Bus Lines; Data Lines; Bus Management Lines; Handshake Lines - Keithley 2500 User Manual

Models 2500 and 2502 User's Manual

Bus lines

The signal lines on the IEEE-488 bus are grouped into three different categories: data
lines, management lines, and handshake lines. The data lines handle bus data and com-
mands, while the management and handshake lines ensure that proper data transfer and
operation takes place. Each bus line is active low, with approximately zero volts represent-
ing 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 (Data 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 interface control and management.
These lines are used to send the uniline commands.
ATN (Attention) — The ATN line is one of the more important management lines. 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 instru-
ments 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 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 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 informa-
tion), while the remaining two lines are controlled by accepting devices (the listener or lis-
teners 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.
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IEEE-488 Bus Overview D-5