As another alternative, you can choose stand-alone DIO modules that plug into a desktop or laptop computer's USB port. Such modules, from firms including Biodata, Intelligent Instrumentation, and IOtech, come with programs that add data-acquisition capabilities to their host computers. The modules also come with drivers that support test-related programming environments such as National Instruments' LabView.
Optical isolation rejects common-mode voltages between the computer and external environment.
Electromechanical relays provide isolation for output signals. Many products offer standard TTL levels, where an input or output of 0 to 0.8 VDC represents a logic low, while 2 to 5 VDC represents a logic high. Such isolation can improve noise immunity, and it is mandatory if common-mode voltages exist between your computer's ground and external equipment's ground. With such a scheme, it's incumbent on the host PC to determine when inputs are valid and when outputs are appropriate. Developed for the high-end VMEbus with its VXIbus instrument extensions, IP mezzanine modules can reside in PC environments when plugged into PCI or CompactPCI carrier boards.