What to consider when buying USB-based
digital I/O

Minature, bus-powered modules

USB Digital I/O Product Selection Made Easy

Tabletop/DIN rail
modules

USB Digital I/O Product Selection Made Easy

USB Digital I/O can be a powerful alternative to the acquisition and transmission of digital signals through PCI or ISA boards. There are a number of factors to weigh when considering this alternative:

  • How many digital bits do you need?
  • What kind of signals will you be acquiring or outputting?
  • What kind of connectors do you want/need?

How many bits do you need?

When selecting a USB Digital I/O product, you must first know how many lines or bits you will need in your application. Since many vendors sell digital I/O products with dedicated input and output lines, you should calculate not only the total number needed, but the number of digital inputs and digital outputs. Measurement Computing’s USB products all come with bi-directional DIO bits, meaning they can be configured by the user as either input or output.

Also important in your specification of a USB I/O product is the issue of expandability. If it is possible that the application’s digital I/O requirements could increase, some people prefer to specify more lines than they need to accommodate potential additional needs. This is a reasonable precaution; however, it does mean buying more DAQ than you actually need. An alternative, or complementary, approach is to select a USB solution that can be easily expanded, if the need arises. For example, many of Measurement Computing’s USB DIO products, such as the USB-PDISO8, a high voltage input/electromechanical relay device, come in aluminum cases that include a USB expansion port that allows multiple units to be daisy-chained together off of a single USB port on the host computer.

What kind of signals do you need?

In addition to selecting a USB product based on the number of lines required for your application, you also need to satisfy your signal type requirements. Will you be measuring and/or transmitting high voltage signals? Do you need standard current for inputs but high-drive current for outputs to control more power-hungry components?

USB I/O products that handle high voltage signals will need some form of signal conditioning. One of the advantages of some USB-based products is that they have integrated signal conditioning, which means that the conditioning function and the data acquisition function occur within a single device. The advantages of this design are that it often reduces the wiring/cabling requirements and eliminates the need for termination boxes. These advantages result in savings of time (through reduced wiring/cabling and increased ease-of-installation and –use) and money (through the elimination of some components). The Measurement Computing USB-ERB24, for example, is the USB-based alternative to a PCI-based system consisting of: a PCI-DIO24 board, a CIO-ERB24 board, a screw terminal enclosure, and cables. The USB-based solution reduces the number of components from four to one, is easier to install, and costs 37% less.

What kind of connectors do you need?

One of the advantages of USB devices over the equivalent PCI-based system is that you don't have to open up your computer to install the USB device. However, the USB device, like the PCI board, still obviously needs to be connected to the signal sources (e.g., sensors, probes). These connections can be achieved in a variety of ways, including screw terminals.

When specifying the best connection method for a USB I/O device, the following factors should be considered:

  • the environmental potential for physical disconnection through movement or physical accidents,
  • the planned lifespan of the application (long-term project vs. short-term test),
  • the desirability of cables,
  • the ease of installation

Most often, the USB digital I/O device will be used in an application or applications that call for one kind of connector. It is therefore preferable to select the best connector, and purchase the USB device with that connector. Purchasing a device with multiple connector types, while apparently offering flexibility, actually simply costs the buyer money. Why pay for connectors you will never use? Many of Measurement Computing’s popular I/O devices, such as the USB-1024LS, the USB-1024HLS, and the USB-DIO96H, are offered with screw terminals. The USB-DIO24/37, USB-DIO24H/37, and USB-DIO96H/50 are offered with standard connectors. You pay for only what you need.