11 
A comparison of Figures 5 and 8 might suggest that we have progressed 
in a circle. In both cases the servo loop is closed inside a computer. 
However, in reality the common trends are again verified: the trend has 
been from specific to general, and the presence of a microprocessor in 
the servo- system is a good example of distributed processing. 
Analytical Plotters 
When discussing here the design trends in analytical plotters there 
is no reason to start with a historical perspective. Much of what was 
said previously about recording systems and servos applies to input and 
output functions of analytical plotters as their development has progressed 
over the years. Let us therefore start from the present. Figure 9 shows 
a typical design approach that may be applied to analytical plotters today. 
All minicomputers in the class capable of handling the analytical plotter 
requirements have an I/O bus. Peripheral equipments communicate with the 
computer via this bus, to which they are connected by a bus interface. 
The bus ipterface contains all the necessary timing and control functions 
to establish the communication link, as well as pertinent address and 
data transfer contacts. The input and output functions of an analytical 
plotter can be tied to the computer by interfacing to its I/O bus. Fig 
ure 9 shows schematically what is involved. Essentially, there is a 
trade-off between the number of bus interface units employed and the 
amount of additional control logic. A practical and cost-effective com 
promise in most cases is to use several bus interface units, as shown in 
the figure. 
Microprocessors are inherently capable of performing various logic 
functions required in interfacing with the computer I/O bus, and with 
other devices. Specifically, they can multiplex data in and out, and 
between these functions also perform data accumulation or "metering out" 
for servos. Figure 10 shows a possible design approach suitable for 
the relatively slow microprocessors readily available today. 
The ever-increasing speed of microprocessors leads to another design 
concept that may in the near future become feasible. As shown in Fig 
ure 11, all input/output functions are channelled through a single micro 
processor. Obviously, in line of what was said about distributed pro 
cessing and servos, the input and servo channels may include dedicated 
microprocessors of their own, as means of implementing necessary logic 
functions. 
Finally, the computer may become a microcomputer, as shown in Fig 
ure 12. It may seem again that we have progressed in a circle. This 
is not so: the system of Figure 12 is a distributed processing system 
with a hierarchy of processors. Such a concept is certainly intellec 
tually pleasing; its significance, however, is in equal measure based 
on practicality. All the microprocessors may be identical, differing