Real World Instrumentation with Python

Chapter 7. Physical Interfaces

Connectors usually cause more failures than any other type of component. Many of these failures are not reported because they can be “fixed” by reseating the connector.

—W. Ireson, Clyde Coombs, and Richard Moss, Handbook of Reliability Engineering and Management, Second Edition, 1995

In this chapter we will examine the types of interfaces, both physical and software, that one is most likely to encounter when attempting to interface Python to data acquisition or control instrumentation. We looked briefly at some of these in Chapter 2, but here we’ll start to get into the details.

But first we’ll take a look at physical connectors, and in particular the types commonly used with the interfaces found in PC-based instrumentation. By the end of this chapter you should be able to determine what type of interface you might expect to find with a given connector type, or at least be able to readily identify the connector. You should also bear in mind that the conventions for connector usage aren’t always followed, so don’t be surprised to find a DB-9 connector being used in an interface for motor control signals, or an aerospace-style circular connector serving as an Ethernet connection.

We will then turn our attention to serial interfaces—namely, RS-232 and RS-485, the two most commonly encountered types of serial interfaces. We then cover the basics of the USB and GPIB/IEEE-488 interfaces, along with a discussion of where one might expect to encounter them.