5 
is needed to implement sequential logic. A RAM module can fulfill this 
function, and also act as a PROM or ROM, if needed. 
PLA (Programmable Logic Array) [8] 
The programmable logic array may be regarded as a combination of 
two ROM's or a ROM in which the addresses are also programmable. In any 
case the PLA approach offers substantial advantages over the ROM. Spe 
cifically, it is feasible for a PLA to examine a great deal of input 
information in parallel, without having specific results for all possi 
ble combinations. In other words, the PLA can have "don't care" input 
conditions. Similarly, the PLA can present the same output from two 
different input combinations, and also outputs from a single input con 
dition. Furthermore, some PLA modules include flip-flop banks for logic 
feedback within the module. This increases the flexibility of the PLA 
even further. 
Microprocessor [9] 
A microprocessor may be defined as a complete central processor 
unit (CPU) of a digital computer implemented on one or on a few inte 
grated circuit (IC) chips. The appearance of microprocessors has caused 
a great deal of soul searching among designers of digital systems. The 
reason is that the microprocessor can be used both to put together com 
puters (often called microcomputers and involving CPU, ROM, and RAM, as 
well as some other components), and also to perform random logic. In 
the latter capacity the microprocessor may be considered as a universal 
logic module. Because of its universality, it can be manufactured in 
large quantities to keep its price down. This is a great advantage. 
Its disadvantages are, at least for the time being, that it performs the 
logic functions relatively slowly, and that its users must learn a new 
way of doing things: implementation of logic by micro-programming. 
TRENDS IN SYSTEM DESIGN 
Designers of digital systems are facing significant transitions in 
their mode of operation, just as are the detail designers who are be 
coming de facto system designers. The list of skills and tools needed 
by such a designer or applications engineer has grown to formidable pro 
portions. He must thoroughly understand the organization, operation, 
and performance capabilities as well as limitations of at least one, 
but preferably several microprocessors, RAMs, ROMs, PROMs, and PLAs; 
control of input and output; analog-to-digital and digital-to-analog 
converters; and operation of priority-interrupt systems, peripheral 
equipment, encoders, servos, control panel circuits, multiplexers, de 
coders, displays, and numerous logic families. In addition, he must now 
understand and have extensive knowledge of microprogramming, as well as 
machine language, and higher level software. On top of all that he must, 
of course, comprehend the requirements of the application, e.g., photo- 
grammetric equipment.