6502 8-bit adventure

  I've been away from my project, but this week I've made a few additions to the SBC. Instead of using an old TRS-80 keyboard, I decided a while back to use a PS/2 keyboard (interfaced through the VIA). Things worked relatively well, except that the SBC froze if no keyboard was present. Last week, I changed from the fully software-driven PS/2 interface, to an MCU-based interface. This offloads a lot of the work, and only sends characters (instead of scan codes) via an interrupt to a buffer.  And today, I decided to add a video component to the SBC. A Texas Instruments TMS9918A VDP (Video Display Processor). It outputs a composite signal to a standard-definition television. Although hard to see in this picture, you can see a bit of text with lots of color fringing. I will need a good monitor to see if it's a problem with my cheapy LCD monitor, or the video signal itself. I did notice some jitter in the oscilloscope, so fixing this might not be so easy.

The time has come to add a keyboard. I scrounged up an old Radio Shack TRS-80 Model III last year. Alas, it arrived broken. The tube was completely smashed! Thanks, UPS! So I decided to repurpose the microcomputer to become my future casing for the 6502 SBC. Not sure what I'm going to do for the final display. I did get my hands on some TMS9918 chips and the Parallax Propeller. But that's another post for another time. For now, the keyboard circuit is all done. It's fairly simple and the 8x8 keyboard matrix is directly connected to port A and port B of a second VIA chip. It did take me a while to map out the keyboard matrix. What is need about this keyboard is the integrated reset button. TRS-80 Model III Keyboard Matrix 12KE010C (or is it I2KEOIOC ?) ALPS ELECTRIC CO., LTD. JAPAN 20 pin connector 9 10 11 12 13 14 15 16 1 @ A B C D E F G 2 H I J K L M N O 3 P Q R S T U V W 4 X Y Z 5 0 1 2 3 4 5 6 7 6 8 9 : * ; + , < - = . > / ? 7 Enter Clear Break Up Down Left Right S