Measurement Computing Digital488/80A - Manual (Page 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