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• How_To
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• Connect USB Headers
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Connect USB Headers

USB Speed Considerations - Don't confuse Full (1.1) and High (Hi/2.0)

• There are three different specifications. Low, Full and High speed, which can allow transmission of data at rates of up to 1.5, 12 and 480 Mbits/sec (or 0.187, 1.5 and 60 Mbytes/sec respectively). In practice the rates achieved are likely to be some 50% of these maxima. High and Full Speeds can only be attained if special attention is given to screening and of having the data wires as a "twisted pair". Some specifics are detailed later in this article under "Shielding Specifications and Notes".

Commonly Used Abbreviations and Standard* Wiring Colours

• VCC = SVCC = VBUS = PWR = USBPWR = POWER = +5V (RED or ORANGE)
  
• -D = SBD- = DATA- (WHITE or YELLOW)
  
• +D = SBD+ = DATA+ (GREEN or GREY)
  
• GND = GRND = GROUND (BLACK or BLUE or BROWN)
  
• NC =  Not Connected
  
• OC = USBOC = Over Current Signal
  
• Key = Empty = No Pin
• Drain = Shield = Shell = Supplementary Ground (usually also BLACK).
  

* We have called the colours "Standard Colours". The USB specification never mentions any specific colouring stating no more than that it is "Vendor Specific". However throughout the specification the authors always use (both in text and actual colour diagrams) RED, WHITE, GREEN AND BLACK, just as outlined above, in all diagrams and descriptions of the power, data and ground wires. Bear in mind too that wiring can be wired incorrectly and that vendors have carte blanche as to what they do decide to use.

Low Speed and Early Header Arrangements

• Low speed is all that is required for HID (Human Interface Device) equipment such as Mice and Keyboards. These were the first USB devices to come on stream and there was even native support for them under Win98SE. In the beginning just four unshielded/untwisted wires were connected from each port to four specific pins on a motherboard header.
• Such unshielded/untwisted 4-wire cables are most commonly identified by being Red (5V Power), White (Data-),  Green (Data+) and Black (Ground) and only support low speeds. At the other end (the case end) these four wires most commonly attach directly through to the four connectors inside series A (Fig 1) or series B receptacles.  Just four (1,2,3 and 5) of the five connections inside a mini-USB series receptacle would be equivalent.
  
• Motherboard headers "typically" have two rows of up to five pins for connecting to two individual ports. Some have them in one long row, some have them in groups of four-pins or even just one four-pin header on its own. Pin 1 is usually the bottom leftmost (or top rightmost). But they are even described as being reversed with Pin 2 being the bottom leftmost (or top rightmost). In our experience this latter orientation is more normal for Firewire headers, but it only exemplifies the importance of looking carefully at your motherboard's specific details. Pin 9 (on both Firewire and USB ports "usually" has an empty/absent pin.
• Take particular care when attaching any wires to these header pins since it is easy to put individual wires onto the wrong pins or a block of 10 over just 8 of the 10 pins, etc, etc. Only attach them with the PC powered down, the power disconnected and then having pressed the power-on switch.
  
• Each complete group of four cable wires can be connected to an equivalent group of four pins - but this is where the first care really needs to be taken. Although 90% of boards use the Intel layout where pins 1, 3, 5, and 7 are equivalent to pins 2, 4, 6 and 8  and would connect to red, white, green and black low speed cable respectively - on some boards (notably Gigabyte boards) they use a sort of flipped configuration with pins 1, 3, 5 and 9 being equivalent to its pins 10, 8, 6 and 2 yet these would connect to the same red, white, green and black wires respectively. BUT CHECK YOUR MOBO'S MANUAL. A few examples follow and the blanked black cells are NC (Not Connected) to any wire. Power and Ground pins are enumerated with Red and Black letters and Data Minus and  Data Plus with White and Green letters respectively. A wide range of variations exists but the following three are perhaps the most common ones seen for early hardware designs.
  

