sburrow wrote:
Did you every happen to post schematics? Can't remember.
You know, I guess I didn't! I had in mind that I did, but looking back I only see a couple of sketches of the glue logic. It's a pretty simple design though; There's a 25.125MHz oscillator feeding a 74HC163 prescaler. There's a 74HC688 8-way comparator that places a 128byte I/O window in memory. This is based on Mike McLaren's moveable and resizable decoding scheme, but I was running low on board space so mine is just relocatable, not resizable.
There's a 74AHC138 to generate the I/O chip selects and a 74AHC139 to do RAM and ROM selects and PHI2 qualify memory writes. There's a 74HC04 for inverting A15, but I'm only using one inverter, so I could replace it with a 74HC00 if I needed some NANDs for some reason. There's also one empty IC socket that is destined to hold a 3-way NAND (HC11, I think) that I will need to generate the SHIFT/LOAD signal for my shift register when I get back to working on the VGA circuit. Right now I'm just using that empty socket as a handy way to get +5V/GND for my I/O devices and probes.
The big chips are a 65C02 (jumper configurable for WDC or non-WDC), one 65C22, a 28-pin ZIF for EEPROM (jumper configurable for 27c256 or 28c256), and a narrow 28-pin SRAM (a nice fast one at 12ns).
Quote:
Is PB-1 the computer you are using with PAGMON?
Yes, although I should be able to move it to Blue April with minimal fuss. The idea of PB-1 is to give me something that's easier to expand / reconfigure / interface with for the VGA terminal without having to have the whole thing on breadboards. When I got to the point of adding the CRT controller, dual-port RAM, processor, SRAM, ROM, plus VGA output stage, etc., it was just too big for breadboards. At least, for my breadboards. No doubt Radical Brad could have done it.
Blue April is made out of RC6502 cards. You would think that a backplane design would make it easy to add / tinker with stuff (I thought that!) but the truth is that it's kind of tricky because everything is broken out on to separate PCBs. It's not so easy to, for example, add a second VIA. The one on the "project board" is jumpered so that it's either on or off the IRQ line, and a second VIA would have to come in on its own card, which means there's no way to AND their IRQ lines together before they go on the bus. I'd have to solder a diode across the jumper, which is less than ideal.
Since the CRTC is controlled by software, I thought it might be useful to have a real machine monitor so I can send commands to it without having to keep reprogramming the EEPROM. That's been kind of lurking in the background for a while, and since I was more or less laid up last week it seemed like a good time to work on getting a really useable version of PAGIMON. At that point (two weeks ago or so) all it could do was scan through memory and jump to an address - no editing features. It's quite a bit more useful now, although I think the code could use some improving. I should get the last few installments out in the next couple of days.
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