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PICList Thread
'[EE:] USB cable length'
2004\01\09@152015 by Bob Blick

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Before I try, has anyone experimented with extending USB cable length on a
cheap webcam?

If so, how far could you go? 10 feet? 20 feet? 30 feet?

I had an old parallel port color quickcam, it seemed that 15 feet was OK,
20 feet started to get flaky. I'd expect USB to work a little farther, but
not much.

Thanks,

Bob

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2004\01\09@153015 by Dave VanHorn

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At 03:18 PM 1/9/2004 -0500, Bob Blick wrote:
>Before I try, has anyone experimented with extending USB cable length on a
>cheap webcam?
>
>If so, how far could you go? 10 feet? 20 feet? 30 feet?
>
>I had an old parallel port color quickcam, it seemed that 15 feet was OK,
>20 feet started to get flaky. I'd expect USB to work a little farther, but
>not much.

You can use hubs or powered repeaters to extend the distance.
Also, IONetworks makes a USBAnywhere that lets you plug USB devices into
your computer over a twisted pair lan.
I have one at home, it's pretty spooky to plug in a mouse across the room,
and watch it running around on the main system screen just like you were
sitting there.

Great prank potential, you could mess with someone's  mouse from a
completely remote place.
It only works when the lan is up, but unplugging the lan would not be my
first guess when having mouse trouble.

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2004\01\09@161411 by Andrew Warren

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Bob Blick <spam_OUTPICLISTTakeThisOuTspammitvma.mit.edu> wrote:

> Before I try, has anyone experimented with extending USB cable length
> on a cheap webcam?
>
> If so, how far could you go? 10 feet? 20 feet? 30 feet?
>
> I had an old parallel port color quickcam, it seemed that 15 feet was
> OK, 20 feet started to get flaky. I'd expect USB to work a little
> farther, but not much.

Bob:

You can go five meters with just a cable, or up to 30 meters with a
series of cables and hubs (or a series of active USB extension
cables).

USB cables are limited to 5 meters because that length is short
enough to allow reflections from the far end of the cable to settle
between bits; if a longer cable is used, reflections won't be damped
as well, and the line voltage could rise high enough to cause
physical damage to the drivers.

You can connect a series of 5 USB hubs and cables to get a maximum
host-to-device length of 30 meters (the "active USB extension cables"
are really just single-port hubs between two cables), but you can't
use more than 5 hubs.  The 5-hub limit on serially-connected hubs is
a consequence of the USB spec for maximum turnaround delay between an
outgoing packet and the incoming response; that spec allows for 70 ns
of propagation delay through each cable/hub combination, and 30 ns
through each cable alone.  The sum of the propagation delays through
5 hubs and 6 cables (and back), plus the delay allowed in the device
itself, beats the full-speed timeout spec by less than half a
nanosecond. Adding even a few inches of cable to a full-length serial
string of USB cables and hubs would violate that spec.

There are more-exotic USB extenders that can give you miles of range,
but they're expensive... They work by putting what looks like a USB
device right next to the PC host, and what looks like a USB host
right next to the device, then communicating betweeen the two using
something other than USB.

-Andy

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2004\01\09@171539 by Josh Koffman

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Dave VanHorn wrote:
> Also, IONetworks makes a USBAnywhere that lets you plug USB devices into
> your computer over a twisted pair lan.

For the record, IONetworks' product is AnywhereUSB. Check out all their
products at: http://www.ionetworks.com/products/pcp.html

Enjoy!

Josh
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2004\01\09@172820 by Robert Rolf

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Andrew Warren wrote:
> Bob Blick <PICLISTspamKILLspammitvma.mit.edu> wrote:
>
> > Before I try, has anyone experimented with extending USB cable length
> > on a cheap webcam?
> >
> > If so, how far could you go? 10 feet? 20 feet? 30 feet?
> >
> > I had an old parallel port color quickcam, it seemed that 15 feet was
> > OK, 20 feet started to get flaky. I'd expect USB to work a little
> > farther, but not much.
>
> Bob:
>
> You can go five meters with just a cable, or up to 30 meters with a
> series of cables and hubs (or a series of active USB extension
> cables).

