A-2
A
A1
IEEE 488 BUS DESCRIPTION (IEEE 488.1, IEEE
488.2, SCPI)
The IEEE Std 488 Bus is a convenient means of connecting instruments and
computers together to form a test system or to transfer data between two
computers. The IEEE Std 488.1 covers the electrical and mechanical bus
specifications and the state diagrams for each bus function. The IEEE Std
488.2 expanded on the original specification and established data formats,
common commands for each 488.2 device and controller protocols. The
SCPI standard developed a tree like series of standard commands for
programmable instruments so that similar instruments by different
manufacturers can be controlled by the same program.
The 488-PC2 card provides an IEEE 488 Interface for any IBM PC
computer or compatible ISA bus clone. When used as a bus controller, the
488-PC2 drivers, driver libraries and windows DDL make the 488-PC2
operate as a 488.2 compatible controller. The controller protocols are built
in to the PC2 drivers. All IEEE 488.2 common commands, queries and
SCPI commands are placed in the output command string by the user and
are not part of the PC2 driver software.
A1.1
IEEE 488.1 Bus
The IEEE Std 488 Bus, or GPIB as it is commonly referred to, provides a
means of transferring data and commands between devices. The physical
portion of the bus is governed by IEEE -Std 488.1 - 1978. The interface
functions for each device are contained within that device itself, so only
passive cabling is needed to interconnect the devices. The cables connect
all instruments, controllers and other components of the system in parallel
to the signal line as shown in Figure A-1. Eight of the lines (DIO1-DIO8)
are reserved for the transfer of data and other messages in a byte-serial, bit-
parallel manner. Data and message transfer is asynchronous, coordinated
by the three handshake lines (DAV, NRFD, NDAC). The other five lines
control Bus activity.
Two types of messages are transferred over the bus:
Interface messages - for bus management
Device-dependent messages - for device control and data
transfer
