complicated than that, really. Computers use parallel
data in the form of bytes, words, long words, and long long words.
Transferring data in their native formats is naturally preferable. Serial
transmission typically takes parallel data converts it to serial, adds a
couple extra bits for error detection/correction, drives it down a wire,
and it is reassembled at the other end and clocked in as parallel data.

Think about an 8 bit vs 1 bit device, if a 1 bit device costs $10, an 8
bit device could cost $80. So, serial wins on cost, even though that $80
interface would be more than 8 times faster.



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