Universal Serial Bus (USB) - Basic Technical Summary
USB Features
- True Plug & Play
- Hot swappable devices
- Ease of use
- Low cost
- Theoretical limit of 127 physical devices
Bandwidth
| Full speed: |
12 Mbps. (Shielded cable) |
| Low speed: |
1.5 Mbps. (Non-Shielded) |
Power Usage
| USB Device |
Power Requirements |
| Bus-Powered Hub |
Draw 100 mA (max) at power up & 500 mA normal operation. |
| Self-powered Hub |
Draw 100 mA (max). Be able to supply 500 mA to each port. |
| Low power, bus-powered function |
Draw 100 mA (max) |
| Self-powered function |
Draw 100 mA (max) |
| Suspended device |
Max 0.5 mA |
Operational Voltages
- Voltage supplied by a host or powered hub must be in the range of
4.75V - 5.25V.
- Max. voltage drop for bus-powered hub is 0.35V from its host or hub to
the downstream port(s)
- All USB devices must be able to send data at 4.4V, but only low
power functions need to be working at this voltage.
- Operational voltage for functions is 4.75V.
USB Motherboard Headers
Unfortunately, not all motherboards have the same USB pinouts on the header
connector. Motherboards can have USB headers with 4, 5, 8, 10 or 16 pins.
This can be confusing to the end user.
We are currently compiling a list of motherboards & the pinouts of the respective
USB header and would welcome any technical information which would help us
in this task, to
tech@starmount.co.uk
However, every functional USB port must have the following pins in the correct
order, regardless of the layout (or orientation) of the motherboard header :-
| Pin |
Name |
Description |
Cable Colour |
| 1 |
Vcc |
+5V DC |
Red |
| 2 |
D- |
Data - |
White |
| 3 |
D+ |
Data + |
Green |
| 4 |
Gnd |
Ground |
Black |
This is standard on all functional USB motherboards. On occasion, there is
a 5th pin. This 5th pin is used for Shielding purposes and is rarely used.
If used, Shield should be connected to Ground at the host only. No USB device
should connect Shield to Ground. The receptacle bracket provides additional
grounding through the computer case.
The orientation of each group of 4 pins is extremely important. Take particular
care to check the location of the Vcc (power) pins. On some motherboards they
are located side by side, and on others they are not.
The most common type of receptacle brackets (ROB) are :-
| ROB Type |
Description |
| 2 x 4 pin |
Two separate connectors, each with 4 pins. |
| 2 x 5 pin |
Two separate connectors, each with 5 pins. |
| 1 x 8 pin |
A single connector, with two rows of 4 pins. |
| 1 x 10pin |
A single connector, with two rows of 5 pins. |
USB Socket A Pinout
USB Cable Power Ratings & Length
Shielded: Data (28 AWG, twisted pair)
Shielded: Power (28 - 20 AWG)
Shielded: Data (28 AWG, non-twisted)
Shielded: Power (28 - 20 AWG)
| Gauge (AWG) |
Max. Length (m) |
| 28 |
0.81 |
| 26 |
01.31 |
| 24 |
2.08 |
| 22 |
3.33 |
| 20 |
3.00 |
