- an overview about the use of USB data acquisition systems, how USB can
be used for data acquisition, its advantages and limitations.
The USB (Universal Serial Bus) provides an ideal method
of providing connectivity between a computer and remote sensors for data
acquisition. As a result USB data acquisition modules and devices are now
widespread on the market. The use of USB has grown in all sectors of the
computer market as a result of its convenience. In line with this the data
acquisition sector has also used the interface for small and cost effective
devices.
USB data acquisition advantages
Not only has USB proved itself an ideal platform for many mainstream
computer applications, but it also offers many advantages for data
acquisition systems. The advantages include aspects including:
- USB allows much faster speeds than RS232
- The power for the "sensor" or data acquisition module can be
obtained from the computer, simplifying many systems, and especially any
portable USB data acquisition systems.
- USB data acquisition modules can be connected and disconnected
without the need to power down the computer
- USB ports are standard on most PCs these days making it an almost
universally available method of connection.
- It is possible to use USB as well as other data acquisition
communication standards together
- It is possible to expand the connectivity using a USB hub so that
several USB data acquisition devices can be connected
USB data acquisition products
There is a considerable number of different USB data acquisition products
that are available. Although dedicated measurement modules may be available,
other items may be capable of taking in a variety of analogue and digital
inputs. These items are particularly useful in applications where a number
of points and sensors need to be monitored. Also, having a multiple input
USB data acquisition module means that only one module may be required.
These modules may also come with their own data acquisition software.
A further advantage of this form of data acquisition
module is that only one module may be required. If several modules are
needed, then ultimately the power supply on the computer may become
overloaded.
Cable for USB data acquisition
While many USB devices may plug directly in to the host computer it is
possible to use an extender cable. The maximum allowable length for an
individual cable is 5 metres (3 metres for slow devices) and this allows the
USB data acquisition module to be located remotely from the computer.
A USB cable has two forms of connector. These are
designated the "A" and "B" connectors. The connections to the connectors are
given below:
| Pin |
Function |
| 1 |
Vbus
4.75 - 5.25 V |
| 2 |
Data - |
| 3 |
Data + |
| 4 |
Ground |
| Shell |
Screen |
USB cable pin assignments
Summary
USB data acquisition cards provide a very attractive solution to many data
acquisition applications. Often compact modules are available and these can
be connected to virtually any PC very easily. Not only is the connection
simple, but a USB data acquisition system can be extended relatively easily
by using a USB hub to enable further units to be connected.
