Automation System
Today, programmable controllers:

None of these
can be cost effective even when replacing just a few relays
are more powerful then the first controllers on the market
All of these
are relatively low cost compared with the original digital equipment available for industrial control

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Automation System
Digital signals:

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

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