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4-6, IEEE 488 Primer
11-20-01
Digital488 User’s Manual
Parallel Poll Configure (PPC)
This configures devices capable of performing a Parallel Poll as to which data bit they are to assert in
response to a Parallel Poll . ($05)
Parallel Poll Unconfigure (PPU)
This disables all devices from responding to a Parallel Poll . ($15)
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 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.
Serial Poll
Suppose the Controller receives a service request. For this example, let's assume there are several devices
that could assert SRQ . The Controller issues an SPE (Serial Poll enable) 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.
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 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.