Serial communication for Encoder Feedback

@arvidb ..
Actually I'm using Ormec Not Omron .
- But in the other hand , when you come to know that the compatible Ormec servo motor is using Yaskawa encoder , will be pretty sure there will be compatibility for signal transfer .
- I thing by creating a circuit to be able to translate the 2 wire signal communication ( data one clock) to differential signal through a Modbus communication will be the solution .. ( but this beyond the range of my brain ..need some who really knows what he have to do ) ..

While many things are possible if given the right amount of effort, I think you are flogging a dead horse.
First of all you would need total insight into the encoder protocol and then you would need to find a way to feed that data into the servo controller. Modbus is not going to be anywhere near fast enough to handle the data required to close a feedback loop of a servo drive.
As an example: My mitsubishi encoders use RS422 at 2.5Mbaud