USB Cable shield continuity

[Paul E. Schoen]

I have been having seemingly random noise problems where my PIC-based USB
device will go into a USBSuspendControl state, and it requires the cable to
be removed and replaced to reestablish communication. The problem usually
occurs in the field where there is switching of high current and high
voltage, but I was able to duplicate it to some extent by running the USB
cable along an AC power line to a current source which I switched on and
off.

Recently I suspected that the USB cable itself might be at fault, because I
had bought a batch of 100 pieces for $0.69 each (but now about $1.50) from
www.CableWholesale.com, and several of these were sent to a customer who
reported problems, while a previous customer with an older cable did not
seem to experience this very much, and another customer replaced his cable
with a longer one, and he said his unit was working OK.

So, I dissected one of the new cables by removing the PVC jacket in the
middle, and I found a substantial tinned copper braid shield, and an
aluminized Mylar wrap under that. When I removed the shield I could see that
the four USB conductors were twisted together, which is generally good for
noise induced by strong magnetic fields. So far, so good.

But when I measured continuity from the connector shells to the exposed
shield, I got intermittent readings of about 3 to 30 ohms and sometimes an
open circuit. Then I measured the continuity from shell to shell on a couple
other USB cables I had been using, and I found that one showed an open
circuit and the others showed intermittent. This was the case for two new
cables from different sources. Yet I measured the shells of a USB cable for
my Nikon digital camera (with a mini-USB on the camera end), and I got a
solid connection of less than 1 ohm.

I still need to do more testing and I may also purchase a high grade cable
with gold plated connectors and better shielding. They are about $20. I will
also have my customer check the continuity and try a better cable. Perhaps a
USB 3.0 cable will work better.

I removed the PVC molded cover for the male type "A" connector, and there is
a metal shell that extends back and tapers to a smaller "neck" where the
cable is clinched or crimped. By bending the ears on the crimp I was able to
separate part of the metal housing to reveal where the shield has been
folded back and exposed so that the inner surface of the housing presses
against it. But it seems that the jacket of the cable is a continuous
molding that fills the shell of the connector, and the crimp mechanism can
only apply light force to the exposed part of the shield. So the actual
connection may degrade with time as a non-conductive film may form on the
metal surfaces, and mechanical flexing may further degrade the connection.

Here are pictures of the cable after dissection and exposure of the crimped
shield connection:

http://cygnus.smart.net/~pstech/photos/USB_Cable_23.JPG
http://cygnus.smart.net/~pstech/photos/USB_Cable_Conn-A_26a.JPG
http://cygnus.smart.net/~pstech/photos/USB_Cable_Conn-A_27.JPG
http://cygnus.smart.net/~pstech/photos/USB_Cable_Conn-A_28.JPG
http://cygnus.smart.net/~pstech/photos/USB_Cable_Conn-A_29.JPG
http://cygnus.smart.net/~pstech/photos/USB_Cable_Conn-A_30.JPG

I think this is a design defect and I should be able to get a refund or
credit for the unused cables. It may not be the reason for the problem but I
should be able to determine that if my customers replace the cables with
high performance versions and their problems are greatly reduced.

Anyone else have experience with this? One member of the Microchip forums
reported that he found the following with some new cables he had on hand:

Poundland 1.8 m A-A(F) yellow 8 ohms
Signalex 1.5 m A-A(F) white OPEN CIRCUIT!
Signalex 1.5 m A-B white OPEN CIRCUIT!
CPC 1.8 m A-B translucent yellow 18.5 ohms
IXIOS 3 m A-A(F) translucent/silver grey gold plated connectors
<0.5 ohms

Paul

 

[baron]

Paul E. Schoen Inscribed thus:

I have been having seemingly random noise problems where my PIC-based
USB device will go into a USBSuspendControl state, and it requires the
cable to be removed and replaced to reestablish communication. The
problem usually occurs in the field where there is switching of high
current and high voltage, but I was able to duplicate it to some
extent by running the USB cable along an AC power line to a current
source which I switched on and off.

