Automating Manufacturing Systems - Process Control and ...

plc analog - 22.18
N9:0
0
1
2
3
4
5
6
7
8
9
10
11
12
D1
D2
D3
D4
f s s s s s s s s p4 p3 p2 p1
L1
U1
L2
U2
L3
U3
L4
U4
D - data value words for channels 1, 2, 3 or 4
f - data format bit (1) binary, (0) BCD
s - scaling factor bits
p - data sign bits for the four output channels
L - lower scaling limit words for output channels 1, 2, 3 or 4
U - upper scaling limit words for output channels 1, 2, 3 or 4
Figure 22.14
Configuration Data for a 1771-OFE Output Card
22.3.2 Pulse Width Modulation (PWM) Outputs
An equivalent analog output voltage can be generated using pulse width modulation,
as shown in Figure 22.15. In this method the output circuitry is only capable of outputing
a fixed voltage (in the figure ’A’) or 0V. To obtain an analog voltage between the
maximum and minimum the voltage is turned on and off quickly to reduce the effective
voltage. The output is a square wave voltage at a high frequency, typically over 20Khz,
above the hearing range. The duty cycle of the wave determines the effective voltage of
the output. It is the percentage of time the output is on relative to the time it is off. If the
duty cycle is 100% the output is always on. If the wave is on for the same time it is off the
duty cycle is 50%. If the wave is always off, the duty cycle is 0%.