Using GPIB / IEEE 488

- notes on using GPIB / IEEE 488 for automatic test and instrumentation, data acquisition and other applications.

GPIB tutorial includes:

    •  GPIB / IEEE 488 tutorial & basics
    •  GPIB / IEEE 488 operation
    •  Using GPIB / IEEE 488
    •  GPIB cables and connectors

In practice the GPIB interface is very easy to use. Ready-made GPIB cables are widely available even if they appear to be a little expensive. However these GPIB cables are fully screened and have the correct lines as twisted pairs. This considerably reduces the susceptibility of the bus to data corruptions. Manufacture of full specification GPIB cables can be difficult in view of the complexity of the cable and having to ensure the integrity of the screening.

A cheap and convenient alternative for GPIB cables is available in the form of insulation displacement connectors. While cables made in this way are much cheaper they are not screened and do not conform to the GPIB / IEEE 488 specifications. In view of this they should only be used where there are a very limited number of instruments, where data rates are likely to be low, for long runs, and where electrical noise is not likely to be a problem. If a cable of this nature is used then it is worth being aware that it could be the cause of random errors when the system is operating.

When setting up a GPIB system linked by the bus few rules need to be observed. The cables can generally be routed as required, linking the instruments as is most convenient. As the connectors can be "piggy backed", this makes linking the instruments very easy. However a little common sense is required, and not too many connectors should be linked to one point.

Before firing up the system, check all the instrument GPIB addresses to ensure that they are correct and match what the software in the controller requires. Also check that none are duplicated. Unfortunately checking the addresses can be a time consuming operation because not all the instruments will have switches that are easily available. To overcome this it is best to have standard addresses for different types of instruments within a factory. This will eliminate the need for any swapping and changing as test stacks are taken down and erected.

Although GPIB normally works very well, occasionally some problems inevitably arise. Sometimes it has been known for the bus to hang, even though all the instruments are operating correctly on their own. Some instruments can be sensitive to their physical position on the bus, particularly if they are at a remote end. In instances like this the topology of the cable routing can be changed to bring the offending instrument closer to the controller.

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