Recently I suspected that the USB cable itself might be at fault,
because I had bought a batch of 100 pieces for $0.69 each (but now
about $1.50) from www.CableWholesale.com, and several of these were
sent to a customer who reported problems, while a previous customer
with an older cable did not seem to experience this very much, and
another customer replaced his cable with a longer one, and he said his
unit was working OK.

So, I dissected one of the new cables by removing the PVC jacket in
the middle, and I found a substantial tinned copper braid shield, and
an aluminized Mylar wrap under that. When I removed the shield I could
see that the four USB conductors were twisted together, which is
generally good for noise induced by strong magnetic fields. So far, so
good.

But when I measured continuity from the connector shells to the
exposed shield, I got intermittent readings of about 3 to 30 ohms and
sometimes an open circuit. Then I measured the continuity from shell
to shell on a couple other USB cables I had been using, and I found
that one showed an open circuit and the others showed intermittent.
This was the case for two new cables from different sources. Yet I
measured the shells of a USB cable for my Nikon digital camera (with a
mini-USB on the camera end), and I got a solid connection of less than
1 ohm.

I still need to do more testing and I may also purchase a high grade
cable with gold plated connectors and better shielding. They are about
$20. I will also have my customer check the continuity and try a
better cable. Perhaps a USB 3.0 cable will work better.

I removed the PVC molded cover for the male type "A" connector, and
there is a metal shell that extends back and tapers to a smaller
"neck" where the cable is clinched or crimped. By bending the ears on
the crimp I was able to separate part of the metal housing to reveal
where the shield has been folded back and exposed so that the inner
surface of the housing presses against it. But it seems that the
jacket of the cable is a continuous molding that fills the shell of
the connector, and the crimp mechanism can only apply light force to
the exposed part of the shield. So the actual connection may degrade
with time as a non-conductive film may form on the metal surfaces, and
mechanical flexing may further degrade the connection.

Here are pictures of the cable after dissection and exposure of the
crimped shield connection:

http://cygnus.smart.net/~pstech/photos/USB_Cable_23.JPG
http://cygnus.smart.net/~pstech/photos/USB_Cable_Conn-A_26a.JPG
http://cygnus.smart.net/~pstech/photos/USB_Cable_Conn-A_27.JPG
http://cygnus.smart.net/~pstech/photos/USB_Cable_Conn-A_28.JPG
http://cygnus.smart.net/~pstech/photos/USB_Cable_Conn-A_29.JPG
http://cygnus.smart.net/~pstech/photos/USB_Cable_Conn-A_30.JPG

I think this is a design defect and I should be able to get a refund
or credit for the unused cables. It may not be the reason for the
problem but I should be able to determine that if my customers replace
the cables with high performance versions and their problems are
greatly reduced.

Anyone else have experience with this? One member of the Microchip
forums reported that he found the following with some new cables he
had on hand:

Poundland 1.8 m A-A(F) yellow 8 ohms
Signalex 1.5 m A-A(F) white OPEN CIRCUIT!
Signalex 1.5 m A-B white OPEN CIRCUIT!
CPC 1.8 m A-B translucent yellow 18.5 ohms
IXIOS 3 m A-A(F) translucent/silver grey gold plated
connectors <0.5 ohms

Paul

Hi Paul,

The figures above don't cause any surprise at all !

Part of the problem with USB cables is that the shield is not needed for
the cable to work but is vital to prevent interference effecting the
signals on the cable and by the same token preventing the cable
radiating and causing interference to other things.

I built a test rig that checks continuity of all four wires and the
screen. I used a 100ma test current from a 12 volt source. I found
that most of the cables tested showed 10 ohms or more between the ends
of the shells and I found a number with varying resistance on the
individual conductors of a similar order.

I also found that some receptacles showed varying degrees of contact
resistance with increasing usage. The type "A" being the most
affected, type "B" being more robust.

 

[Paul E. Schoen]

Paul E. Schoen Inscribed thus:


Hi Paul,

The figures above don't cause any surprise at all !

Part of the problem with USB cables is that the shield is not needed for
the cable to work but is vital to prevent interference effecting the
signals on the cable and by the same token preventing the cable
radiating and causing interference to other things.

I built a test rig that checks continuity of all four wires and the
screen. I used a 100ma test current from a 12 volt source. I found
that most of the cables tested showed 10 ohms or more between the ends
of the shells and I found a number with varying resistance on the
individual conductors of a similar order.

