Programming for Microprocessors

9

Control of peripheral devices

(2)

Polling, the method of peripheral control described in Chapter 8, is

suitable for systems where the peripheral devices all have similar charac-

teristics of data rates and crisis times. Sometimes the load

is

unbalanced;

one or two devices may have a much higher speed and a much shorter

crisis time than the rest. For example, a plant control system that nor-

mally polls round 500 observation points, taking a leisurely cycle time

of several seconds, may need to respond to an 'alarm' input within 10

milliseconds. If the alarm is merely placed in the polling loop, and

interrogated in its turn, the response tim