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?
The old one had 24V on two extra terminals, yeah, that's the 4-wire. I capped those two wires off when I pulled it. The new one has two terminals total. So it's got nothing. Not sure, is the IF8 supposed to supply it?
The IF8 doesn't source loop power. Some analog cards do, pretty sure the IF8 isn't one of them.
Correct, and joeb has it right. The loop is: 24V supply plus, through the transmitter, into the module, back to supply minus. The transmitter regulates the current, the module measures it. So take your capped 24V, plus to the transmitter's positive terminal, transmitter negative to IN-3 on the IF8, supply minus to i RTN-3. Single ended wiring. The IF8 manual, 1756-UM009, has exactly this drawing for 2-wire transmitters, and the differential one next to it which you don't want here. Prove it with the meter first, 4 to 20mA in series before you look at the tag. The old 4-wire drove the loop itself, that's why it just worked. Writeup on loop powered versus 4-wire on the input side: https://plctr.com/understanding-the-basics-of-input-output-modules-in-automation/
Wired it the way marekw said, the capped 24V through the transmitter into IN-3, minus to i RTN-3. Meter in series 5.2mA with the chiller off, tag reads 1.5 bar, which is about right for static head. Underrange bit dropped the moment the loop had current.
So it was loop power all along. The 2-wire transmitter has none of its own, the IF8 doesn't provide any, and the old 4-wire drove the loop itself. 24V in series through the transmitter and it reads.
Only thing I don't get is why the 4-wire used the same two signal terminals and the new one needs the supply minus tied to i RTN. Reading the manual page tonight.
Thanks marekw, mikko, joeb.
5.2mA at zero flow is a good sign. Loop proved.