Hey guys, swapped a pressure transmitter on the chiller loop yesterday. Old one was a 4-wire from the 90s that died, the new one's a 2-wire 4-20mA, channel 3 of a 1756-IF8.
The channel shows underrange, Ch3Underrange is 1 and Ch3Data sits at 0. Module's OK green, no fault in the tree, and the old transmitter read fine on that channel the day before. I put the two new wires where the old signal pair was, checked the scaling in module properties, and moved it over to channel 5, same underrange. Meter on the transmitter terminals reads 0V, and in series I get 0mA.
Is the module input dead on that half, or is the new transmitter DOA? Before I send the new transmitter back as faulty, is there something about a 2-wire one I should have checked at the terminals?
Meter in series reads 0mA and 0V across the transmitter. So nothing is driving the loop. Before the PLC: what is powering the transmitter? A 2-wire unit has no supply of its own, it lives on the loop current. Did the old one have its own 120V or 24V feed?
So a 2-wire transmitter needs a power supply in the loop and a 4-wire one has its own? I guess that's the difference?