Bench tank rig, CPU 1214C with an SM 1231 AI 4x13 bit, TIA V17. Channel 0 is configured as current, 0 to 20 mA, and the 4 mA offset is taken out in NORM_X.
I replaced the old 4-wire level transmitter with a 2-wire one, same 4 to 20 mA output, plus to channel 0 plus and minus to channel 0 minus, the way the old one was. Now the raw value just sits at 0. No wire break diagnostic, no underflow, just 0, where the old transmitter gave about 16700 at half tank. I checked the NORM_X and SCALE_X blocks and moved it to channel 1, still 0.
Why doesn't the module see the new transmitter when the old one worked on the same terminals? Pretty sure the difference is the 2-wire part, but I can't see what it needs that the old one didn't.
Two things it could be, and the module configuration isn't one of them since it worked before. Either the loop has no power in it, or the terminals are wrong for a 2-wire device. Post how it's wired, terminal by terminal, and put a meter in series to read the loop current.
Transmitter plus to 0+, transmitter minus to 0-, nothing else connected to it. Meter in series reads 0.0 mA. The old transmitter had four wires, two went to the 24V supply and two to the channel.
There is your zero. The 4-wire transmitter powered itself from its two supply wires and pushed 4 to 20 mA into the channel. The 2-wire one takes its power from the loop, and the SM 1231 puts no power into the loop, it only measures what flows through.
Wire it as a loop: panel 24V plus to transmitter plus, transmitter minus to channel 0 plus, channel 0 minus to the supply M. The transmitter sits in series with module and supply and regulates 4 to 20 mA through all three. The S7-1200 system manual has that drawing on the SM 1231 wiring page, 2-wire and 4-wire side by side. Someone did a decent article on the module side: https://plctr.com/understanding-the-basics-of-input-output-modules-in-automation/
Rewired it as a loop through the 24V supply, meter reads 12.1 mA at half tank, raw value about 16700, same as the old one gave.
So the 2-wire transmitter had no power in the loop and the module puts none in, it only measures what flows through it. Putting the 24V supply in series with the transmitter and the channel was all it needed.
Why did the module give no wire break diagnostic with 0 mA in it? I'd have expected an underflow at least, not sure why it stays quiet. Thanks marekw and ines31.
Wire break diagnostics is a tick box per channel. Off by default.