Information Technology Reference
In-Depth Information
22
IEEE-488, VME and VXI
22.1 Introduction
The IEEE-488, VME and VXI busses have all been used to interface to programmable in-
strumentation and controllers. They were at one time very popular, but with the increasing
development of PCI, SCSI and RS-232 they have been replaced in many applications. IEEE-
488 has the advantage of being very robust and simple to use, but suffers from a lack of
speed. The VME bus has the opposite problem. It has fast data transfers and is powerful, but
it is also difficult to use.
22.2 IEEE-488 bus
The IEEE-488 bus was developed in the 1970s as an answer to problems in interfacing with
programmable instruments and controllers. Its history is as follows:
•
1975 - the IEEE published the IEEE-488 standard, which was based on work done by
Gerald Nelson and David Ricci at Hewlett-Packard.
•
1978 - the IEEE published a revised specification known as the ANSI/IEEE standard
488-1978. IEC (International Electrotechnical Commission) then adopted the specifica-
tion as an international standard (IEC 625-i).
•
1987 - revised standard, known as ANSI/IEEE standard 488.l-l987. This specification
deals with the mechanical interconnection and the electrical protocol.
•
1987 - ANSI adopted the ANSI/IEEE standard. 488.2-1987 which standardises the soft-
ware interfaces in terms of codes, formats, protocols and common commands.
The IEEE-488 bus is an excellent interface to programmable instruments as it is relatively
simple to add to, operates at reasonable speeds and is available from various manufacturers.
It is also know as the general-purpose interface bus (GPIB).
The IEEE-488 bus allows for the interconnection of instruments using a standard cable
and a standard interface. It supports different data-transfer rates and also different message
lengths. The maximum transmission path length is 20 m, and it is recommended the maxi-
mum interconnection length between instruments is limited to 2 m, each. A maximum of 15
instruments can be connected to the bus, although bus extender can increase this number. All
devices connect to a common bus (a party-line) using 16 lines. These are shown in Figure
22.1 and are:
