Prior to Ethernet's dominance in plant, control and field networking the norm was serial datacomms. Point to point communication was typically achieved using the RS232 standard (up to 50ft), longer distances RS422 and longer distances in a multi drop topology RS485.
At the same time the reliability of intelligent electronic devices (PLCs, RTU, RIO, Instruments,etc) improved well beyond any prediction. Factor in the major cost of software engineering and the cost will be significant.
Using devices that convert signals from serial RS232/485/422 to Ethernet 10/100BaseT can put this cost off indefinitely. The devices, called serial to Ethernet (S2E) device servers, provide many approaches to making the data transformation. This can be a comm port redirector that adds virtual comm ports to the host computer that map to the (now remote) serial interface. A more integrated approach uses native LAN transmission protocols (telnet or ssh) or proprietary TCP 'sockets' to achieve the desired communication.
Where a specific protocol (modbus, profibus, etc) operates across the legacy serial interface, a serial gateway can provide the necessary protocol and interface conversion.
This gateway may simply convert from one variant of the same protocol to another I.e modbus RTU to modbus TCP or ProfiBUS to ProfiNET. More advanced gateways can map one protocols functions to another I.e. Modbus TCP to ProfiBUS.
Caution is needed with device selection. While electrical interfaces may be the same, peculiarities of the transmission protocol may make the lowest cost device servers unsuitable; Modbus conversion is one such case.
There are many variants of device servers, some may extend the serial interface across the mobile 3/4G data network or WIFI. Options also exist for harsh environments where robustness and protection are critical considerations.