When creating a HART network, assuming the instruments are run in parallel, will the instruments still give a 4-20 mA signal up to the backbone? I have a tank bay with 8 tanks. I need to get the level data to the control room, but I have no analog inputs left on the PLC, nor the room to add cards. I was thinking I could put all of the levels in parallel and use a HART to Modbus TCP gateway, but I need to retain the local panel displays for the tank levels. This is what I have in mind; would it work?

 

Multidrop HART has all the transmitter outputs connected together. To do that, all the transmitters must be set to output fixed current, typically 4 mA, otherwise they would be fighting against each other.

You may be able to use a gateway that has separate channels for each transmitter, but that will not be Multidrop HART.

 

I saw a similar tank farm situation with all the 4-20mA level transmitters wired to a Precision Digital Consolidator Plus box that had its own HMI screen. The location of the Consolidator in the tank farm made it suitable as a local panel readout for four of the tanks. A Modbus/TCP client in the control room polled the Consolidator box which functioned as a Modbus/TCP server sourcing the level data.

For the tanks that were further away, a couple tanks used loop powered panel meter displays, but others had DC powered local panel meters driven by the retransmit 4-20mA analog outputs from the back of the Consolidator panel. The operators preferred the powered displays because they're easier to read than the gray displays on the loop powered meters.

 

Phoenix Contact has a HART to Ethernet (Modbus TCP, Profinet, etc) gateway with multiple channels that will enable you to keep your individual loops intact so you can run the displays.
Here's a link for some info, feel free to contact me if you'd like to chat about it!

https://www.phoenixcontact.com/assets/2018/interactive_ed/101_318527/index.html#8