Let's Get Serial!
This week was a bit busy between work for travel and my anniversary, but I did manage to get some time in on the NEON Sunrise.
I had fully intended to get the 6551 installed and working, but I guess that was biting off more than I can chew.
I didn't clear out the old 4 button setup, I just swapped out breadboards. I think you can even see it off to the side in one shot. But I did set it up to use the WDC 65C51N Asynchronous Communications Interface Adapter (ACIA). That meant picking a space in the memory map for it, which for the moment I went with $6800, cutting the wasted space from the VIA in half.
I picked that space because it seemed like a reasonably setup for the chip select logic. There's already a NAND gate used for the inverter for the RAM addressing, so using that meant not adding three chips. I needed an CS Low chip that would work by combining the OR of A12 and A15, and then I could invert that and use a 4 input AND gate. I got it down to 3 chips, one of which already exists, so I'm pretty happy with that.
I do intend to learn to program a GLA or PLA later down the road and use that to eliminate some of the gates, hopefully to increase speed. Remember, I'm aiming for a full 10MHz in the end, but I'll be happy at 8MHz. For testing I'll still be running at 1MHZ or with the 555 timer.
One thing I didn't realize until later was that RS-232 uses a 12V data line! So since I didn't know I needed the line drivers, I didn't have any of the line drivers on hand. Those have been ordered from Jameco (#NotSponsored) and will be here this week. Then Episode 3 can be getting that wired in, and writing the code just to ping it over USB.
Looking at the MAX232 datasheets, it can either take the 5V and use caps to ramp up to 12V, or just take a 12V reference line. Since this will be based on ATX in the end, we will have a free 12V rail to use for this in the long run, and that makes it seem like an even better idea now. So go me for planning ahead.
I did, off camera, learn about making my own footprints for items in Eagle, and have since updated the 65xx footprints and the ones for the ROM/RAM to have slightly larger holes and the bigger pads to solder to. So the next revision of the board will be much easier to work on. I didn't bother with the 555 timer pads, since that wasn't so bad and I'm hoping to move to a full can crystal soon.
One thing I do want to make, is the Full Can to 555 Timer Board, which will plug into a Full Can Oscillator slot and let you do the full Ben Eater 555 timer setup in it's place. I'm sure someone's already made this, but it'll be something I want to offer up as a diagnostic tool.
Also on the learning block, adding graphics to the silkscreen in Eagle.