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The role of digital components in photogrammetric instrumentations

as standard modules. Therefore, the detail designer has in most cases
become a system designer. His task is to design a system that performs
the specific functions he is concerned with, by using standard modules
as system components whenever possible.
Any digital logic function can be implemented as a combination of
a number of simple logic building blocks (e.g., NOR gates, or AND/OR
gates). One may think about a standard module as a "black box" that
has a set of inputs and outputs. The outputs are a function of the in
puts, either in a fixed fashion (combinatorial logic), or in a variable
way (sequential logic). A functioning operational system usually includes
many such modules, which are selected, interconnected, and "programmed"
by the designer to do the job.
ROM (Read Only Memory) [6]
These modules come in many forms, the primary variable being their
capability to store bits of information. In all cases, the module has
an address (input) that specifies which word is to be selected and pre
sented as its output. The word, once it is put into its particular
address location, cannot be changed or moved. Thus, the name Read Only
ROM modules have found wide use in computers and calculators for
storage of constants, function tables, and permanent computation se
quences such as microprograms. However, it is obvious that ROM's can
also be used to perform combinatorial logic functions; for a given input
a specified output is obtained. ROM's are very fast. The access time
to the stored data is normally less than 200 nanoseconds and can be con
siderably less.
PROM (Programmable Read Only Memory) [7]
These modules are functionally similar to ROM modules, except that
the contents of the memory can be erased and altered by using special
techniques. Thus, PROM's are often used in prototype equipment when user
requirements and other reasons (including design mistakes!) may still
call for changes.
RAM (Random Access Memory)
These modules could be used to implement combinatorial logic.. How
ever, such an application would be wasteful. The RAM modules have their
own significant function: they store information in a fashion that per
mits rapid alteration. RAM modules are widely used as normal random-
access computer memory, in direct competition with magnetic core memories.
However, fundamentally a rapid access storage of some kind (e.g., a register)