Building an IBM 3270 terminal controller

The IBM 3270 type terminal is a mainframe computer terminal. It's difficult to find the necessary IBM 3174 terminal controller with Ethernet or Token Ring interface, version of the software that supports TCP/IP and in working condition. My initial search uncovered a description of the cabling used and the 3270 data stream, that is well-documented, but nothing about the protocol used between the terminal and the controller. The controller handles much of the intelligence people associate with 3270 terminals, meaning the CUT terminal logic is simpler than a comparable VT100. DFT type terminals can process the 3270 data stream themselves, and the controller passes this on to the terminal. The diagnosis program did display menus on the terminal, which was sufficient to capture some traffic between the terminal and the controller for analysis. All I had to do was poll the terminal for keypresses, pass these to the process, update the pyte emulator with the process output and then write the emulated terminal screen to the terminal - some mapping required.