VISA defines an architecture consisting of many resources that encapsulate device functionality. In VISA, every defined software module is a resource. In general, the term resource is synonymous with the word object in object-oriented architectures. For VISA, resource more specifically refers to a particular implementation or instance, in object-oriented terms, of a resource class, which is the definition for how to create a particular resource.
A specialized type of resource class is a VISA instrument control (INSTR) resource class , which defines how to control a particular device. An INSTR resource class encapsulates the various operations for a particular device together (reading, writing, triggering, and so on) so that a program can interact with that device through a single resource. TekVISA supports two kinds of devices associated with the INSTR resource class: GPIB and ASRL (serial) devices. Applications that use VISA can access device resources by opening sessions to them. A session is a communication path between a software element and a resource. Every session in VISA is unique and has its own life cycle. VISA defines a locking mechanism to restrict access to resources for special circumstances.
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