There are a number of devices particularly in Plant automation and SCADA that communicate on serial protocols like RS-485/RS-422/RS-232 etc. Typically extending these serial communication signals on shielded cables has distance limitations apart from susceptibility to electromagnetic interference(EMI). To get over this limitation, we can incorporate Serial to Fibre optic converters that accept RS485/422/232 signals and convert them to optical signals for transmission over 2 Kms in the case of Multimode and 20 Kms in the case of Singlemode. The converters are transparent to the software protocols such as MODBUS-RTU, MODBUS-ASCI etc. Here is a picture of a typical serial to optical converter:
Know more about this please visit http://www.yaminiengineers.com. We are all about passive and active fiber optic products.