Microcontroller SDLC interface
Use 2 full duplex RS-485 chips (e.g MAX488). One for TX, RX lines and the other chip for clocks.

Send data over through bit banging. And also use bit banging to receive and decode the data. With these implementation communication over SDLC protocol would be possible.

Obviously one need to have correct understanding of the SDLC protocol so as to be able to send right packets and decode data through bit banging.