HMISCU8B5 4-20ma input wiring question.

[Anonymous user]

Hi All,

I have a question about the input wiring on a HMISCU8B5. I am so close to having what I am after yet it seems so far away. I have attached a picture of what I am doing and it works fine as long as I hook only one of the 4-20ma inputs at a time. I want the HMISCU8B5 to take the place of the Kele LPI-1C's. I have to be missing something on the attached wiring diagram. How can terminal (D12) C2 be the same for both 4-20ma feedbacks? Like I said either or works fine by itself, meaning my JCI controller sees the same thing the HMI sees when either input is hooked up by itself. I just don't understand how this wiring schematic could work for two different 4-20ma signals. If anyone has any information or suggestions please let me know. Thanks.

[BevanWeiss]

You need to work out if your analog input is sourcing or sinking.  Which might also be called two-wire, or three/four-wire mode.

In two-wire mode, the analog input will actually output a smallish voltage (often up to 20V or so) and will let the field sensor consume some of the power, and return a 4-20mA signal.  This is only used on 4-20mA instruments that I'm aware of.

In this form, often the + 'input' will actually be an output for the voltage, with the current returning back in the '-' terminal.  But it pays to check with the IO module on this (normally they will show an example layout).

In three/four-wire mode, then the field device needs to supply its own power supply, and output its own 0-20mA current.

In this mode, the + terminal is almost always where the conventional current flow will go into.  The - terminal is where the power supply negative will be commoned to the input module.

In three/four wire mode, the + of the power supply will always feed the field sensor.