Microcomputer I/O devices

I N THE DISCUSSION of thestructure of a microcomputer,we described the various data paths in a microcomputer, includ-ing data input, data output, exter-nal device addressing, in and out function pulses, and interrupt sig-nals. These are the vital lines of communication between the micro-computer and the "outside world," i.e., those signal lines that are neces-sary to interface the micro-processing unit (MPU) to the in-put/output (I/O) devices that you would like to control. I/G devices Some useful definitions pertain-ing to I/O devices are given in the table. The traditional view of an I/O device is that it is somewhat large or complex. Card readers, magnetic tape units, cathode-ray tube dis-plays, and Teletypes certainly fit such a description. However, a sin-gle integrated circuit chip, such as a latch, shift register, counter, or small memory, can also be consider-ed an I/O device to a computer. Another important point is that several device select pulses (synchronization pulses) may be required to interface a single I/O device. For example, a 74198 shift register has a pair of control inputs that determine whether the register shifts left, shifts right, or parallel-loads eight bits of data. This chip also has a clock input and a clear input. Thus, a single 74198 chip, when serving as an output device, may require up to four device select lines from the microcomputer. Therefore, the fact that we can generate 256 different input and 256 different output device select pulses does not necessarily mean that we can address 512 different "devices. " A more reasonable num-ber is of the order of 50 to 100 different devices. Device select pulses are inexpen-sive and easy to implement. We encourage their use as often as possible as you attempt to substi