I also found that some receptacles showed varying degrees of contact
resistance with increasing usage. The type "A" being the most
affected, type "B" being more robust.

The overall resistance of the four conductors should be much more consistent
if they are formed with a proper metal-to-metal crimp. Solder or tack weld
would be even better but would be very costly. Here is the USB spec:

Universal Serial Bus Specification

6.6.3 Electrical Characteristics
...
The DC resistance from plug shell to plug shell (or end of integrated
cable) must be less than 0.6 ohms.

http://alsa.cybermirror.org/manuals/usb/usbcableecn_final.pdf

The cables may work, but they do not meet specification. Your test at 100 mA
is better than a simple ohmmeter check, but lower voltage might show poor
connections better.

I'm wondering if I could charge up a large capacitor and then discharge it
through the cable shield so that a spot weld might occur at the junction of
shield and shell. But it probably would not be very reliable long-term under
normal use.

Another "fix" might be to expose the shield near the connector and add a
jumper to the shell, but that is ugly, time-intensive, and just plain wrong
except to test if shield ground integrity improves the noise problem.

I found some good deals on gold plated USB cables at www.monoprice.com and
www.TriangleCables.com. Even if the same crimp was used, the gold plating
might reduce the chance for oxides or other coatings to form at the
junction. But the shield braid would also need plating.

The manufacturer might be able to apply a drop of conductive paste at the
junction point that would provide a decent connection and also exclude
moisture and other contaminents. But there may be no reliable way to fix
these cables as they are.

Paul

 

[IanM]

The overall resistance of the four conductors should be much more
consistent if they are formed with a proper metal-to-metal crimp. Solder
or tack weld would be even better but would be very costly. Here is the
USB spec:

Universal Serial Bus Specification

6.6.3 Electrical Characteristics
...
The DC resistance from plug shell to plug shell (or end of integrated
cable) must be less than 0.6 ohms.

http://alsa.cybermirror.org/manuals/usb/usbcableecn_final.pdf

The cables may work, but they do not meet specification. Your test at
100 mA is better than a simple ohmmeter check, but lower voltage might
show poor connections better.

I'm wondering if I could charge up a large capacitor and then discharge
it through the cable shield so that a spot weld might occur at the
junction of shield and shell. But it probably would not be very reliable
long-term under normal use.

Another "fix" might be to expose the shield near the connector and add a
jumper to the shell, but that is ugly, time-intensive, and just plain
wrong except to test if shield ground integrity improves the noise problem.

I found some good deals on gold plated USB cables at www.monoprice.com
and www.TriangleCables.com. Even if the same crimp was used, the gold
plating might reduce the chance for oxides or other coatings to form at
the junction. But the shield braid would also need plating.

The manufacturer might be able to apply a drop of conductive paste at
the junction point that would provide a decent connection and also
exclude moisture and other contaminents. But there may be no reliable
way to fix these cables as they are.

Paul

If the braid is pigtailed and crimped into the housing separately from
the overall strain relief crimp there would be no problem.
Unfortunately that would require an extra step on the assembly line.

Sturgeon's Law applies . . . .

 

[Baron]

Paul E. Schoen Inscribed thus:

The overall resistance of the four conductors should be much more
consistent if they are formed with a proper metal-to-metal crimp.
Solder or tack weld would be even better but would be very costly.
Here is the USB spec:

Universal Serial Bus Specification

6.6.3 Electrical Characteristics
...
The DC resistance from plug shell to plug shell (or end of
integrated cable) must be less than 0.6 ohms.

Grief ! I never saw one less than an ohm.

http://alsa.cybermirror.org/manuals/usb/usbcableecn_final.pdf

The cables may work, but they do not meet specification. Your test at
100 mA is better than a simple ohmmeter check, but lower voltage might
show poor connections better.

I measured the voltage drop across the cable. (1.5mt nominal length)

I'm wondering if I could charge up a large capacitor and then
discharge it through the cable shield so that a spot weld might occur
at the junction of shield and shell. But it probably would not be very
reliable long-term under normal use.

You might find that it just blows the bad joint open.

