Fluke PM6681R, PM6685R How does Scpi Work in the Instrument?, Message Exchange Control protocol

Introduction to SCPI

How does SCPI Work in the Instrument?

The functions inside an instrument that control the operation provide SCPI com- patibility. Figure 3-3shows a simplified logical model of the message flow inside a SCPI instrument.

When the controller sends a message to a SCPI instrument, roughly the following happens:

–The GPIB controller addresses the instru- ment as listener.

–The GPIB interface function places the message in the Input Buffer.

–The Parser fetches the message from the Input Buffer, parses (decodes) the message, and checks for the correct syntax. The in- strument reports incorrect syntax by send- ing command errors via the status system to the controller. Moreover, the parser will detect if the controller requires a response. This is the case when the input message is a query (command with a “?” appended).

The Parser will transfer the executable messages to the Execution Control block in token form (internal codes). The Exe- cution Control block will gather the infor- mation required for a device action and will initiate the requested task at the ap- propriate time. The instrument reports ex- ecution errors via the status system over the GPIB and places them in the Error Queue.

–When the controller addresses the instru- ment as talker, the instrument takes data from the Output Queue and sends it over the GPIB to the controller.

Message Exchange Control protocol

Another important function is the Mes- sage Exchange Control, defined by IEEE 488.2. The Message Exchange Control protocol specifies the interactions between the several functional elements that exist between the GPIB functions and the device-specific functions, see Figure 3-3 .

The Message Exchange Control protocol specifies how the instrument and control- ler should exchange messages. For exam- ple, it specifies exactly how an instrument shall handle program and re- sponse messages that it receives from and returns to a controller.

This protocol introduces the idea of com- mands and queries; queries are program messages that require the device to send a response. When the controller does not read this response, the device will gener- ate a Query Error. On the other hand, commands will not cause the device to generate a response. When the controller tries to read a response anyway, the de- vice then generates a Query Error.

The Message Exchange Control protocol also deals with the order of execution of program messages. It defines how to re- spond if Command Errors, Query Errors, Execution Errors, and Device-Specific er- rors occur. The protocol demands that the instrument report any violation of the IEEE-488.2 rules to the controller, even when it is the controller that violates these rules.

The IEEE 488.2 standard defines a set of operational states and actions to imple- ment the message exchange protocol. These are shown in the following table:

3-4 How does SCPI Work in the Instrument?