Agilent Technologies 8753ET Uživatelská příručka stáhnout pdf (Strana 319)

Chapter 3 3-9

GPIB Programming

GPIB Operation

Data Bus

The data bus consists of 8 bidirectional lines that are used to transfer data from one device

to another. Programming commands and data transmitted on these lines are typically

encoded in ASCII, although binary encoding is often used to speed up the transfer of large

arrays. Both ASCII and binary data formats are available to the analyzer. In addition,

every byte transferred over GPIB undergoes a handshake to insure valid data.

Handshake Lines

A three-line handshake scheme coordinates the transfer of data between talkers and

listeners. To ensure data integrity in multiple-listener transfers, this technique forces data

transfers to occur at the transfer rate of the slowest device connected to the interface. With

most computing controllers and instruments, the handshake is performed automatically,

making it transparent to the programmer.

Control Lines

The data bus also has five control lines. The controller uses these lines to address devices

and to send bus commands.

IFC (Interface Clear) This line is used exclusively by the system controller. When this

line is true (low), all devices (whether addressed or not) unaddress and revert to an idle

state.

ATN (Attention) The active controller uses this line to define whether the information on

the data bus is command-oriented or data-oriented. When this line is true (low), the bus is

in the command mode, and the data lines carry bus commands. When this line is false

(high), the bus is in the data mode, and the data lines carry device-dependent instructions

or data.

SRQ (Service Request) This line is set true (low) when a device requests service and the

active controller services the requesting device. The network analyzer can be enabled to

pull the SRQ line for a variety of reasons such as requesting control of the interface, for the

purposes of printing, plotting, or accessing a disk.

REN (Remote Enable) This line is used exclusively by the system controller. When this

line is set true (low), the bus is in the remote mode, and devices are addressed by the

controller to either listen or talk. When the bus is in remote mode and a device is

addressed, it receives instructions from the system controller via GPIB rather than from

its front panel (pressing returns the device to front-panel operation). When this

line is set false (high), the bus and all of the connected devices return to local operation.

EOI (End or Identify) This line is used by a talker to indicate the last data byte in a

multiple-byte transmission, or by an active controller to initiate a parallel-poll sequence.

The analyzer recognizes the EOI line as a terminator, and it pulls the EOI line with the

last byte of a message output (data, markers, plots, prints, error messages). The analyzer

does not respond to parallel poll.

Local

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Agilent Technologies 8753ET Uživatelská příručka Strana 319