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page 511

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D

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u16

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v16

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d1

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D - diagnostics

u - under range for input channels v - over range for input channels d - data values read from inputs

s - sign of data

29.2 ANALOG OUTPUTS

After we have used a controller equation to estimate a value to put into our process, we must convert this from a digital value in the computers memory, to a physical voltage.

This voltage is typically limited to 20mA in most computer boards, and drawing near this current reduces accuracy and life of the board.

A simple circuit is shown below for a simple digital to analog converter.

page 512

MSB bit 3

bit 2

Computer

bit 1

LSB bit 0

 

 

 

 

5K

10K

V

 

-

Vss

 

 

 

 

20K

V

+

+

+

 

 

 

 

 

 

0

 

 

 

 

Vo

40K

 

 

 

 

 

 

 

 

-

80K

 

 

 

 

First we write the obvious,

V + = 0 = V

Next, sum the currents into the inverting input as a function of the output voltage and the input voltages from the computer,

 

Vb3

Vb2

Vb1

Vb0

Vo

10--------------K

+ 20--------------K

+ 40--------------K + 80--------------K

= 5-----------K

 

Vo

= 0.5Vb3

+ 0.25Vb2

+ 0.125Vb1 + 0.0625Vb0

Consider an example where the binary output is 1110, with 5V for on,

 

Vo

= 0.5( 5V) + 0.25( 5V) + 0.125( 5V) + 0.625( 0V) = 4.375V

The calculations for the A/D converter resolution and accuracy still apply.

Consider the example below,

page 513

We need to select a digital to analog converter for an application. The output will vary from -5V to 10V DC, and we need to be able to specify the voltage to within 50mV. What resolution will be required? How many bits will this D/A converter need?

What will the accuracy be?

29.2.1 Analog Outputs With A PLC

An example of an output card is 1771-OFE.

To output a value we only need to write a single value to the output card

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