accessing and communicating with instruments. We will present only the functions provided by the NI-VISA libraries that were used in building the Web service (Figure 12.41). A complete documentation of all the functions provided by the NI-VISA implementation can be found on the National Instruments Web site. A central concept in the NI-VISA library is the VISA Resource Manager. A powerful feature of VISA is the concept of a single interface for find- ing and accessing devices on various platforms. The VISA Resource Manager does this by exporting services for controlling and managing resources. Tele-Measurement Services for m-Learning CNI-488 C VISA INSTR LabVIEW NI-488 LabVIEW VISA Device Operation Device Function INSTR Operation Function ibdev viOpen <no equivalent> VISA Open ibonl viClose equivalent: VISA close ibwrt viWrite GPIB Write VISA Write viRead GPIB Read VISA Read Figure 12.41 Functions provided by the NI-VISA library. These services include, but are not limited to, assigning unique resource addresses and unique resource IDs, locating resources, and creating sessions. Each session contains all the information necessary to configure the communication channel with a device, as well as information about the device itself. [Quote from NI-VISA User Manual] When trying to access any of the VISA resources, the first step is to get a reference to the default Resource Manager by calling viOpenDefaultRM(). The application can then use the session returned from this call to open sessions to resources controlled by that Resource Manager, as shown in the following examples. 1. GetInstrumentList operation: GetInstrumentList operation will return a list of all available instruments. The returned list contains the VISA address of the instrument, formatted using the VISA standard notation for resource descrip- tor. The implementation of this service operation uses the functions provided Service Delivery Platforms by the NI-VISA library, to discover the available resources. As mentioned above, we should open the default