The comms board 4-20mA current loop input consists of a 75Ω shunt resistor across one of the analog input pins.
You need to connect an appropriate voltage source to your device and the other end of your device to the comms board input. Connect the ground of your voltage source to the Analog Ground pin. Divide the measured voltage by 75 to obtain the current. The cape does not provide a voltage source, Many devices use 24V excitation, but this is not universal. Thus the choice of voltage source is left to you. Hope this helps someone. Schematic can be found here if you have any doubts: https://github.com/beagleboard/capes/blob/master/beaglebone/Comms/Comms_Cape_sch.pdf <https://github.com/beagleboard/capes/blob/master/beaglebone/Comms/Comms_Cape_sch.pdf> > On Feb 2, 2021, at 8:14 PM, evilwulfie <[email protected]> wrote: > > you can use an current to voltage converter then an opamp to level shift it > > On 2/2/2021 5:03 PM, Dwayne Zimmerman wrote: >> Can anyone tell me how to connect a sensor to the 4-20mA input on the comms >> board.I need temperature and all 4-20mA temperature sensors I'm familiar >> with are 24 VDC not 1.8 >> >> On Monday, February 1, 2021 at 7:29:46 PM UTC-5 Dennis Bieber wrote: >> On Mon, 1 Feb 2021 15:46:51 -0800 (PST), in >> gmane.comp.hardware.beagleboard.user Dwayne Zimmerman >> <[email protected] >> <applewebdata://8B272222-29C4-408D-A708-C1CB73D4CD56>> wrote: >> >> >So the pull down is to keep the voltage under 1.8 correct? My confusion is >> >I have one screw per input, 4-20mA is two wire where is the other half of >> >the circut? >> > >> >> I suspect the pull-down is only ensuring that the floating voltage is >> tied to 0V; feeding anything with a voltage over 1.8V will still fry the >> input. And since 0V likely also means 0mA, it indicates a fault condition >> in the loop. >> >> https://www.fluke.com/en-us/learn/best-practices/test-tools-basics/process-tools/what-is-a-4-20-ma-current-loop >> >> <https://www.fluke.com/en-us/learn/best-practices/test-tools-basics/process-tools/what-is-a-4-20-ma-current-loop> >> >> https://www.predig.com/indicatorpage/back-basics-fundamentals-4-20-ma-current-loops >> >> <https://www.predig.com/indicatorpage/back-basics-fundamentals-4-20-ma-current-loops> >> >> {Many others| >> >> The input to the BBB likely needs to consist of a current->voltage >> tranform, so the ADC can measure the voltage corresponding to the current. >> Which might be built-into the cape (I've not looked at it). >> >> >> -- >> Dennis L Bieber >> >> -- >> For more options, visit http://beagleboard.org/discuss >> <http://beagleboard.org/discuss> >> --- >> You received this message because you are subscribed to the Google Groups >> "BeagleBoard" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to [email protected] >> <mailto:[email protected]>. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/beagleboard/5fd03d5f-b21f-43b8-b3d3-ae464ff8d1bdn%40googlegroups.com >> >> <https://groups.google.com/d/msgid/beagleboard/5fd03d5f-b21f-43b8-b3d3-ae464ff8d1bdn%40googlegroups.com?utm_medium=email&utm_source=footer>. > > > -- > For more options, visit http://beagleboard.org/discuss > <http://beagleboard.org/discuss> > --- > You received this message because you are subscribed to the Google Groups > "BeagleBoard" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected] > <mailto:[email protected]>. > To view this discussion on the web visit > https://groups.google.com/d/msgid/beagleboard/9654dc5a-2d31-6f0f-a745-ae085db30859%40gmail.com > > <https://groups.google.com/d/msgid/beagleboard/9654dc5a-2d31-6f0f-a745-ae085db30859%40gmail.com?utm_medium=email&utm_source=footer>. -- For more options, visit http://beagleboard.org/discuss --- You received this message because you are subscribed to the Google Groups "BeagleBoard" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/beagleboard/2C68CEE2-3A98-4D73-95E2-1851938D8C73%40gmail.com.
