Measurement-computing Digital HS Manuel d'utilisateur télécharger pdf (Page 50)

4.8.2 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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Measurement-computing Digital HS Manuel d'utilisateur Page 50