Keithley 7999-6, Instruction Manual

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3-8 Operation
Message exchange protocol
Two rules summarize the message exchange protocol:
Rule 1: You must always tell the relay unit what to send to the computer.
Perform the following two steps to send information from the relay switch to the
computer:
1. Send the appropriate query command(s) in a program message.
2. Address the relay switch to talk.
Rule 2: The computer must receive the complete response message before another pro-
gram message can be sent to the relay unit.
Status model
The relay unit provides status registers and queues allowing the operator to monitor and
manipulate the various instrument events. The status structure is shown in Figure 3-3. The heart
of the status structure is the status byte register. This register can be read by the user’s test pro-
gram to determine if a service request (SRQ) has occurred, and what event caused it.
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