And the absurd thing is that the LOW SPEED USB devices have
a much SHORTER cable limit than the high speed ones?
Any rational explanation for that would be appreciated.

> USB cables are limited to 5 meters because that length is short
> enough to allow reflections from the far end of the cable to settle
> between bits; if a longer cable is used, reflections won't be damped
> as well, and the line voltage could rise high enough to cause
> physical damage to the drivers.

It take it that the USB designers never heard of 'terminations'
to prevent reflections? Or 'transient clamp diodes'?

{Quote hidden}

Would it not help to use a 'low capacitance' cable, which has
a higher propagation velocity?

I've read of a videomaker in Montreal who runs firewire links
to his camera that are WAY over spec (150' he claims) by using
Cat5 wire. I've been meaning to do this with my firewire
camera, but have not been able to find a Male-Female firewire
extension cable I could hack up.

> There are more-exotic USB extenders that can give you miles of range,
> but they're expensive... They work by putting what looks like a USB
> device right next to the PC host, and what looks like a USB host
> right next to the device, then communicating betweeen the two using
> something other than USB.

So what's the point of using USB then?

How long before we see ethernet mice/keyboards and cameras?
Looks to me like a market opportunity, particularly for cameras.
If I can get a 100BT card for under $10, putting the chip IN
the camera can't be THAT expensive.

Robert

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2004\01\09@173033 by Josh Koffman

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I guess this is mainly a question for Andy, as he's likely pretty up to
date on the USB standard. There is a local store here that I've seen USB
A-B cables in 25 foot lengths. Now, the packages could be mislabeled,
I'm not sure. These are the no name, made in China imported jobbies.
These cables would seem to be definately out of spec. They are not
powered extensions, they don't have a big enough "hump" on the cable. If
I read your post correctly, having a cable that long could damage the
drivers, correct? So why would anyone make one that long? Is it possible
that it might work?

Josh
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Andrew Warren wrote:
> USB cables are limited to 5 meters because that length is short
> enough to allow reflections from the far end of the cable to settle
> between bits; if a longer cable is used, reflections won't be damped
> as well, and the line voltage could rise high enough to cause
> physical damage to the drivers.

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2004\01\09@183846 by Andrew Warren

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Josh Koffman <.....PICLISTKILLspamspam.....mitvma.mit.edu> wrote:

> There is a local store here that I've seen USB A-B cables in 25
> foot lengths. Now, the packages could be mislabeled, I'm not sure.
> These are the no name, made in China imported jobbies. These cables
> would seem to be definately out of spec. They are not powered
> extensions, they don't have a big enough "hump" on the cable. If I
> read your post correctly, having a cable that long could damage the
> drivers, correct?

   It could, yes.  In the real world, it probably won't, but it's
   certainly out of spec.

   The trademarked USB logo is legitimately carried only by products
   (including cables) that have passed USB compliance testing.  Lack
   of a logo is a good clue that you're looking at a non-compliant
   (or at least non-tested) product.

> So why would anyone make one that long?  Is it possible that it
> might work?

   Sure, it could work.

   Using a 25-foot cable is loosely analogous to overclocking a PC
   video card (or a PIC):  In any specific system, it may work well
   enough for a particular application, but it may not meet all of
   its specifications... And under those conditions, it's certainly
   not guaranteed to do so by its manufacturer.

   -Andy

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2004\01\09@183847 by Bob Ammerman

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It is actually outside the USB spec to make any 'extension cord' or overlong
cable. For example a A-male to A-female or B-male to B-female cable are
"illegal" because they could be used to make a connection that is too long.

The design decisions made in the USB spec were intended so that no matter
what you did, if you could hook something up, it would work. (The only
exception is that you could hook up a chain of more than five hubs, which is
relatively unlikely to happen).

