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I am trying to comunicate with a Yokogawa vortex meter. Its on the test bench and i have succes talking to it when I break the loop and insert the E+H commubox in series. I cannot make a connection however, when I connect the Commubox across the terminals of the transmitter. Does anyone know why i can't connect without breaking the ps loop?

If it anything like the HART modem, you'll need to use a 250ohm resistor in series and hook up the Commubox across it.

Or maybe i'm out to lunch.

Yes, 250ohm resistor should be hooked up.
if you didn't connect it, then the reason why it doesn't work is because in series you don't apply PS voltage across the commubox and the current is circulating ok. But in paralleling you apply PS voltage that would counteract with commubox voltage creating extra current to your 4-20, which must degenerate operation of your vortex device. Resistor drops the voltage to commubox V, thus normalizing working conditions of device.

In case you have resistor, you need to go thru the datasheets again and check any inconsistensies in V-I of CMBx and PS.

Btw, use the suggested setup by your cmbx manufacturer, don't try setting it in series. Having comm doesn't mean correct comm.
Good luck.

When I connect the Comm box up in series on the negative terminal to the power supply, there is a 250 Ohm resistor there. It tells me not to add one when goin across the terminals, but im still not comunicating. I have tried it with and without the resistor as well and neither work. Any other ideas?

Silly question, but are you sure the YEW vortex is a Hart instrument and not their Brain protocol?? I may be mistaken, but the E&H software is Hart only........