1.2 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.

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

• Consider the example below,

1.2.1 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

 

 

• The format for the block that is to be written to the card is shown below.