Another design decision involved having low speed devices requiring captive
cables (or cables with a custom (non-USB connector) end at the device) that
are not longer than a certain limit. I forget the details on this, but I do
remember that it was a requirement to ensure reliable operation (otherwise
the user could use a cable designed for full speed operation and get
problems). And yes, low speed devices are limited to shorter cables than
full speed devices because of the simpler signaling scheme used.

Unfortunately idiotic companies don't follow the spec properly, leaving
support folks to deal with the grief.

Bob Ammerman
RAm Systems

{Original Message removed}

2004\01\09@185133 by Herbert Graf

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> > You can go five meters with just a cable, or up to 30 meters with a
> > series of cables and hubs (or a series of active USB extension
> > cables).
>
> And the absurd thing is that the LOW SPEED USB devices have
> a much SHORTER cable limit than the high speed ones?
> Any rational explanation for that would be appreciated.

       My guess is low speed devices are expected to be very low power, i.e.
keyboards, mice, etc. Low power means weaker drivers. TTYL


> So what's the point of using USB then?

       Originally? To replace the parallel, keyboard, mouse, serial and other
legacy ports. In most cases these devices are all low power, low speed, and
close to the PC.

> How long before we see ethernet mice/keyboards and cameras?
> Looks to me like a market opportunity, particularly for cameras.
> If I can get a 100BT card for under $10, putting the chip IN
> the camera can't be THAT expensive.

       Not necessary. Necessity is the mother of all invention. Lack of need means
something won't catch on. It seems you don't understand what USB was
originally meant for and expect more from it then you should. TTYL

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2004\01\09@193156 by Andrew Warren

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Robert Rolf <PICLISTspamspam_OUTmitvma.mit.edu> wrote:

> the absurd thing is that the LOW SPEED USB devices have a much
> SHORTER cable limit than the high speed ones? Any rational
> explanation for that would be appreciated.

   Reading the USB spec (available for free at http://www.usb.org)
   would explain it.  See Chapter 7.

   Paraphrased from the spec:

   Low-speed USB signalling has slow rise and fall times (no less
   than 75 ns and no more than 300 ns under specific test
   conditions).  The 3-meter maximum cable length is to avoid
   transmission-line effects:  Limiting the cable length to 3
   meters (18 ns propogation delay) ensures that reflections will
   appear during the first half of the rise or fall, which allows
   the cable to be approximated by a lumped capacitance.

> It take it that the USB designers never heard of 'terminations' to
> prevent reflections? Or 'transient clamp diodes'?

   Yeah, I'm sure they never heard of any of that.  Sounds too
   complicated for those dumb USB guys to understand.

> > The sum of the propagation delays through 5 hubs and 6 cables
> > (and back), plus the delay allowed in the device itself, beats
> > the full-speed timeout spec by less than half a nanosecond.
> > Adding even a few inches of cable to a full-length serial string
> > of USB cables and hubs would violate that spec.
>
> Would it not help to use a 'low capacitance' cable, which has a
> higher propagation velocity?

   The USB spec is already written and allows 30 ns for the cable
   propagation delay.  If you have a cable that meets all the other
   requirements and has a propagation velocity closer to the speed
   of light than what the USB spec assumes, you can make a cable
   longer than 5 meters.  If you can exceed the speed of light,
   that'd be even better.

> I've read of a videomaker in Montreal who runs firewire links to
> his camera that are WAY over spec (150' he claims) by using Cat5
> wire. I've been meaning to do this with my firewire camera, but
> have not been able to find a Male-Female firewire extension cable I
> could hack up.

   Firewire's significantly different from USB; there's no
   necessary correlation between what works for one and what'll
   work for the other.

> > There are more-exotic USB extenders that can give you miles of
> > range, but they're expensive... [they communicate] using
> > something other than USB.
>
> So what's the point of using USB then?

   The point is mostly contained in the phrase "but they're
   expensive".  There's a LOT more, but cost is a big-enough deal
   that further explanation is usually unnecessary.

> How long before we see ethernet mice/keyboards and cameras?

   Ethernet cameras?  There are already some; they have an
   absolutely insignificant share of the webcam market.

   You'll never see ethernet mice and keyboards.

