Automation System
Digital signals:

have a frequency measured in units of hertz
can have three possible levels
are assigned values of 1 or 0 depending on the voltage level
None of these
have a sinusoidal shape.

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Automation System
Time proportioning control:

cannot use a TM990 micro¬computer
a and b above.
can be used on a PM550 programmable process controller
None of these
is less efficient than normal PID control

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