Bus Lines - Keithley 6517A User Manual

IEEE-488 Bus Overview
TO OTHER DEVICES
DEVICE 1
ABLE TO
TALK, LISTEN
AND CONTROL
(COMPUTER)
DEVICE 2
ABLE TO
TALK AND
LISTEN
6517A
DEVICE 3
ONLY ABLE
TO LISTEN
(PRINTER)
DEVICE 4
ONLY ABLE
TO TALK
Figure D-1
IEEE-488 bus configuration
.
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, 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 31,
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. For example, if the pri-
D-2
DATA BUS
DATA BYTE
TRANSFER
CONTROL
GENERAL
INTERFACE
MANAGEMENT
D IO1 ... 8 DATA
(8 LINES)
DAV
NRFD
HANDSHAKE
NDAC
IFC
ATN
SRQ
BUS
REN
MANAGEMENT
EOI
mary address is $27, the actual listen address is $47 ($47 =
$27 + $20). In a similar manner, the talk address is obtained
by ORing the primary address with $40. With the present ex-
ample, the talk address derived from a primary address of 27
decimal would be $67 ($67 = $27 + $40).
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,
including the Model 6517A, do not use secondary addressing.
Once a device is addressed to talk or listen, the appropriate
bus transactions take place. For example: if the instrument is
addressed to talk, it places its data string on the bus one byte
at a time. The controller reads the information and the appro-
priate software can be used to direct the information to the
desired location.

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 operation takes place. Each bus line is ac-
tive low, with approximately zero volts representing a logic
1 (true). The following paragraphs describe the operation of
these lines.
Data lines
The IEEE-488 bus uses eight data lines that transfer data one
byte at a time. DIO1 (Data Input/Output) through DIO8 (Da-
ta Input/Output) are the eight data lines used to transmit both
data and multiline commands and are bidirectional. The data
lines operate with low true logic.
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.
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 from the bus.
REN (Remote Enable) —The REN line is used to place the
instrument on the bus in the remote mode.