Bus Lines; Data Lines - Keithley 6430 Instruction Manual

D-4 IEEE-488 Bus Overview
A device is placed in the talk or listen state by sending an appropriate talk or listen com-
mand. These talk and listen commands 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 programmed from the front panel of the instrument. The actual listen address value
sent out over the bus is obtained by ORing the primary address with #H20. For example, if the
primary address is #H16, the actual listen address is #H36 (#H36 = #H16 + #H20). In a similar
manner, the talk address is obtained by ORing the primary address with #H40. With the present
example, the talk address derived from a primary address of 16 decimal would be #H56
(#H56 = #H16 + #H40).
The IEEE-488 standards also include another addressing mode called secondary addressing.
Secondary addresses lie in the range of #H60-#H7F. Note, however, that many devices, includ-
ing the SourceMeter, 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 appropriate 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 handshake lines. The data lines handle bus data and commands, 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 representing a logic 1 (true). The fol-
lowing 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.