Keithley 708A, Инструкция по эксплуатации

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IEEE-488 Bus Overview
C-3
C.3 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
proper data transfer and operation take place. Each bus line
is active low, with approximately zero volts representing a
logic 1 (true). The following paragraphs describe the opera-
tion of these lines.
C.3.1 Data lines
The IEEE-488 bus uses eight data lines that transfer data one
byte at a time. DIO1 (Data Input/Output) through DIO8
(Data Input/Output) are the eight bi-directional data lines
used to transmit both data and multiline commands. The data
lines operate with low true logic.
C.3.2 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 state of the ATN line
determines how information on the data bus is to be
interpreted.
• IFC (Interface Clear) — 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.
• EOI (End or Identify) — The EOI line is used to mark
the end of a multi-byte data transfer sequence.
• SRQ (Service Request) — The SRQ line is used by
devices when they require service from the controller.
C.3.3 Handshake lines
The bus handshake lines operate in an interlocked sequence.
This method ensures reliable data transmission regardless of
the transfer rate. Generally, data transfer will occur at a rate
determined by the slowest active device 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(s)
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.
• NRFD (Not Ready For Data) — The acceptor controls
the state of NRFD. It is used to signal to the transmitting
device to hold off the byte transfer sequence until the
accepting device is ready.
• NDAC (Not Data Accepted) — NDAC is also con-
trolled by the accepting device. The state of NDAC tells
the source whether or not the device has accepted the
data byte.
The complete handshake sequence for one data byte is
shown in Figure C-2. Once data is placed on the data lines,
the source checks to see that NRFD is high, indicating that
all active devices are ready. At the same time, NDAC should
be low from the previous byte transfer. If these conditions are
not met, the source must wait until NDAC and NRFD have
the correct status. If the source is a controller, NRFD and
NDAC must be stable for at least 100ns after ATN is set true.
Because of the possibility of a bus hang up, many controllers
have time-out routines that display messages in case the
transfer sequence stops for any reason.
Once all NDAC and NRFD are properly set, the source sets
DAV low, indicating to accepting devices that the byte on the
data lines is now valid. NRFD will then go low, and NDAC
will go high once all devices have accepted the data. Each
device will release NDAC at its own rate, but NDAC will not
be released to go high until all devices have accepted the data
byte.
The previous sequence is used to transfer both data, talk, and
listen addresses, as well as multiline commands. The state of
the ATN line determines whether the data bus contains data,
addresses, or commands as described in the following
paragraphs.
DATA
DAV
SOURCE
SOURCE
VALID
NRFD
NDAC
ACCEPTOR
ACCEPTOR
ALL READY
ALL ACCEPTED
Figure C-2
IEEE-488 handshake sequence
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