If you head on over to the SBC-85 project website, the documentation and build files for the cassette tape interface are available.  Note that this interface is not limited to the SBC-85 project since the port to the computer is simple TTL and could be driven by any system.

For the software, I will be uploading the current version of the SBC-85 Utilities that has functions for reading and writing to the tape as well as adding the leader.  In the SBC-85_Utility_Call_Examples.A85 file there are examples on how to write to a tape, read 100 bytes from the tape, and dump the received data to the terminal.

Enjoy !