Measurement-computing Digital488/80A Instrukcja Użytkownika Pobierz pdf (Strona 42)

The IEEE 488 interface transfers information in a similar way, with a method of data transfer called

handshaking. For data transfer on the IEEE 488 interface, the Active Controller must:

• Unlisten all devices to protect against eavesdroppers.

• Designate a device who will talk by addressing it to Talk.

• Designate all the devices who will listen by addressing them to Listen.

• Indicate to all devices that the data transfer can take place.

Bus Lines & Bus Commands

Bus Management Lines

For the general control and coordination of bus activities, five hardware lines on the IEEE 488 interface are

used for bus management. Signals on these lines are often referred to as uniline (single line) commands.

The signals are active low (i.e., a low voltage represents an asserted logic of “1”, and a high voltage

represents an unasserted logic of “0”).

• Interface Clear (

IFC): The IFC line is used only by the System Controller. It is used to place all bus

devices in a known state. Although device configurations vary, the

IFC command usually places the

devices in the Talk and Listen Idle states (neither Active Talker nor Active Listener).

• Remote Enable (

REN): When the System Controller sends the REN command, bus devices will

respond to remote operation. Generally, the

REN command should be issued before any bus

programming is attempted. Only the System Controller has control of the Remote Enable line.

• Attention (

ATN): The ATN line is one of the most important lines for bus management. If Attention is

asserted, then the information contained on the data lines is to be interpreted as a multiline command.

If it is not, then that information is to be interpreted as data for the Active Listeners. The Active

Controller is the only bus device that has control of this line.

• End Or Identify (

EOI): The EOI line is used to signal the last byte of a multibyte data transfer. The

device that is sending the data asserts

EOI during the transfer of the last data byte. The EOI signal is

not always necessary, for the end of the data may be indicated by some special character such as

carriage return (

CR) and/or line feed (LF). The Active Controller also uses EOI to perform a Parallel

Poll by simultaneously asserting

EOI and ATN.

• Service Request (

SRQ): When a device desires the immediate attention of the Active Controller, it

asserts

SRQ. It is then the responsibility of the controller to determine which device requested service.

This is accomplished with a Serial Poll or a Parallel Poll.

36 IEEE 488 Background Digital488/80A User's Manual

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Measurement-computing Digital488/80A Instrukcja Użytkownika Strona 42