Keithley 6512 Instruction Manual page 136

IEEE-488 Bus Overview
There are two categories of controllers: system controller,
and basic controller. Both are able to control other instru-
ments, but only the system controller has the absolute au-
thority in the system. In a system with more that one
controller, only one controller may be active at any given
time. Certain protocol is used to pass control from one con-
troller to another.
The IEEE-488 bus is limited to 15 devices, including the
controller. 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 for obvious reasons.
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 pro-
grammed in from the front panel of the instrument. The ac-
tual listen address value sent out over the bus is obtained by
ORing the primary address with 20
mary address is 27 (1B ), the actual listen address is 3B
H
(3B = 1B + 20 ). In a similar manner, the talk address is
H H H
obtained by ORing the primary address value with 40
the present example, the talk address derived from a primary
address of 27 decimal would be 5B
The IEEE-488 standards also include another addressing
mode called secondary addressing. Secondary addresses lie
in the range of 60 -7F . Note, however, that many devices,
H H
including the Model 6512, do not use secondary addressing.
Once the device is properly addressed, appropriate bus trans-
actions are set to take place. 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 (usually the controller)
will then read this information.
A.3 IEEE-488 bus lines
The signal lines on the IEEE-488 bus are grouped into three
different categories: data lines, management lines, and hand-
shake lines. The data lines handle bus data and commands,
while the management and handshake lines ensure that prop-
er data transfer and bus operation takes place. Each bus line
is active low, with approximately zero volts representing a
logic 1 (true). The following paragraphs describe the purpose
of these lines, which are shown in Figure A-1.
A-2
. For example, if the pri-
H
H
. With
H
(5B = 1B + 40 ).
H H H H
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 DIO1-DIO8 instead of the
more common D0-D7. DIO1 is the least significant bit, while
DIO8 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.
Bus management lines
The five bus management lines help to ensure proper inter-
face control and management. These lines are used to send
the uniline commands that are described in paragraph B.4.
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
controls clearing of instruments (removing talkers and listen-
ers) from the bus.
REN (Remote Enable)—The REN line is used to place in-
struments on the bus in the remote mode.
EOI (End or Identify)—The EOI line is usually used to mark
the end of a multi-byte data transfer sequence.
SRQ (Service Request)—This line is used by devices when
they require service from the controller.
Handshake lines
The bus uses three handshake lines that operate in an inter-
locked sequence. This method ensures reliable data trans-
mission regardless of the transfer rate. Generally, data
transfer will occur at a rate determined by the slowest active
listener on the bus.
One of the three handshake lines is controlled by the source
(the talker sending information), while the remaining two
lines are controlled by accepting devices (the listener or listen-
ers receiving the information). The three handshake lines are:
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.