87
CHAPTER 8: IEEE 488 Primer
8.3 Send It To My Address
In the previous discussion, the terms “Addressed to Talk” and “Addressed to Listen” were used. These terms
require some clarification.
The IEEE 488 standard permits up to 15 devices to be configured within one system. Each of these devices
must have a unique address to avoid confusion. In a similar fashion, every building in town has a unique
address to prevent one home from receiving another home’s mail. Exactly how each device’s address is set is
specific to the device’s manufacturer. Some are set by DIP switches in hardware, others by software. Consult
the manufacturer’s instructions to determine how to set the address.
Addresses are sent with universal (multiline) commands from the Active Controller. These commands
include My Listen Address (MLA), My Talk Address (MTA), Talk Address Group (TAG), and Listen
Address Group (LAG).
8.4 Bus Management Lines
Five hardware lines on the GPIB are used for bus management. Signals on these lines are often referred
to as uniline (single line) commands. The signals are “active low”: a low voltage represents a logical “1”
(asserted), and a high voltage represents a logical “0” (unasserted).
8.4.1 A
TTENTION
(ATN)
ATN 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.
8.4.2 I
NTERFACE
C
LEAR
(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).
8.4.3 R
EMOTE
E
NABLE
(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.
8.4.4 E
ND OR
I
DENTIFY
(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, as the end of the
data may be indicated by some special character such as carriage return.
The Active Controller also uses EOI to perform a Parallel Poll by simultaneously asserting EOI and ATN.
8.4.5 S
ERVICE
R
EQUEST
(SRQ)
When a device desires the immediate attention of the Active Controller, it asserts SRQ. It is then the
Controller’s responsibility to determine which device requested service. This is accomplished with a
Serial Poll or a Parallel Poll.