  • 2	4	6	8	10
    1	3	5	7	9

    Early "Intel type"

  • 2	4	6	8	10
    1	3	5	7	9

    Early One-Port "Gigabyte type" (eg GA-6WMM)
    

  • 2	4	6	8	10
    1	3	5	7	9

    Early Two-Port "Gigabyte type"

  • 2	4	6	8	10
    1	3	5	7	9

    Another reported variation

Full-Speed and Hi-Speed

• Adequate shielding of cables and the use of a "twisted pair" of data cables for each port is required to allow these faster USB data communications. It is not mandatory (as some believe) to run a shielding drain wire to a specific motherboard header pin to achieve this. Full Speed and High Speed can both function quite adequately with just one (for a single port) or two (for a pair of ports) 4-wire assemblies. Modern systems (both cases and motherboards) now usually have an additional wire and this wire and the extra (or ninth) pin it may attach to on the header pin-outs is probably the least understood and the most common cause of confusion over just how to connect a front panel to a USB header.
• If in any doubt then just use the four fundamental wires from each port and attach them to their corresponding four header pins - just as above. It is usually obvious as to the correct group of four wires but, with no apologies for repetition, do check your motherboard's manual to confirm the correct pin layouts and the pin-out designations.

The Extra Pin

• The extra pin often now found (in 9/10-pin or 5-pin or 2x5-pin layouts) - is MOST COMMONLY an extra ground and can usually be ignored - especially if designated as NC. Don't use it if unsure, as if the input should happen to come directly from any downstream overcurrent (for example an abnormal current from a defective powered peripheral or cable), it would be best to not short it directly to ground by this route. There are reports of USB connections become hot and melting plastic so if anything like that is happening we suggest you just use the four mandatory wires per port.
  
• If your USB2 HiSpeed is not up to the mark, read and re-read the description of the wires from the ports and the details of the motherboard pin-outs. If the motherboard ships with its own back-brackets and matching composite 9/10-pin plugs then they should be quite safe to attach to the motherboard headers.
  
• Over-Current (OC/USBOC) commonly gets described as Open Connection - and very often intrepreted as meaning the same as NC or No Connection or Normally Closed. We treat such comments (and indeed all abbreviations) with suspicion and note that Intel recommends the following advice (our own comments are in italics within brackets) to motherboard manufacturers:- "It is recommended that the over current protection device for the front panel header be included on the motherboard to prevent damage to the motherboard. This should be implemented only if the front panel solution does not include a connector card (daughterboard) that includes over current protection. If a specific front panel cable solution is not provided (for example by attaching directly to a USB device), it is safer to assume the front panel solution won’t provide over current protection. With this being the case, include the over current protection on the board and clearly document that over current protection is provided on the board. Be sure to place the over current protection device downstream of the bypass bulk storage capacitor so not to cause droop failure (which relates to two adjacent ports affecting each other's effectiveness)". Note that any overcurrent from downstream is a different issue than mis-connecting wires to the header, which can cause damage even if there is an on-board fuse-element for that header block.
  
• On any front-panel daughterboards that we have examined the normal ground wires and any shielding drain wire have been connected either directly or quite simply to one another via a resistor. Once sure of this arrangement we have never had any worries about attaching the 9th wire to pin10 on a "Late Intel Type" header as below. What you decide to do is up to you.
  

• If the 9th wire from a two-port header block is a drain/shell/shield wire and the motherboard header's has a drain pin (usually pin10) running to ground then these two can certainly be connected to one another. This has now become the most standard layout and even the Gigabyte boards now seem to be adopting this with this 9th wire running to pin10 on the header.

  • 2	4	6	8	10
    1	3	5	7	9

    Late "Intel type" (now the most common arrangement seen). Pin 9 is usually removed from the header which, then being empty, means that its equivalent wire socket can be filled with plastic to minimize the chances of mixing-up USB and Firewire connections since the nine available slots can then only fit over the nine equivalent remaining pins. Pin 10 is normally a supplementary ground but also may be designated as NC.

  • 2	4	6	8	10
    1	3	5	7	9

    The same Late "Intel type" but using a different combination of "standard" wiring colors.
    

• If pin10 is designated as OC then it is a specific ground that can be used detect a specialised low logic signal and thus be capable of indicating that an overcurrent situation exists. We have yet to come across equipment that provides such a signal so unless you have a wire from a panel or other device that specifies that it can carry an OC signal then it is probably a redundant and unnecessary feature. If, having attached 9 wires from a front-panel, the port doesn't function try removing or cutting the "ninth wire". The metal sleeve inside each slot will usually pull straight out if its little plastic flange is teased slightly open with a large needle or the point of a small knife.
  

