Standard Commands for Programmable Instruments

The Standard Commands for Programmable Instruments (SCPI; often pronounced 'skippy') defines a standard for syntax and commands to use in controlling programmable test and measurement devices, such as automatic test equipment and electronic test equipment. The Standard Commands for Programmable Instruments (SCPI; often pronounced 'skippy') defines a standard for syntax and commands to use in controlling programmable test and measurement devices, such as automatic test equipment and electronic test equipment. SCPI was defined as an additional layer on top of the IEEE 488.2-1987 specification 'Standard Codes, Formats, Protocols, and Common Commands'. The standard specifies a common syntax, command structure, and data formats, to be used with all instruments. It introduced generic commands (such as CONFigure and MEASure) that could be used with any instrument. These commands are grouped into subsystems. SCPI also defines several classes of instruments. For example, any controllable power supply would implement the same DCPSUPPLY base functionality class. Instrument classes specify which subsystems they implement, as well as any instrument-specific features. The physical hardware communications link is not defined by SCPI. While it was originally created for the IEEE-488.1 (GPIB) bus, SCPI can also be used with RS-232, RS-422, Ethernet, USB, VXIbus, HiSLIP, etc. SCPI commands are ASCII textual strings, which are sent to the instrument over the physical layer (e.g., IEEE-488.1). Commands are a series of one or more keywords, many of which take parameters. In the specification, keywords are written CONFigure: The entire keyword can be used, or it can be abbreviated to just the uppercase portion. Responses to query commands are typically ASCII strings. However, for bulk data, binary formats can be used. The SCPI specification consists of four volumes: Volume 1: 'Syntax and Style', Volume 2: 'Command Reference', Volume 3: 'Data Interchange Format', Volume 4: 'Instrument Classes'. The specification was originally released as non-free printed manuals, then later as a free PDF file. First released in 1990, SCPI originated as an additional layer for IEEE-488. IEEE-488.1 specified the physical and electrical bus, and IEEE-488.2 specified protocol and data format, but neither specified instrument commands. Different manufacturers, and even different models, of the same type of instrument would use different command sets. SCPI created a standard which could be common across all manufacturers and models. It requires use of the IEEE-488.2 data formats, but does not mandate the IEEE-488.1 bus. In 2002-2003, the SCPI Consortium voted to become part of the IVI Foundation (Interchangeable Virtual Instruments). In 1987, IEEE introduced IEEE 488.2-1987 specification 'Standard Codes, Formats, Protocols, and Common Commands', it was later revised in 1992 as IEEE 488.2-1992. While IEEE 488.2 provided a device-independent syntax, there was still no standard for instrument-specific commands. Commands to control the same class of instrument, e.g., multimeters, would vary between manufacturers and even models. The United States Air Force, and later Hewlett-Packard, recognized this problem. In 1989, HP developed their TML language which was the forerunner to SCPI.