What's the difference between an ECS and an Arc: Modbus output?

The ECS interface mimics a traditional polarographic DO sensor output signal (nA). The Arc: Modbus output provides Modbus digital outputs. By adding an Arc Wi 2G Adapter BT (Ref 243470) to the top of the VisiFerm RS485-ECS sensor head, it is possible to output an analog 4-20 mA signal from the digital Modbus.