Testing/Checking the Designated Front Panel Wires

• If you are seriously worried (because you doubt the color coding or for any other reason) about which wire goes to which USB contact then an ohm-meter/multi-meter can be used to check things out. If you don't have a meter you can still check which wire is which using a battery in series with a bulb in series with the wire in question. This can save having to dismantle the front panel to follow the wires to their destinations. Looking-into a TypeA Port Receptacle with the narrow part at the top the contacts from left to right would be 1, 2, 3 and 4 respectively and with any shielding running to the metal rectangular surround:-
  
  • Fig 1

Overcurrent Review

• The PCB Daughterboard or Connector-Card on which the Front-Panel ports are placed may or may not contain overcurrent protection.

• Intel recommends to motherboard manufacturers that the VCC to Vbus (the 5V-Main to 5V-Front-Panel-Header) circuit(s) should contain an on-board Fuse-Unit Thermistor (1.5A for two ports) for the greatest protection against a Vbus-to-Ground short as the cause of an overcurrent draw. This is recommended even though it is unlikely to be necessary if, even an unfused, front-panel daughterboard uses another overcurrent protection scheme. An on-board thermistor would however protect the motherboard from a damaged header-to-front-panel cable or assembly.
• Not, by any means, is every Pin10 demarcated for OC (Over-Current) detection and an OC signal wire should only be attached to this pin if it is demarcated as an OC solution from the front panel daughterboard. Both the normal Ground wires and any OC wire are designed to run to ground but they are separated downstream by electronics that allow a logic signal to be picked up on the OC pin and thus allow the overcurrent to be detected and any on-board protection instituted or warnings given.
• If Pin10 is demarcated as NC (No Connection) or if it is not clear-cut that the motherboard can set in place OC protection then no wires should be connected to this pin-out and making any such connection can result in failed USB port access. In such circumstances any such block should have the  inappropriate OC wire cut or removed fromt the block and only 8 pins (for a two port assembly) utilised.

Shielding Specifications and Notes

• Both High and Full-speed cables consist of one 28 to 20 AWG non-twisted power pair and one 28 AWG twisted data pair with an aluminum metallized polyester inner shield, 28 AWG stranded tinned copper drain wire, > 65% tinned copper wire interwoven (braided) outer shield, and PVC outer jacket. The interwoven (braided) tinned copper wire outer shield must encase the aluminum metallized PET shielded power and signal pairs and must be in direct contact with the drain wire.
• The minimum marking information for High and Full-speed cables must include: "USB SHIELDED <Gauge/2C + Gauge/2C> UL CM 75".
• The shield's drain wire should be terminated to the receptacle-casing and this in turn grounded by one of a number of schemes. Peripherally connected USB devices should not be grounded separately. The grounding scheme for USB devices, and cables must be consistent with accepted industry practices and regulatory agency standards for safety and EMI/ESD/RFI.
• Many front panel daughterboards ground the shielding drain wire via "the ninth wire" to its relevant motherboard header-pin but others, particularly back-panel brackets, can discharge to ground via other routes that attach to the case. In some instances the grounding may have an inductor or capacitor interposed to dampen RF interference.
• The fact that both overcurrent detection and grounding of the drain wire can use a common pathway is just one other cause of confusion of just what the function of the specialised 10th header-pin entails. So yet another nomenclature for this pin is to sometimes call it a shield or drain pin. As should have been seen already, it is not mandatory to use this pin or this routhe to ground the shielding; shielding that is a requirement for the support of USB High and Full speed data transmission.
  

References

1. USB 2.0 Specification
2. Power Delivery Design Issues for Hi-Speed USB on Motherboards ("Intel Whitepaper" in pdf format)
3. High Speed USB Platform Design Guidelines ("Intel Guidelines" in pdf format)
4. Chapter 6 of the USB 2.0 Specification: Mechanical ("The mechanical and electrical specifications for the cables, connectors, and cable assemblies used to interconnect USB devices" in pdf format)

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