Measurement-computing Serial488A Manual de usuario descargar pdf (Pagina 70)

5.11

5.8.1 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.

5.8.2 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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