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. Acromag is one firm making both DIO IP mezannine modules as well as PC-compatible carrier cards.