Comparison Between NI-VISA and NI-488.2 APIs

For GPIB users who are familiar with NI-488.2, the following table shows several common, but not all, NI-488.2 device-level function calls and the corresponding VISA operations. As you can see, the APIs are almost identical. The difference is that VISA is extensible to additional hardware interfaces. Therefore, if you are programming multiple devices that communicate over more than one bus type, it might be easier to use VISA for your entire system.

NI-VISA and NI-488.2 Functions and Operations

C NI-488.2 Device Function	C VISA INSTR Operation	LabVIEW NI-488.2 Device Function	LabVIEW VISA INSTR Operation
ibdev	viOpen	<no equivalent>	VISA Open
ibonl	viClose	<no equivalent>	VISA Close
ibwrt	viWrite	GPIB Write	VISA Write
ibrd	viRead	GPIB Read	VISA Read
ibclr	viClear	GPIB Clear	VISA Clear
ibtrg	viAssertTrigger	GPIB Trigger	VISA Assert Trigger
ibrsp	viReadSTB	GPIB Serial Poll	VISA Read STB
ibwait	viWaitOnEvent	Wait for GPIB RQS	Wait for RQS
ibconfig	viSetAttribute	GPIB Initialization	VISA Property Node

One difference in the event mechanism between NI-488.2 and VISA is worth noting. In VISA, you must always call viEnableEvent() prior to being allowed to receive events. While this was not the case with NI-488.2, this is required in VISA to avoid the race condition of trying to wait on events for which the hardware may not be enabled. Thus, you should enable the session for events not just immediately before calling viWaitOnEvent(), but before the device has even been triggered or configured to generate a service request event.