There are two categories of controllers: system controller,
and basic controller. Both are able to control other in-
struments, but only the system controller has the absolute
authority in the system. In a system with more that one con-
troller, only one controller may be active at any given time.
Certain protocol is used to pass control from one controller to
another.
The IEEE-488 bus is limited to 15 devices, including the con-
troller. Thus, any number of talkers and listeners up to that
limit may be present on the bus at one time. Although several
devices may be commanded to listen simultaneously, the bus
can have only one active talker, or communications would be
scrambled.
A device is placed in the talk or listen state by sending an ap-
propriate talk or listen command. These talk and listen com-
mands are derived from an instrument’s primary address. The
primary address may have any value between 0 and 30, and is
generally set by rear panel DIP switches or programmed in
from the front panel of the instrument. The actual listen ad-
dress .value sent out over the bus is obtained by ORing the
primary address with $20. For example, if the primary ad-
dress is 27 ($lB), the actual listen address is $3B ($38 = $lB
+ $20). In a similar manner, the talk address is obtained bv
3.3.1 Data Lines
The IEEE-488 bus uses eight data lines that allow data to be
transmitted and received in a bit-parallel, byte serial manner.
These lines use the convention DIOl-DIOS instead of the
more common DO-D7. DIOl is the least significant bit, while
DIOS is the most significant bit. The data lines are bidirec-
tional (with most devices), and, as with the remaining lines,
low is considered to be true.
3.3.2 Bus Management Lines
The five bus management lines help to ensure proper interface
control and management. These lines are used to send the
uniline commands that are described in paragraph. 3.4.1.
ATN (Attention)-The ATN line is one of the more impor-
tant management lines in that the state of this line determines
how information on the data bus is to be interpreted.
IFC (Interface Clear&As the name implies, the IFC line con-
trols clearing of instruments from the bus.
OR& the primary address value with $40. With the present
example, the talk address derived from a primary address of
REN (Remote Enable&The REN line is used to place instru-
27 decimal would be $5B ($SB = $lB -t $40)
ment on the bus in the remote mode.
EOI (End or Identify&The EOI line is usually used to mark
The IEEE-488 standards also include another addressing mode
the end of a multi-byte data transfer sequence.
called secondary addressing. Secondary addresses lie in the
range of $60-$7F. Note, however, that many devices do not
SRQ (Service Request)-This line is used by devices when
use secondary addressing.
they require service from the controller.
Once the device is properly addrwed, appropriate bus tran-
3.3.3 Handshake Lines
sactions are set to take place. For example, if an instrument is
addressed to talk, it will usually place its data byte on the bus
one byte at a time. The listening device (frequently the con-
The bus uses handshake lines that operate in an interlocked
x*e lines that operate in an mterlockza
troller) will then read this information.
sequence. This method ensures reliable data transmission ms method ensures reliable data transmission
..6YA....00 ,f the transfer rate. Generally, data transfer will regardless of the transfer rate. Generally, data transfer will
occur at a rate determined by the slowest active device on the occur at a rate determined by the slowest active device on the
3.3 IEEE-488 BUS LINES
bus
The signal lines on the IEEE-488 bus are grouped into three
One of the three handshake lines is controlled by the source
different categories: data lines, management lines, and hand-
(the talker sending information), while the remaining two
shake lines. The data lines handle bus data and commands,
lines are controlled by accepting devices (the listener or
while the management and handshake lines ensure that
listeners ret-‘..‘-- ’ S~~UZS rhe information
0. The three handshake
proper data transfer and bus operation takes place. Each bus
lines are:
iine.is active low, with approximately zero v&s representing
a logic 1 (true). The following paragraphs describe the pur-
pose of these lines, which are shown in Figure 3-l.
DAV (Data Valid&The source controls the state of the DAV
line to indicate to any listening devices whether or not data
bus information is valid.
3-2
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