What does someone do who wants to use computers for data acquisition? No matter which way the goal is pursued, it's necessary to know how computers communicate with the external circuits which make measurements. This chapter summarizes choices and introduces the approach used in this book.

Ports and Expansion Slots

Measurement Circuits (MC's) can be connected to computers through game ports, serial ports and expansion slots. In each case, software must send values to and get values from the MC's. The following sections review and compare the approaches.

Game Port Communication

On an Apple II, communication between its game port and an MC occurs as follows. Up to three digital signals generated by an MC are connected to the three game port push buttons. The computer ascertains a signal's high or low state by peeking a specified address and then seeing if the value is greater than 127 which means the signal is high. Similarly, one or more of the game port's four enunciator outputs are connected to an MC. The computer makes an output high by peeking a specified address and low by peeking a different address.

Game port data acquisition is practical when the number of input and output bits can be limited which is the case with the few commercially available systems. They are typically self-contained units starting with, for instance, a temperature probe and ending with a data plot. Their low cost, reliability and operational simplicity make them attractive for instructional laboratories.

Serial Communication

The most widely adopted standard in computers is RS-232 serial communication. For virtually any machine, serial ports either are or can be installed (at low cost).