Measurement Computing Digital488 - Manual (Page 64)

Section 4 IEEE 488 Primer
4.4
4.3 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).
4.4 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).
4.4.1 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.