I've been curious about building a 6502 system with a pair of VIA's, rather than the usual 6551 paired with VIA's. The idea is two fold: first, side step the 65C51 bug. second, the hardware forces you to try self-hosting, rather than being reliant on a host PC. As luck would have it, there was an auction for an APATCO NCS-EB50 on eBay which has this hardware configuration. So I purchased it to give it a try.
But there is a problem. The documentation included with the NCS-EB50 is thin and missing obvious information like a parts list. I assume that's because APATCO intended for the PCB to be an add-on to their NCS-2056T breadboard computer kit, rather than a standalone product. However, the NCS-2056T is perpetually out of stock, and APATCO's contact us email link is broken. So I am somewhat on my own with the PCB.
The documentation did include a memory map and a schematic, so I am using the latter to build a parts list. I've also uploaded everything to Google drive including my parts list in progress.
See:
https://drive.google.com/drive/folders/ ... sp=sharingLooking at the memory decoding logic, it looks identical to Daryl Rictor's SBC2.
But there are some things I find odd about this schematic compared to other 6502 SBC's I've seen:
* The diodes for the VIA's IRQ line are 1N5817 rather than the usual 1N4148 or 1N914. I have a bunch of the latter so I am wondering if I can substitute?
* Each IC has a pair of capacitors in parallel between Vdd and ground. I assume this is for decoupling the IC's. Normally most schematics only have a single 0.1 uF capacitor. But the VIA 1 for example has C7 0.047 uF and C11 1.0 uF. Likewise the RAM has C3 performing a similar function, but it has a value of 0.056 uF!
* If the small capacitor is a decoupling cap, what is the purpose of the 1.0 uF?
* Resistors R3 through R13 look like pull up or pull down resistors, but the 3K value is slightly less than the 3.3K used in the SBC 2. Is this reasonable? But that raises the question why 3K versus 10K which I've seen used as pull ups in other applications?
* The data sheet claims you can use either a W65C02 or a W65C816S. But I thought the W65C816 wasn't a drop in replacement to the W65C02 due to asserting the high address lines on the data bus. Would this actually work?
* Having the choice of a TTL oscillator or crystal oscillator circuit is a bit odd.But I guess it is related to the educational aspect of the kit.