It is possible to mix two-wire current loop and active-source devices in a multidrop scheme, but because of their different methods of connection, a third wire is needed to the field, as shown in the diagram below. Current flow is shown by the arrows. The upper transmitters are two-wire loop-powered; the lower transmitters are separately-powered active-source devices. If "twisted triple" cable is not available, such a mixed system should be constructed using two separate twisted pairs, connected together at the load resistor. A communicating device can still be connected either across A and B, or across B and C, or across a field device, for communication to any field device.

Make sure that, with the parked currents added, the voltage drop across the HART load resistor is not too high for the transmitters to source into. I would expect that up to 20mA total current would be fine. (Some transmitters will park at less than 4mA, or might conceivably park at more than 4mA.)

Remember that up to HART revision 5, using command #6 to set a non-zero polling address for multidrop also parked the output current. In revision 6 or later, "loop current mode disabled" must be specified in the second data byte of the command, to park the output current.