IEEE 488 instrument bus speeds up

The Institute of Electrical and Electronics Engineers has upgraded its IEEE 488.1 standard so that IEEE 488 buses can transfer data at speeds as high as 8 MB/sec.

The need for higher-performing buses in laboratory and manufacturing instruments has prompted the Institute of Electrical and Electronics Engineers to upgrade its IEEE 488.1 standard so that IEEE 488 buses can transfer data at speeds as high as 8 MB/sec (megabytes per second), up from 1 MB/sec.

The increased throughput rests on the use of two-wire handshaking and packed data streaming. The prior version only allowed three-wire handshaking and handshaking after each byte.

The revised standard, IEEE 488.1-2003, ‘Standard for Higher Performance Protocol for the Standard Digital Interface for Programmable Instrumentation,’ addresses the operation of the IEEE 488 bus or General Purpose Interface Bus (GPIB), which has been in use for over 25 years and is built into millions of instruments world-wide.

These buses allow communication among controllers (usually a computer) and instruments in research, test and production settings, such as oscilloscopes, spectrum analysers and digital multimeters.

The new specification defines a ‘non-interlocked’ handshaking protocol between devices using only two control lines, which transfers data faster while still ensuring data integrity. The existing ‘interlocked’ handshaking protocol depends on three control lines for handshaking. Devices that comply with the new version of IEEE 488.1 will be backward compatible with those operating under the existing protocol and can be used with those older instruments on the same physical bus.

Newer devices will default to the three-wire, 1-MB/sec mode when working with older systems. In addition, existing IEEE 488 instruments are fully compatible with the revised standard but cannot take advantage of the higher data transfer rates.

‘This revision, the first since 1987, reflects the high interest in a better-performing IEEE 488 bus,’ says Robert Canik, IEEE 488 Working Group Chair. ‘It extends the usefulness of this tried and true bus standard and makes it even more viable for years to come.

‘Although alternate connectivity options exist, such as those based on Ethernet and USB protocols, the IEEE 488 bus offers a range of benefits that favour its use in many situations. For instance, it is designed specifically for test and measurement applications. The bus also uses rugged, industrial cabling and connectors and avoids the security and other issues that can accompany options associated with broad networks.’

In complying with the new standard, instrument makers need not make hardware or panel labelling changes because the high-speed capability is optional. Instrument manufacturers who would like to support the new high-speed IEEE 488 transfer protocol can do so by adding a readily available controller IC to their design.

Canik notes that several instrument vendors participated in creating the new specification because the IEEE 488 bus is integral to such a large installed-instrument base.

‘After decades of use, this bus is nearly universal in the test and measurement industry,’ he says. ‘The updated standard reinforces the use of the GPIB in current and future systems.’