[time-nuts] ``direct'' RS-232 vs. RS-232 via USB vs. PPS decoding cards

Chris Albertson albertson.chris at gmail.com
Tue Feb 14 00:24:59 EST 2017

Pretty dramatic difference between a "real" serial port and USB.  Like two
orders of magnitude or more.

If you computer lacks a serial port, just buy a new computer.  The
Raspberry Pi or the like costs about $40.   But the money you save on
electric power will pay off that $40 in less than a year.

Why is this?  the serial port has a pin that is tied to an interrupt.  If
you read the code associated with that interrupt it is like maybe 4 or 6
lines of C and VERY simple.   USB on the other hand is packetized.
Nothing happens on till a block of data comes in and then it is quite
complex so the time from the pin going active to the internal counter being
sampled is quite variable   I said "two orders of magnitude"  it might even
be three orders.

On Mon, Feb 13, 2017 at 8:07 AM, Ruslan Nabioullin <rnabioullin at gmail.com>

> Hi, generally speaking, what are the performance differences between the
> following: 1. direct RS-232 (i.e., what I believe is a standard PCI card
> offering RS-232---essentially UARTs interfaced more-or-less directly to the
> PCI bus); 2. RS-232 via USB; 3. PPS decoding PCI cards (which might also
> have an IRIG input or even an onboard GNSS receiver).
> Thanks in advance,
> Ruslan
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Chris Albertson
Redondo Beach, California

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