Ravens Fan wrote:
What you show won't work at all.
I'm wondering why... Actually, NI RECOMMENDS to wire Positive voltage to Vsup, but also the scheme with Vsup unwired should work, looking at the internal diagram. The Vout terminal should only switch, from closed to open, on COM terminal. Since COM is wired, it shouldn't matter if Vsup is or not! I'm trying to understand...
On the other hand, I don't know if it'll be good practice to wire a modulatedVoltage to Vsup terminal, form diagram you see a pure DC alim connected to Vsup...
When I wrote that, I believe I was thinking of a sourcing digital output rather than a sinking digital output. So I was looking at it as a +24 V source on Vout being connect to the bult across from another Voltage source. If anything, I would say that calling the Vout terminal by that name is a bit misleading, that it really should be called Vin considering it is a sinking terminal. So now I can't figure out why it wouldn't work like you have it shown. Perhaps having Vsup wired up as well will help establish the approriate voltage levels for the transistor circuitry????
Have you tried connecting it as they have shown in the manual and see if your system behaves any differently?
If you look at figure 4 of the manual http://www.ni.com/pdf/manuals/373354b.pdf it shows that V terminal is connected internally to the buffer driving the output transistor in the 403 module. Without the potential on the V pin, the transistor will not be completely turned off. This a very common configuration when dealing with opto isolators.
Suggest you connect the modulated 28V signal to the V terminal of the module to provide the needed potential.
So you suggest that this is the "guilty" node... If you're right, not connecting Vsup also shorten life of DO-403, since transistor is not completely switched OFF in OFF state...