VT100 BeagleBone Mod

Recently, I acquired a working DEC VT100 Terminal. After getting it set up, and connected to an old laptop running Ubuntu, I tried to find a good place for it. However, I quickly found that the extra cables and laptop made it awkward to place where I'd like. What was I to do?
VT100 Running Ubuntu VT100 Running Ubuntu

Solution: Add a BeagleBone!

When reading through the VT100 manual, I came across the STP Connector. Eureka! Here is the solution I was looking for. The port is designed to allow accessories to intercept and modify, or replace the serial communications between the external serial device, and the terminal CPU. I could design an add on board that would allow me to tap into this connection, and use a small Linux computer (in this case the BeagleBone Black). This would allow me to run the terminal with no external computer attached. The best part was that I didn't even have to modify the VT100's hardware in any way.

The STP Connector

The STP connector is a pass-through edge connector designed to allow accessories to intercept serial data before it goes to the screen, or send data on the terminals behalf. This allowed printers, disk drives, and even an internal PDP-8 to be added to the terminal. This port was available on many of the VT100 series terminals, and was intended as a well documented interface for 3rd party vendors. An overview of the connector, and it's pinout can be found here.

The BegleBone black would sit between the VT100 main CPU, and the external connector. This allowed the VT100 to run without an external computer. Since the RS232 level converter IC I chose could handle two ports, I opted to also connect the BeagleBone to the external serial port.

Diagram of the BeagleBone

Board Design

The board design was fairly simple. The most difficult part was creating the edge connector. It's something I had not done before, and getting the dimensions right was very important. Fortunately, I managed to get that part right the first time. The other problem I had was that the VT100 didn't power up cleanly. There was usually a small glitch on the 5V rail at power-up, To work around this problem, I added a power on reset chip.

Schematic Board V1 Board An image of an unpopulated V1 board. Unfortunately, the silkscreen did not come out on the V2 Board.
Board Sitting in the VT100 main board An image of the BeagleBone adapter board sitting in the STP connector on the VT100 main board.

Conclusion

All in all the project went well. The second revision of the board worked, and no modification of the VT100 was necessary. The next step is to get a Wifi dongle working to get the VT100 on-line.

GitHub URL: https://github.com/brendan0powers/vt100-addon