RaspTI: Convert a Vintage Computer (TI-99/4A) Into a RaspPi Workstation - Part 1 - Keyboard

This is the first in a series of Instructables that document the process of converting a vintage computer (in this case, a TI-99/4A) into a RaspberryPi workstation. You should know that the only thing we're preserving about the old computer is its physical form. We will not be using any original hardware or software, with the exception of the awesome keyboard. 

Part 1 will show you how to convert the TI's passive keyboard matrix to a USB keyboard. We will be re-installing it into the TI-99 body, but once it's been programmed, it can be used for anything.

NOTE: this process can be used to convert any push-button matrix into a USB keyboard input device.

Thanks and Salutes:
Ido Gendel for his advice on how the code should be structured.
Sean McIntyre for his help with debugging the code.

Code for the Teensy can be found here: https://github.com/matthewepler/raspTI_keyboard


Supplies:
Teensy++ 2.0 microcontroller
Micro USB cable
Bread board and standard gauge wire
30x female headers
55x male headers
ribbon cable (regular wire will do but this will make your project less of a wire nest. you need at least 7 wires in the ribbon)
10x 10K resistors
Wire cutters and stripper
Solder and Iron
4x small zip ties
TI-99/4A or any vintage computer keyboard with wired output similar to the one shown here.

If you have questions, feel free to contact me via the address on my site: http://mepler.com

The screws aren't hidden. Just unscrew the ones on the case to get inside. Then liberate the logic and power boards by unscrewing them from the plastic base. 

Carefully unhook the keyboard as shown in the picture. Your connector cable may not be exactly like mine, but it will work the same. 

Save all the pieces and screws!!!

[image coming]

1. Solder male headers to every pin on your Teensy.

2. Add wires to the bread board that will connect the power and ground lines on either side.

3. Add wires for power and ground to the Teensy.

4. Insert Teensy, aligning the power and ground pins with the wires you put in the breadboard.

5. Insert a row of 15 male headers further down on the breadboard.

6. Insert a 10K resistor for each of these headers and connect it to ground.

7. Make two sets of cable using the ribbon cable and female headers. One should have 7 wires, the other 8. Solder the stripped ends of the wire into the female end of the header.

8. Connect the first 8 male header lines (counting from left) to the Teensy:
    Header 1 -> C0
    Header 2 -> C1
    Header 3 -> C2
    Header 4 -> C3
    Header 5 -> C4
    Header 6 -> C5
    Header 7 -> C6
    Header 8 -> C7

9. Connect the last 7 male header lines to the bottom
    Header 9   -> F7
    Header 10 -> F6
    Header 11 -> F5
    Header 12 -> F4
    Header 13 -> F3
    Header 14 -> F2
    Header 15 -> F1 

10. Connect the keyboard's connector cable to the male headers.

11. Connect the Teensy to your computer.

Before we get to the code, let's take a look at how this keyboard works. As you'll see in the diagram (found here), the keyboard is made up of 8 rows and 7 columns in the circuit. The connector pins are for the original chip, NOT the Teensy. Ignore the pin numbers shown in the diagram.

Notice on the diagram the columns and rows are attached to pins that are numbered 1 through 15 from top to bottom. We are doing the exact same thing, only from left to right. The reason we split the keyboard's connector wire into 2 groups, one with 8 connections and the other with 7 connections, is to mirror this column/row setup shown in the diagram. Half of the wires will be Outputs, the other half Inputs. 

Here's the plain text version of the diagram. This appears in the header of the code for your reference:

       #12  #13  #14  #15  #9  #8  #6
#5    =         .       ,        M    N    /  
#4  spac    L      K       J     H    ;
#1  entr     O       I       U     Y   P
#2              9       8       7     6   0
#7  fctn      2      3        4     5   1  lock
#3  shft      S      D       F    G   A
#10 ctrl     W      E       R    T   Q
#11            X      C       V    B   Z

Watch the video for an explanation of how these circuits are used to detect a connection.

Here is a demo of the keyboard working. 

Hope it worked for you. If not, just email me at the address on my site: http://mepler.com