The first thing you'll need to do is to determine whether you want to use the module in 16 channel single ended mode or 8 channel differential mode. How you'll wire the module depends upon this module configuration option. Single ended mode is more efficient, while differential mode provides better noise rejection. Configuring the module for single ended mode assigns one input amplifier to each channel, while configuring for differential mode assigns two input amplifiers to each channel.
All analog input modules measure voltage. To measure a current input, you must series the current loop through a 250 ohm resistor. The module then reads the voltage drop across that resistor. 4-20 mA = 1-5 VDC.
The RTN terminals are the common terminals for voltage inputs. For current inputs, you connect the instrument common to an iRTN terminal and then install a jumper between that iRTN terminal and a RTN terminal in order to place the module's built in 250 ohm resistor into the current loop and convert the transmitter output from a current input to a voltage input. The RTN terminals are all interconnected internally.
This module does not provide loop excitation power, so if you are using a 2 wire transmitter, you must provide an external loop excitation power supply.