0
Not a bug

MC43 Digital Inputs staying true with nothing connected

Michael Carlyle 3 years ago in IQANdesign updated by Gustav Widén (System support) 2 years ago 6

All inputs are configured as Digital inputs – AM, AN, AO, and AP and are staying true no matter what is connected (we pulled out the pins and they are still true).

 

These are Pins C1- 11, C1-12, C1-13, and C1-27. They are all staying true and seem to be outputting 4.6V.

 

Have you seen this before? 

 

We have tried 2 different MC43’s with the same results.

 

I am using the latest version of Iqandesign 4.07 if that matters.

+1

I believe it's because of the internal pull up resistor that is in place on some of the inputs. 


we had that problem and had to move some inputs around to pins that do not have internal pull up resistors


Not a bug

As David points out, these four pins have internal pull-up, as DIN they are useful for sensors that connect to ground. 

So to see a change of value from True to False, connect these pins to ground. 


IQANdesign 4 does not show any information on DIN type, but there are ideas for IQANdesign version 5 on how to make differences between I/O more visible in the block diagram, see this topic :
http://forum.iqan.se/topics/858-identify-pull-up-pull-down-digital-inputs-on-mcxx-hardware-in-iqan-design/

Also had the same on MC41 (1st time we trying these units out as multi master with md4). could not figure out why these pins would stay true after uploading, if I run the simulation it all works perfectly fine as normal digital in, so I would assume it would work equally well on the hardware then as is. Good thing I had a chance to actually test the physical hardware before wiring took place. This could have ruined some technicians day looking for some sort of wire breakage that could cause these pins to go high. so after properly reading the manual I also noticed "w/pull-up"... 


So basically what this comes down to is just connect these pin to ground and they will work the same as the rest of the DIN?

That snapshot appears to be the only place where -BAT is advised as the negative reference for a digital input (with a pull-up to 5 VDC no less), and it appears to contradict with a strong recommendation elsewhere in the user manual to use the pin -VREF for all negative sensor references.  Can you clarify if some inputs should use one negative reference instead of the other?

It is still important to use a good ground connection, but it it is more critical on analog sensors connected to e.g. VIN, where even a small ground voltage potential difference influence the value you read. 


For a digital input with a pull-up, you have more margins for a signal change, what is important is that the voltage on the input is below the logic low level when the switch is closed. The instruction book specifies -BAT which should be good enough; if the switch is connected to chassis ground you are more likely to get an incorrect reading.