GBS Elektronik MCA-527 Choosing the correct USB cable - Manual (Page 2)

Technical Note
Choosing the Correct USB Cable for your MCA527
Rev.: 00/01.2012
Page 2 of 2
resistance but the resulting voltage drop of 0.4V
violates the USB specification which defines a
maximum voltage drop for detachable cables of
0.125V. Therefore, a USB compliant cable for this
scenario should have a maximum DC resistance of:
R
max
=
V
BUSD
I
max
=
0.125V
0.1A
= 1.25Ω
In the second scenario a high-power device is
connected to a host or powered hub. According to the
USB specification the maximum allowed conductor
resistance will decrease to:
R
max
=
V
BUSD
I
max
=
0.125V
0.5A
=
0.025V
0.1A
= 0.25Ω
This is also the maximum allowed resistance in
the third scenario where a low-power device is
connected to a bus-powered hub. In this case the
maximum allowed voltage drop is 0.025V.
In summary it can be said that three classes of
USB cables are possible, but only two of them are
compliant with the USB specification. We have tested
six arbitrary USB cables and have found cables of all
three classes. Table 1 shows the results.
Table 1: Measured Conductor Resistances for Different
USB cables
USB Cable R
GND
R
VCC
USB compliant
Cable1, 0.9m 0.10 Ω 0.10 Ω full
Cable2, 1.0m 0.12 Ω 0.11 Ω full
Cable3, 1.8m 0.20 Ω 0.19 Ω full
Cable4, 1.8m 0.51 Ω 0.65 Ω limited
Cable5, 1.8m 1.91 Ω 1.95 Ω no
Cable6, 3.0m 1.02 Ω 0.95 Ω limited
Unfortunately, the cables are not classified by
the manufacturers. Therefore, it is the users task to
find the appropriate cable for their application. This is
not easy, because most data sheets specify the
conductor's cross-section and material instead of its
DC resistance. Table 2 simplifies selection of an
appropriate cable. Some cables have different cross-
sections for data and power conductors. Usually the
conductors with the larger cross-section are used for
the power lines. Not all USB cables have copper
conductors. Specially some low-cost cables are
made with copper clad steel (CCS) conductors,
maybe other materials are also used. The
conductivity of CCS varies very much. For conductors
used in data cables it may be about 20% relative to
copper!
Table 2: Calculated Resistances for Different Conductors
Conductor R @ 1m R @ 2m R @ 5m
AWG20, Cu 0,03 Ω 0,07 Ω 0,17 Ω
AWG22, Cu 0,05 Ω 0,11 Ω 0,27 Ω *
AWG24, Cu 0,09 Ω 0,17 Ω 0,43 Ω *
AWG26, Cu 0,14 Ω 0,28 Ω * 0,70 Ω *
AWG28, Cu 0,22 Ω 0,44 Ω * 1,11 Ω *
AWG30, Cu 0,35 Ω * 0,71 Ω * 1,77 Ω **
AWG28, CCS 20% 1,11 Ω * 2,21 Ω ** 5,53 Ω **
* only usable for low-power devices connected to powered hubs
** not compliant with USB specification
Conclusion
Use only USB1.1 or USB2.0 cables with a
standard-B plug on one side and a type-A plug which
mates to your host computer on the other side to
operate the MCA527. The cable shouldn't be longer
than 5 meters because longer cables are not
compliant with the USB specification. Avoid to use
extension cords, they are not compliant with the USB
specification. When used, the power conductor's
overall DC resistance must not exceed the value
required by the application (0.25
Ω ). Furthermore, the
overall length must not exceed 5 meters. Make sure
that the conductor cross-section of the two power
conductors corresponds with the cable length. Take
care about the conductor material. For the MCA527
only copper cables are suitable. We recommend to
use a standard USB2.0 cable from TE Connectivity
(
www.te.com ), for example part number 1487596-3
(1.8m) or 1487598-2 (5m).
Do not use active cables (repeater cables).
Usually they are bus-powered hubs and cannot
operate high-power devices. Do not operate the
MCA527 on a bus-powered hub. Actually, this should
not work but most bus-powered hubs on the market
identify themselves as powered hubs. Under some
circumstances the input voltage on the MCA527 can
fall below 4.5V and may cause malfunction.
When only a bad USB cable is available, operate
the MCA527 with its battery charger. Then the current
on the USB power lines decreases to a few
milliamperes and therewith the voltage drop on the
power lines is negligible.
References
1. Universal Serial Bus Specification, Revision 2.0,
April 27, 2000, page 175,
http://www.usb.org/developers/docs/usb_20_101111.zip
2. Wikipedia, www.wikipedia.org/wiki/usb/
3. TE Connectivity, www.te.com
TN_Choosing_USB_Cable.odt