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I have an Allen Bradley Micro820 (2080-LC20-20QWB) with four sinking 24 VDC analogue inputs and eight 24 VDC sink/source discrete inputs. I need one more discrete input for my project than the 8 that I have to work with (not counting the analogues).

Can I simply use one of the analogue inputs as a sinking discrete input?

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  • \$\begingroup\$ The Allen Bradley Micro820 (2080-LC20-20QWB) have 12 digital inputs (4 Inputs shared with analog inputs) link. So it is possible. \$\endgroup\$ – AltAir Apr 11 '17 at 7:26
  • \$\begingroup\$ If your inputs are intended for 4-20mA then you can use some combination of switched resistors and stabilised power supply or more sophisticated switched current sources to provide two or more distinct current levels that you can test for in the logic. Offering 6 and 18mA should give you error free detection of two states. \$\endgroup\$ – KalleMP Apr 11 '17 at 13:57
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To elaborate on @AltAir's comment, the Micro820 controller you have has 12 inputs, 4 of which (I-00 - I-03) can be used as either 0-10v analog inputs or 24VDC digital inputs.

There are a few considerations:

  • Unlike the other digital inputs (I-04 - I-11), the shared digital/analog inputs are not isolated from the controller's power supply. The common for these inputs is tied to the power supply common.

  • These shared inputs, when used as digital inputs, can only be used in a sinking wiring configuration. (The other digital inputs can be wired either as sourcing or sinking.)

For wiring diagrams and technical specifications, see the manual.

Alternatively, your controller has 2 expansion slots for plug-in modules, so you could add more discrete inputs by adding an IQ4, for example.

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