Measurement Computing DAC488 v.1 - Manual (Page 45)

DAC488 User’s Manual IEEE 488 Background 39
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Serial Poll Disable ( SPD ): The command disables all devices from sending their Serial Poll status
byte.
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Serial Poll Enable ( SPE ): This command, when ATN is unasserted, will cause a device that is
addressed to Talk, to output its Serial Poll status byte.
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Take Control ( TCT ): This command passes bus control responsibilities from the current Controller to
another device which has the ability to control.
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Unlisten ( UNL ): This command places the device in the Listen Idle state.
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Untalk ( UNT ): This command places the device in the Talk Idle state.
More On Service Requests
Most of the commands covered, both uniline and multiline, are the responsibility of the Active Controller to
send and the bus devices to recognize. Most of these happen routinely by the interface and are totally
transparent to the system programmer. Other commands are used directly by the user to provide optimum
system control. Of the uniline commands,
SRQ is very important to the test system and the software
designer has easy access to this line by most devices. Service Request is the method by which a bus device
can signal to the Controller that an event has occurred. It is similar to an interrupt in a microprocessor
based system.
Most intelligent bus peripherals have the ability to assert SRQ . A digital multimeter (DMM) might assert it
when its measurement is complete, if its input is overloaded or for any of an assortment of reasons. A
power supply might
SRQ if its output has current limited. This is a powerful bus feature that removes the
burden from the System Controller to periodically inquire, "Are you done yet?". Instead, the Controller
says, "Do what I told you to do and let me know when you're done" or "Tell me when something is wrong."
Since SRQ is a single line command, there is no way for the Controller to determine which device requested
the service without additional information. This information is provided by the multiline commands for
Serial Poll and Parallel Poll.
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Serial Poll: Suppose the Controller receives a service request. For this example, let us assume there
are several devices which could assert
SRQ . The Controller issues a Serial Poll Enable ( SPE ) command
to each device sequentially. If any device responds with DIO7 asserted, it indicates to the Controller
that it was the device that asserted SRQ . Often times the other bits will indicate why the device wanted
service. This Serial Polling sequence, and any resulting action, is under control of the software
designer.
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Parallel Poll: The Parallel Poll is another way the Controller can determine which device requested
service. It provides the who, but not necessarily the why. When bus devices are configured for Parallel
Poll, they are assigned one bit on the data bus for their response. By using the Status bit, the logic level
of the response can be programmed to allow logical
OR/AND conditions on one data line by more than
one device. When SRQ is asserted, the Controller (under the user's software) conducts a Parallel Poll.
The Controller must then analyze the eight bits of data received to determine the source of the request.
Once the source is determined, a Serial Poll might be used to determine the why.
Of the two polling types, the Serial Poll is the most popular due to its ability to determine the who and why.
In addition, most devices support Serial Poll only.