Another "fix" might be to expose the shield near the connector and add
a jumper to the shell, but that is ugly, time-intensive, and just
plain wrong except to test if shield ground integrity improves the
noise problem.

I found some good deals on gold plated USB cables at www.monoprice.com
and www.TriangleCables.com. Even if the same crimp was used, the gold
plating might reduce the chance for oxides or other coatings to form
at the junction. But the shield braid would also need plating.

The manufacturer might be able to apply a drop of conductive paste at
the junction point that would provide a decent connection and also
exclude moisture and other contaminents. But there may be no reliable
way to fix these cables as they are.

Paul

I agree, they can't be fixed. All the cables that I built the tester
for were of doubtful far east origin. The distributer had a financial
agreement with what ever supplier, because the cables that were
rejected didn't go back to the east.

 

[Baron]

Paul E. Schoen Inscribed thus:

The manufacturer might be able to apply a drop of conductive paste at
the junction point that would provide a decent connection and also
exclude moisture and other contaminents. But there may be no reliable
way to fix these cables as they are.

Paul

I forgot to mention, a temporary fix was to give the bit of plastic,
where the cable enters the plug, a tap with a hammer. It doesn't last
long though, a few flexes and its as bad again.

 

[Martin Riddle]

I have been having seemingly random noise problems where my PIC-based
USB device will go into a USBSuspendControl state, and it requires the
cable to be removed and replaced to reestablish communication. The
problem usually occurs in the field where there is switching of high
current and high voltage, but I was able to duplicate it to some
extent by running the USB cable along an AC power line to a current
source which I switched on and off.

Recently I suspected that the USB cable itself might be at fault,
because I had bought a batch of 100 pieces for $0.69 each (but now
about $1.50) from www.CableWholesale.com, and several of these were
sent to a customer who reported problems, while a previous customer
with an older cable did not seem to experience this very much, and
another customer replaced his cable with a longer one, and he said his
unit was working OK.

So, I dissected one of the new cables by removing the PVC jacket in
the middle, and I found a substantial tinned copper braid shield, and
an aluminized Mylar wrap under that. When I removed the shield I could
see that the four USB conductors were twisted together, which is
generally good for noise induced by strong magnetic fields. So far, so
good.

But when I measured continuity from the connector shells to the
exposed shield, I got intermittent readings of about 3 to 30 ohms and
sometimes an open circuit. Then I measured the continuity from shell
to shell on a couple other USB cables I had been using, and I found
that one showed an open circuit and the others showed intermittent.
This was the case for two new cables from different sources. Yet I
measured the shells of a USB cable for my Nikon digital camera (with a
mini-USB on the camera end), and I got a solid connection of less than
1 ohm.

I still need to do more testing and I may also purchase a high grade
cable with gold plated connectors and better shielding. They are about
$20. I will also have my customer check the continuity and try a
better cable. Perhaps a USB 3.0 cable will work better.

I removed the PVC molded cover for the male type "A" connector, and
there is a metal shell that extends back and tapers to a smaller
"neck" where the cable is clinched or crimped. By bending the ears on
the crimp I was able to separate part of the metal housing to reveal
where the shield has been folded back and exposed so that the inner
surface of the housing presses against it. But it seems that the
jacket of the cable is a continuous molding that fills the shell of
the connector, and the crimp mechanism can only apply light force to
the exposed part of the shield. So the actual connection may degrade
with time as a non-conductive film may form on the metal surfaces, and
mechanical flexing may further degrade the connection.

Here are pictures of the cable after dissection and exposure of the
crimped
shield connection:

http://cygnus.smart.net/~pstech/photos/USB_Cable_23.JPG
http://cygnus.smart.net/~pstech/photos/USB_Cable_Conn-A_26a.JPG
http://cygnus.smart.net/~pstech/photos/USB_Cable_Conn-A_27.JPG
http://cygnus.smart.net/~pstech/photos/USB_Cable_Conn-A_28.JPG
http://cygnus.smart.net/~pstech/photos/USB_Cable_Conn-A_29.JPG
http://cygnus.smart.net/~pstech/photos/USB_Cable_Conn-A_30.JPG

I think this is a design defect and I should be able to get a refund
or credit for the unused cables. It may not be the reason for the
problem but I should be able to determine that if my customers replace
the cables with high performance versions and their problems are
greatly reduced.

