Sealevel synchronous serial expansion boards and the Relio R2 sync server have an SDLC mode that implements many low-level behaviors of SDLC such as frame opening and closing flags, frame check sequence calculation and zero-bit insertion behavior.
The SeaMAC and Route 56 APIs for these devices provide application software with tight control of the devices behavior. These APIs allow software applications to implement any higher-level SDLC behaviors as needed, as well as implement non-SDLC protocols that conform to the low-level behaviors of SDLC mentioned above. HDLC, PPP, Frame Relay, and X.25 protocols all conform to these low-level behaviors. Other protocols derived from HDLC commonly do so as well.