Tabor 8020 - IEEE-488 Bus Lines; Bus Managment Lines

Any given system can have only one controller
(control may be passed to an appropriate device
through a special command). Any number of talkers
or listeners may be present up to the hardware
constraints of the bus. The bus is limited to 15
devices, but this number may be reduced if higher
than normal data transfer rates are required or if
long interconnect cables are used.
Several devices may be commanded to listen at
once, but only one device may be a talker at any
given time. Otherwise, communications would be
scrambled much like an individual is trying to select
a single conversation out of a large crowd.
Before a device can talk or listen, it must be
appropriately addressed. Devices are selected on
the basis of their primary address. The addressed
device is sent a talk or listen command derived from
its primary address. Normally, each device on the
bus has a unique primary address so that each may
be addressed individually. The bus also has another
addressing mode called secondary addressing, but
not all devices use this addressing mode.
Once the device is addressed to talk or listen,
appropriate bus transactions may be initiated. For
example, if an instrument is addressed to talk, it will
usually place its data on the bus one byte at a
time. The listening device will then read this infor-
mation, and the appropriate software is then be used
to channel the information to the desired location.
4-3. IEEE-488 BUS LINES
The signal lines on the IEEE-488 bus are grouped
into three general categories. The data lines handle
bus information, while the handshake and bus man-
agement lines assure that proper data transfer and
bus operation takes place. Each of the bus lines is
“active low” so that approximately zero volts is a
logic “one”. The following paragraphs describe the
purpose of these lines, which are shown in Figure
4-1.
4-3-1. Bus Management Lines
The bus management group is made up of five
signal lines that provide orderly transfer of data.
These lines are used to send the uniline commands
described in paragraph 4-8-1.
1. ATN (Attention) - the ATN line is one of the
more important management lines. The state of the
ATN line determines whether controller information
on the data bus is to be considered data or a
multiline command as described in paragraph 4-8-2.
Figure 4-1. IEEE Bus Configuration
IEEE-488.2 Operation Model 8020
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