Anyone else have experience with this? One member of the Microchip
forums reported that he found the following with some new cables he
had on hand:

Poundland 1.8 m A-A(F) yellow 8 ohms
Signalex 1.5 m A-A(F) white OPEN CIRCUIT!
Signalex 1.5 m A-B white OPEN CIRCUIT!
CPC 1.8 m A-B translucent yellow 18.5 ohms
IXIOS 3 m A-A(F) translucent/silver grey gold plated
connectors <0.5 ohms

Paul

Interesting, I have the same problem with a PIC project. Right now I'm
moving to the latest Mchip USB stack.
I did find a well placed Ferrite bead suppressed the problem, but it
still occurs once in a while.
Eventually I plan on programmatically Detaching the device and
re-enumerating, after a set period of inactivity.

This problem does not occur with other USB devices in the system.

Cheers

 

[Paul E. Schoen]

Interesting, I have the same problem with a PIC project. Right now I'm
moving to the latest Mchip USB stack.
I did find a well placed Ferrite bead suppressed the problem, but it still
occurs once in a while.
Eventually I plan on programmatically Detaching the device and
re-enumerating, after a set period of inactivity.

This problem does not occur with other USB devices in the system.

I found that I had to put a jumper from an unused pin (24 = RB3) on the
PIC18F2550 to the D+ line of the USB connector to effect a reliable detach
and reattach. When the device is stuck in the USBSuspendContol state, I use
a countdown which eventually disables interrupts, changes the pin from an
input to an output and drives it low to force a disconnect. Then I do a
SoftDetach(), run a delay loop for about 5 seconds, and then a program
reset.

However, this causes a problem on a new installation where the driver must
be installed. I found this from a customer. I already had the driver
installed, so it enumerated before the timeout, but without the driver it
went into a loop which prevented it from being installed. There may be a way
to pre-install a driver, but that could be tricky.

I switched to the new stack (2.6) recently, and found several changes needed
to be made. There also seems to be a bug in the code if you are using a
PIC2450 as I have on one of my older boards.

I have two versions of PIC code, one for a CDC and another that uses the
generic mchpusb.sys driver. There is supposedly a bug in the Microsoft
supplied usbser.sys driver, but I have found that both work equally well (or
not well), and the errors seem to be random. These are comm errors where the
USB port seems to stop receiving data for a second or longer, or where
characters are skipped. The SuspendControl lockup is a separate problem
which seems noise related.

Paul

 

[Martin Riddle]

I found that I had to put a jumper from an unused pin (24 = RB3) on
the PIC18F2550 to the D+ line of the USB connector to effect a
reliable detach and reattach. When the device is stuck in the
USBSuspendContol state, I use a countdown which eventually disables
interrupts, changes the pin from an input to an output and drives it
low to force a disconnect. Then I do a SoftDetach(), run a delay loop
for about 5 seconds, and then a program reset.

However, this causes a problem on a new installation where the driver
must be installed. I found this from a customer. I already had the
driver installed, so it enumerated before the timeout, but without the
driver it went into a loop which prevented it from being installed.
There may be a way to pre-install a driver, but that could be tricky.

I switched to the new stack (2.6) recently, and found several changes
needed to be made. There also seems to be a bug in the code if you are
using a PIC2450 as I have on one of my older boards.

I have two versions of PIC code, one for a CDC and another that uses
the generic mchpusb.sys driver. There is supposedly a bug in the
Microsoft supplied usbser.sys driver, but I have found that both work
equally well (or not well), and the errors seem to be random. These
are comm errors where the USB port seems to stop receiving data for a
second or longer, or where characters are skipped. The SuspendControl
lockup is a separate problem which seems noise related.

Paul

I'm using the 4550, as a HID device. A hard reset is not an option for
me. But the USBSuspendControl state is an annoyance. Have you tried a
pull down resistor on D+ ? I could detach my device and reattach it,
since I don't need a driver with the HID model and my application will
happily ignore it.

