Sunday, September 13, 2015

Metrology - USB part 2

Continuing with the discussion relating to Metrology and Universal Serial Bus (USB) cables.

Metrology -
In the first discussion it raised the observations iso9001 has been mentioned and this standard provides a useful guide on record keeping. In most cases users take for granted that the cable/lead/plug is ok and just swap it out if it is deemed not working? Simple questions:

1) Is there a cable/lead tester on the market?
2) What results can be obtained?
3) How to determine output results?
4) Compare manufacturing guidelines for MTTF and MTBF?
5) Can the results scrutinised be improved?
6) Can a minimum standard be achieved.

Metrology - USB part 1 -
Later the discussion raised the notion that smartphones, tablets and other devices fitting the description Size-Scaled Digital Technology (SSDT) using USB physical connectivity provides for the simplest of examination DUT illustrations e.g. the combination of three separate entities involved in inter-connection during an examination:

1) DUT (the target device (SSDT) containing suspected evidence
2) The physical medium (USB) to carry the source data to the examination tool
3) The examination tool (ET) used to extract and harvest evidence

And ended with the point that the discussion started out by referring to the physical medium USB to carry the source data from the DUT to the examination tool (ET). The relevance of doing so is that if the examiner eliminates the medium as the cause for failure or corrupted evidence then the logical conundrum that remains, is the DUT at fault, is the ET at fault or are both DUT/ET together faulty?

In order to eliminate the USB cable's involvement in the acquisition process as the source of causing corrupted data or inducing faults into the DUT requires expanding the investigation of what is known about USB tolerances or identified faults.

Mechanical Failures

Types of USB connector left to right (ruler in centimetres): micro-B plug, UC-E6 proprietary (non-USB) plug, mini-B plug, standard-A receptacle (upside down), standard-A plug, standard-B plug

The procedure required to dissect and strip back a USB plug from its cable. In itself, there is nothing special in this task being performed other than for revelation purposes to allow observations of what is happening underneath the main moulded cable covering, due to the fact that the human eye does not possess x-ray vision. This USB cable was chosen as it had visible signs of wear and tear at the USB plug end that connects to the device (DUT) and charging of a DUT was known to be intermittent.

The USB cable was terminated at either end with a mini-B plug and standard-A plug. The photo below shows the mini-B plug end has been dissected and stripped back.

 The standard coloured wiring is expressed as:

Pin 1VCC (+5 V, red wire)
Pin 2Data− (white wire)
Pin 3Data+ (green wire)
Pin 4Ground (black wire)

It was noticeable from a study of the separate coloured internal wire covers - Green, Red, Black and White ( For a quick reference source refer to ) - that the Red wire cover was in fact a Pink in colour with deterioration (more brittle, easy to pull off covering) than the other coloured coverings.

Given that the mini-B plug is the end that is connected into the DUT raises concerns as to whether the wear and tear could cause damage to the DUT, too. As the Red(Pink) coloured cover concerns the power VCC (+5 V, red wire) it is not difficult to speculate the potential for damage or failure and that on the balance of probability (at one of the end of the scale) the quality assurance programme should have identified this as a problem or issue to be addressed, (and at the other end of the scale) that beyond reasonable doubt the quality control processes should have removed this physical medium (USB cable) from the pool of tools/devices that could be used during an examination process.

The sampling rates for conducted Vbus and Vcc etc tests can be deduced from the USB standards. Full USB compliance test equipment maybe expensive for those who are trading as a one-man business. There are some simple test rigs out there which require the use of a digital multimeter and test cables that may offer a lower cost solution worth investigating.

One such rig is USB Tester from Fried Circuits

Another rig from the same source is USB Tester and Phone Charging

Inexpensive rigs like these should not be a problem but it is essential to carefully document their use in your QA procedures and their requirement to be calibrated.

There are still numerous matters to discuss that have been identified regarding Metrology and USB, which shall be published shortly. The total sum of these discussion Parts build eventually to an identified set of criteria that examiners may wish to apply for QA purposes to reduce or remove the medium USB as having an adverse impact during data acquisition between a DUT and the ET.