> Looks to me like a market opportunity, particularly for cameras. If
> I can get a 100BT card for under $10, putting the chip IN the
> camera can't be THAT expensive.

   Keep studying the problem; you'll see why USB is better.

   -Andy

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2004\01\09@193822 by Robert Rolf

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Bob Ammerman wrote:
>
> It is actually outside the USB spec to make any 'extension cord' or overlong
> cable. For example a A-male to A-female or B-male to B-female cable are
> "illegal" because they could be used to make a connection that is too long.
>
> The design decisions made in the USB spec were intended so that no matter
> what you did, if you could hook something up, it would work. (The only

Yeah, right. Some crap driver s/w won't work even with a short cable!
(WaveX MP3 player for one).

> exception is that you could hook up a chain of more than five hubs, which is
> relatively unlikely to happen).

Happened at a Comdex demo of early USB. "Look how many USB devices we can
hook up". They supposedly made it out to 250 something before
the software broke. With the typical 4 port hubs of that day, that
would mean 3**5 =243 maximum devices.

> Another design decision involved having low speed devices requiring captive
> cables (or cables with a custom (non-USB connector) end at the device) that
> are not longer than a certain limit. I forget the details on this, but I do
> remember that it was a requirement to ensure reliable operation (otherwise
> the user could use a cable designed for full speed operation and get
> problems). And yes, low speed devices are limited to shorter cables than
> full speed devices because of the simpler signaling scheme used.

And yet they use type A connectors, for which extension cords are readily
available.

Part of the distance issue is that manufactures use the cheapest cables
they can get away with. Higher grade (low capacitance) cables
(as used on IEEE 1288 printer cables) extend the range markedly.

> Unfortunately idiotic companies don't follow the spec properly, leaving
> support folks to deal with the grief.

You're talking mostly about MicroSloth I presume...

Q: How many MicroSoft programmers does it take to change a light bulb?

A: NONE. They just define "Darkness" to be the new standard.


R

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2004\01\10@015707 by Mike Singer

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Andrew Warren wrote:
> > Looks to me like a market opportunity, particularly for cameras.
> > If I can get a 100BT card for under $10, putting the chip IN the
> > camera can't be THAT expensive.
>
>     Keep studying the problem; you'll see why USB is better.

  Standalone webcams with built-in Linux based web-server connected to intranet. How USB is supposed to be better than
ethernet to this very much real case?

Mike.

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2004\01\10@021406 by Herbert Graf

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> Andrew Warren wrote:
> > > Looks to me like a market opportunity, particularly for cameras.
> > > If I can get a 100BT card for under $10, putting the chip IN the
> > > camera can't be THAT expensive.
> >
> >     Keep studying the problem; you'll see why USB is better.
>
>    Standalone webcams with built-in Linux based web-server
> connected to intranet. How USB is supposed to be better than
> ethernet to this very much real case?
>
> Mike.

       Very simple: price.

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2004\01\10@024310 by Robert Rolf

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Herbert Graf wrote:
>
> > Andrew Warren wrote:
> > > > Looks to me like a market opportunity, particularly for cameras.
> > > > If I can get a 100BT card for under $10, putting the chip IN the
> > > > camera can't be THAT expensive.
> > >
> > >     Keep studying the problem; you'll see why USB is better.
> >
> >    Standalone webcams with built-in Linux based web-server
> > connected to intranet. How USB is supposed to be better than
> > ethernet to this very much real case?
> >
> > Mike.
>
>         Very simple: price.

Only if the camera is near a PC.
If you need a 150' cable run, it gets awfully expensive,
Or you're not -really- running USB on the long path.

Robert

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2004\01\10@065255 by William Chops Westfield

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On Friday, Jan 9, 2004, at 14:23 US/Pacific, Robert Rolf wrote:

> How long before we see ethernet mice/keyboards and cameras?
> Looks to me like a market opportunity, particularly for cameras.
> If I can get a 100BT card for under $10, putting the chip IN
> the camera can't be THAT expensive.
>
>
However, the software stack to support something like video
over ip over ethernet is pretty expensive, once you add the
baggage "necessary" to ensure reasonable Quality Of Service
over a protocol that was not designed to provide it.  USB
took the desire for video into account during its design,
and requires fewer fixes...

