Measurement-computing Isolator488 Bedienungsanleitung PDF herunterladen (Seite 32)

Section 3 IEEE 488 Primer

3-11

3.8.1 Serial Poll

Suppose the

Controller

receives a service request. For this

example, let's assume there are several devices which 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.

3.8.2 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.

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