The best replacement for opc is modbus.

The best alternatives to opc are rated as following:
★★★★★  modbus
★★★★★  pca
★★★★  dde
★★★★  dcr
★★★  ppc
★★  ppc cement
★  bacnet
★  ppc which is better
★  scada
★  golf 7 gti
★  topps
  gtc
  resveratrol
  golf 5 gti

Your alternative for opc is not listed? Suggest!
Speaking about opc, one probably shouldn't forget about plc.

Other search terms related to opc and their ratings are:
★★★★★ plc
You have in mind a companion for opc and it is not yet here? Propose!



Google Trends

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opc
modbus
pca