1-5 IEEE-488 Interface Bus Description General GPIB Information
1-8 PN: 10370-10374 Rev. F MG369xC GPIB PM
Data and Instrument Status Messages
These messages are sent by the device to the external controller via the GPIB. They contain measurement
results, instrument status, or data files that the device transmits over the data bus in response to specific
requests from the external controller. The contents of these messages are instrument specific and may be in
the form of ASCII strings or binary data.
In some cases data messages will be transmitted from the external controller to the device. For example,
messages to load calibration data.
An SRQ (service request) is an interface function message sent from the device to the external controller to
request service from the controller, usually due to some predetermined status condition or error. To send this
message, the device sets the SRQ line of the General Interface Management Bus true, then sends a status byte
on the data bus lines.
An SRQ interface function message is also sent by the device in response to a serial poll message from the
controller, or upon receiving an Output Status Byte(s) command from the controller. The protocols associated
with the SRQ functions are defined in the ANSI/IEEE Std 488.2 document.
The manner in which interface function messages and device-specific commands are invoked in programs is
implementation specific for the GPIB interface used with the external controller. Even though both message
types are represented by mnemonics, they are implemented and used in different ways.
Normally, the interface function messages are sent automatically by the GPIB driver software in response to
invocation of a software function. For example, to send the IFC (Interface Clear) interface function message,
one would call the ibsic function of the National Instruments software driver. On the other hand, the
command *RST (Reset) is sent in a command string to the addressed device. In the case of the National
Instruments example, this would be done by using the ibwrt function call.
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