1. Any 4-20mA output has a (+) and a (-) terminal.
For active outputs (powered internally by the field device), the (+) connects to the AI
I and AI
V terminals, the (-) connects to the COM terminal.
The installation guide notes the jumper between the V terminal and the I terminal for current input wiring.
2. The wiring diagram assumes an active, powered 4-20mA signal.
If the field device generating the 4-20mA signal is loop powered, then field wiring for that device would have to include a DC power supply.
Connections for a loop powered field device:
Power supply (+) connects to field device (+)
Field device (-) connects to AI I and AI V
AI COM connects to power supply (-)
3. There is no such thing as a "negative 4-20mA" signal. For example, vacuum pressure from atmosphere to -1.0Bar is a range of all negative values. But it the signal is a conventional 4-20mA signal.
The 4-20mA can represent a range or scale of negative engineering unit values but there is no (-)4-20mA signal.