(and we won't talk about the relative costs of administering
an IP ethernet network vs a group of USB devices, either.)

BillW

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2004\01\10@095618 by Mike Singer

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William Chops Westfield wrote:
> > How long before we see ethernet mice/keyboards and cameras?
> > Looks to me like a market opportunity, particularly for cameras.
> > If I can get a 100BT card for under $10, putting the chip IN
> > the camera can't be THAT expensive.
> >
> However, the software stack to support something like video
> over ip over ethernet is pretty expensive, once you add the
> baggage "necessary" to ensure reasonable Quality Of Service
> over a protocol that was not designed to provide it.  USB
> took the desire for video into account during its design,
> and requires fewer fixes...


How much is "software stack to support something like video
over ip over ethernet is pretty expensive"?
From http://www.aegi.com/store/netipcam.html

+++++++++++++++++++++++++++++++++++++++++++++
Intellinet Network IP Camera
$365.00

. No special software or additional hardware required. Simply type the
camera's assigned IP address into your browser and view live, full
motion video . Access the camera from any PC on your LAN . Assign a "live" IP address for anytime, anywhere viewing over the
Internet . Ideal for monitoring buildings, offices, daycare, schools, homes, etc.
- anywhere with a network connection . Embedded operating system streams live video directly to your website . Records streaming video or snapshots on your PC . Automatically uploads images to FTP server or sends images out via
email, fully configurable . Built-in Motion Detection Function . Lifetime warranty  Specifications: . JPEG compression, 10 levels . Image Rate: Up to 30 frames/second, European version 25 frames/second

. Resolution: 640 x 480, 320 x 240, 160x120 . Supported Protocols: TCP / IP, UDP, ARP, FTP, TFTP, HTTP, DHCP, SMTP,
SNTP, SNMP, etc. . Output of 12V to signal external devices, max 50mA . Embedded operating system . CMOS lens +++++++++++++++++++++++++++++++++++++++++++++

Mike.

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2004\01\10@114807 by Herbert Graf

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{Quote hidden}

       Which is what USB was NOT meant for, I don't know why you STILL don't get
that. For most users a webcam WILL be near to PC, for the others there is
ethernet, which is more expensive, or other means (such as WiFi which is
even more expensive).

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2004\01\10@151417 by Mike Singer

picon face
Herbert Graf wrote:

> > > > > > Looks to me like a market opportunity, particularly for
cameras.
> > > > > > If I can get a 100BT card for under $10, putting the chip IN
the
{Quote hidden}

  Because he is talking about "market opportunity". Look at the top
of this post. Is he allowed to talk a bit about that?


> For most users a webcam WILL be near to PC, for the others
> there is ethernet, which is more expensive, or other means (such
> as WiFi which is even more expensive).

Who are you to forecast future? There is an essential difference
between potential webcam usage  and usage of keyboard,  mouse,
monitor and similar I/O devices. These devices are needed to operate a computer, But webcam is/will be used to gather video
data under or not computer control.
Perhaps it's against the law in Canada to use webcams at some distance from a computer?

Linux and Linux-based web servers are open source -
Boa (http://www.boa.org/) and thttpd for example. SOCs are getting cheaper and cheaper. It's a thankless business to forecast what "most users" will prefer in the future.

Best Wishes,
Mike.

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2004\01\10@160356 by Herbert Graf

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> > > Only if the camera is near a PC.
> > > If you need a 150' cable run, it gets awfully expensive,
> > > Or you're not -really- running USB on the long path.
> > >
> > > Robert
>
> >         Which is what USB was NOT meant for, I don't know why you
> > STILL don't get that.
>
>    Because he is talking about "market opportunity". Look at the top
> of this post. Is he allowed to talk a bit about that?

       What does that have to do with complaining about a technology that doesn't
work because it's being used out of spec?

