In Fig. 13.15 the furnace temperature is converted into a digital signal and
transmitted to the computer in the PLC. The digital temperature signal Dt and
the digital reference signal Dref are compared in a digital error detection circuit
and the difference signal is sent as a digital signal to the controller, where it is
converted into a pulse width modulated (PWM) signal or a DAC is used to control
the heater. The control loop in this case would be considered as a digital control
loop. PWM will be discussed later in this section.