6502.org

Well, this is embarrassing: I finished the PS2 keyboard state machine and ready to test it out, but the board current-limited at 1amp when the PS2 CPLD is inserted. Poke around a bit and realized the PS2 CPLD's VCC are connected to ground and grounds are connected to VCC! It turns out I've rotated the symbol 180 degree to facilitate the hooking up of the JTAG programming signals, but didn't realize ground terminals are now at the top and VCC terminals are at the bottom. Oops. Probably fried the CPLD and definitely need a new revision of pc board. In the meantime I'll cut out the VCC/GND connections and rewire them so I can continue testing.
Bill

Ouch is right. This is worse than reversing power leads to a board, which I've also done a few times. I need to cut several traces and patch twice as many wires to undo the power reversal. At least it is a two-layer board with exposed power/ground traces that I can cut into.
Bill

I'm able to swap VCC and ground connections with 5 cuts and 9 jumpers. The CPLD actually survived the reverse voltage, setting current limit at 1amp probably had saved it from destruction. The PS2 keyboard controller appears to be working. I'll start a new topic about CPLD-based PS2 keyboard controller.
Bill

Received rev2 of VGA6448 from JLCPCB. It fixed the power/ground mix up, but still has one missing connection which can be easily patched with a jumper wire. It is working well. I now have a 2-board standalone 6502 computer using CRC65 + VGA6448 on a RC2014 backplane driving a VGA monitor and PS2 keyboard. I'll create a separate topic for the standalone 6502 project.
Bill

It is 4K dual port RAM, IDT71342. Available from utsource for about $2.50 each in small quantity. One side of the dual port RAM connected to 6502 as write-only memory and the other side is connected to CPLD as read-only video data and font.
Bill

I showed a "piggyback VGA" prototype on May 26, 2021 post of this thread. I like the way it looked and thought about how to make pc board version. This is what I come up with after 7 months.

It is two board assembly. The bottom board is 128Kx8 RAM base that plugs into 128Kx8 DIP RAM socket. The top board is CPLD, 4K dual port RAM, 25.175MHz oscillator, and VGA connector. It works, but sadly I still like the prototype better, but at least the pc board version is more producible.
Bill

Edit, I'm working on a new 128Kx8 RAM base that has another CPLD in quad flat pack format. That CPLD will handle the PS2 keyboard interface. This way the 2-board assembly can generate text VGA output and receive PS2 keyboard input.

I totally get it, Bill. The new version is a lovely piece of work. But I, too, prefer the original. "Who says I can't just do it like this...!"

-- Jeff

The prototype harks back to the earlier days when I did lots of prototyping. Those were the Wild Wild West days when we had tried many different ideas, some even worked. That's how innovations came about--trying a lot of ideas. I remembered your initial comment was: I got that a lots, especially the "hideous" part.
Bill