Measurement Computing Isolator488 - Manual (Page 25)

Section 3 IEEE 488 Primer
3-4
3.3 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 product'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).
3.4 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, i.e. a low voltage represents a logic
"1" (asserted), and a high voltage represents a logic "0" (unasserted).
3.4.1 Attention (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.