> > For most users a webcam WILL be near to PC, for the others
> > there is ethernet, which is more expensive, or other means (such
> > as WiFi which is even more expensive).
>
> Who are you to forecast future?

       Not the future. Look at today: what percentage of users of webcams want
them more then say 10 meters from their PC?

>There is an essential difference
> between potential webcam usage  and usage of keyboard,  mouse,
> monitor and similar I/O devices. These devices are needed to
> operate a computer, But webcam is/will be used to gather video
> data under or not computer control.

       Yes, which is why there are other options. What point are you trying to
make?

> Linux and Linux-based web servers are open source -
> Boa (http://www.boa.org/) and thttpd for example. SOCs are
> getting cheaper and cheaper. It's a thankless business to
> forecast what "most users" will prefer in the future.
>
> Best Wishes,
> Mike.

       What point are you making? Or are you back to your old habits?

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2004\01\10@184013 by William Chops Westfield

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On Saturday, Jan 10, 2004, at 06:55 US/Pacific, Mike Singer wrote:

> How much is "software stack to support something like video
> over ip over ethernet is pretty expensive"?
>
> From http://www.aegi.com/store/netipcam.html
> Intellinet Network IP Camera $365.00
>
That makes it out to be about the same price as a typical
USB camera, plus a low-end PC clone for the SW.  Yeah,
that's about right...

And I didn't see anything in the protocol list about RSVP,
IPRTP, or any of those 'fancy' protocols.  Running ONE
ethernet camera across a 100Mbps ethernet without much
other traffic is comparitivly easy.  Try sharing your 10M
ethernet with a bunch of other peripherals and the video
camera (which is analogous to what USB tries to do out of
the box) and you may start to miss the extra capabilities.

BillW

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2004\01\10@193111 by William Chops Westfield

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On Saturday, Jan 10, 2004, at 08:47 US/Pacific, Herbert Graf wrote:

> Which is what USB was NOT meant for, I don't know why you STILL
> don't get that. For most users a webcam WILL be near to PC,

A forum like PICList is likely to rate the merits of a comm port
based somewhat on how easily the defined "protocol" can be "bent"
to other purposes.  Use the printer port for input?  Cool.  Put a
disk drive on it?  Excellent!  And still have the printer work?
Uh...  The bidirectionally-communicating printer?  ummm...
And the SERIAL port, well.  Hardware flow control.  115200bps.
Deeper and deeper FIFOs, but don't get incompatible with that
venerable 8250 - too much legacy SW!  Generating Vpp from the
rs232 voltages, and using the modem signals for IO.  Back to cool!

I don't know who said that the legacy ports "just worked" - that
certainly doesn't match MY memories of struggling with (especially)
serial port cables and adaptors.  Things have settled down a bit now,
thanks to ... extinctions of entire species, essentially, but I think
the most common serial comm problem is STILL a wrong cable somewhere.

And I don't want to talk about lovely external interfaces like HPIB
and SCSI, where the cables often out-weighed the peripherals, and had
their own problems with chaining and termination...

Now, USB was designed (as I see it) to:

1) address the requirements of the sorts of devices people were
   mis-using serial and parallel ports to connect.
2) cut down on the scope and magnitude of the user errors that
   could occur in connecting devices to the computer.
3) permit a greater number of devices to be connected without
   interfering with each other.

They did pretty good, IMO.

USB2.0, OTOH, is a rather embarassing afterthought.  A sort of "oops,
here are the things we forgot about (or specifically excluded), cause
after all we don't want firewire to own those pieces..."

There's no doubt in my mind that USB can be abused.  Longer cables
are clearly possible, especially if you're willing to give up things
like hot-swap out at the end of that cable.  (yeah.  termination.
looks to me like USB is designed to keep working even if one or more
ends of the max-specified cable suddenly become un-terminated.  What
a good idea!)  It would probably be good for the computing world in
general if intentional abuses of the USB spec do not become rampant
the way they did with serial and parallel.  It's bad enough that
"evil" extension cables are widely available.  No selling USB cameras
with 50 foot of cable on them, please...

BillW

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