I have a early USB stack, and the 2.7 seems to be really finicky. I just
got it to enumerate, but that's all it does at the moment. Have yet to
debug it, but I haven't seen any comm errors while using the HID.

Yes it is noise related, I can reproduce it by discharging a hand held
Tesla coil to earth ground near by

Cheers

 

[Martin Riddle]

I found that I had to put a jumper from an unused pin (24 = RB3) on
the PIC18F2550 to the D+ line of the USB connector to effect a
reliable detach and reattach. When the device is stuck in the
USBSuspendContol state, I use a countdown which eventually disables
interrupts, changes the pin from an input to an output and drives it
low to force a disconnect. Then I do a SoftDetach(), run a delay loop
for about 5 seconds, and then a program reset.

However, this causes a problem on a new installation where the driver
must be installed. I found this from a customer. I already had the
driver installed, so it enumerated before the timeout, but without the
driver it went into a loop which prevented it from being installed.
There may be a way to pre-install a driver, but that could be tricky.

I switched to the new stack (2.6) recently, and found several changes
needed to be made. There also seems to be a bug in the code if you are
using a PIC2450 as I have on one of my older boards.

I have two versions of PIC code, one for a CDC and another that uses
the generic mchpusb.sys driver. There is supposedly a bug in the
Microsoft supplied usbser.sys driver, but I have found that both work
equally well (or not well), and the errors seem to be random. These
are comm errors where the USB port seems to stop receiving data for a
second or longer, or where characters are skipped. The SuspendControl
lockup is a separate problem which seems noise related.

Paul

I tried some experiments with decoupling (2.2uf tant) the Vusb output
pin. Some improvement which leads me to believe there is a noise related
problem with the internal USB core. ( I don’t use the Vusb pin, and
apparently the USB core runs on 3.3v ) Also I see that there is another
lockup beside the SuspendControl, I haven't dug deeper since I was
successful in detaching and Attaching to the USB bus. But this is a
bandaid fix, and I'm sure most will find this not acceptable, but in my
system its acceptable.

I was able to detach by setting UCON=0 and UIEF=0 ,waiting 1sec and
reattaching by setting USBEN=1 and UPUEN=1 and initializing the stack.
I left my version 2.1 stack in the code, since 2.7 required a little
more work and we don't have any other problems with it. I added a com
timeout to handle the unknown lockup, since it is not related to the
SuspendControl lockup.
Now everything happily recovers, without a hard reset, when that 120vac
relay switches in the system.


Cheers

 

[JosephKK]

Paul E. Schoen Inscribed thus:


I forgot to mention, a temporary fix was to give the bit of plastic,
where the cable enters the plug, a tap with a hammer. It doesn't last
long though, a few flexes and its as bad again.

That sounds like a foil shield instead of braid.

 

[Baron]

JosephKK Inscribed thus:

That sounds like a foil shield instead of braid.

I belive most of them probably are. The only ones that I saw that were
braided had clear jackets.

 

[Paul E. Schoen]

JosephKK Inscribed thus:


I belive most of them probably are. The only ones that I saw that were
braided had clear jackets.

The cables I had bought for $0.69 each from www.cableswholesale.com had an
inner foil shield and a substantial outer shield of tinned copper braid.
Most other cables at least appeared to have a braided shield. I don't think
it would be possible to meet the USB spec of 0.6 ohms with just a foil
shield. I am returning 80 pieces of the 100 I had bought that are still
unopened and unused.

I also contacted www.monoprice.com because they have very good prices, and
some with gold plating, but my technical questions have remained unanswered
even though several people have responded. I asked for a free sample (list
price is only about $1), but nothing yet, although they all say they will
issue a refund if not satisfied. But I'd have to take the time to place an
order, pay maybe $5 S$H, then test it, and then pack it up and spend a
couple dollars more to ship it back to get my $1 refund.

I found www.trianglecables.com, and they say that their cables are custom
made to their specs. They have some that have ferrite filters built into the
cable, so I ordered an assortment of six cables with and without (6ft and
10ft, black, white, clear) for a total of about $25 including shipping. I
should get the shipment in a couple days and I'll report back with findings.

It seems that the USB3.0 cables have a larger type B connector so I can't
use them.

Paul

Paul