Hey folks. :D I am getting ready to (try to) assemble a 6551 board to interface to the Rpi, since I have no super serial card and the A2pi board is out of stock.
So, on the schematic for the A2pi here: http://schmenk.is-a-geek.com/wordpress/?p=88
... I see no capacitors. However, looking further down at the photos of the device on perfboard, I see two capacitors soldered on there. I also see them on the commercial board here: https://www.ultimateapple2.com/catalogzen155a/index.php?main_page=product_info&cPath=1_31&products_id=126 and here (although there is only one on this board): https://www.callapple.org/hardware/ultimate-apple-ii-announces-last-apple2pi-run-for-2017/
I assume these are just decoupling capacitors for the ICs? But they are not very close to the ICs on the prototype...... If so, what values would be appropriate here? 100 nF?
Sorry for the dumb questions. I know a bit (enough to get in trouble, haha) about digital logic in theory, but I'm fairly ignorant when it comes to implementation.
Use a 0.1uf decoupling capacitor across each IC. (there are 2)
I don't come to AppleFritter very often. Thanks for posting the correct answer! A bigger issue is finding a working 6551. You will be better off buying a working pull or older 6551 rather than a new WDC 65C51 as they have a bug that renders them pretty much useless.
FWIW, a lot of hand drawn schematics leave out little details like the decoupling capacitors because they are just kind of assumed by people. I don't really think that's a great practice, but especially back in the old day, it was a common one.
The problem with getting working 6551 chips has led me to start work on a design for an Apple II RPi interface board which uses a completely different interface chip. The chip I'm using is readily available and also significantly faster (at least 2x) than a 6551. The only downside is it will require a bunch of work on the software on both the Apple II and RPi side because the chip works completely differently than the 6551.
I have 5 Motorola-stamped S6551 chips. The place I bought them from only had 50-some in stock, so I bought 5 so as to have some extras in the future if they disappear, heh. Makes me sad that 6502 stuff seems to have stopped being made while I was out of the loop. :(
Surely microcontrollers are getting fast enough to interface directly to the Apple ][ bus now, too, and one could make a generic uC card that could be programmed to emulate whatever board one wanted, within reason. Or maybe not, or everyone would be doing it, haha. This is something I'd like to try, but I fear I am too dumb to do the hardware side of it correctly.
Microcontrollers, not really. Most of them you still need some kind of interface between them and the Apple II data and address busses. However, I think what you are really looking for is an FPGA. That has been done, like the Carte Blanche card. With the right FPGA code you can make it do what a lot of other cards do. However, that card was fairly expensive and never made in quantity and as far as I know is not only no longer being made, I believe that the FPGA chip it used is out of production and getting scarce. Now, the same idea could be done with FPGA chips that are currently in production which are less expensive and more powerful to boot. The problem is virtually all of the current chips are 3.3V or lower and most are not 5V tolerant, which means you have to provide level conversion (74LVC245 typically) on the inputs and outputs. That can be done. I've actually been contemplating doing something like that with the TinyFPGA products which are based on the Lattice FPGA chips (other popular FPGA brands are Xilinx and Altera).
The other thing that I think has kept something like the Carte Blanche from taking off is that FPGA programming is non-trivial to learn and requires some dedicated software plus the tools are fairly hefty (not something that would ever run on the Apple itself I'm afraid). I'm certainly not there yet. I've got a couple of CPLD and FPGA developer's boards but I'm in the beginning stages of learning it. I haven't really mastered PAL/GAL programming yet and that is simpler than CPLD and FPGA.
Now if you wanted to use a microcontroller like an Arduino (Atmel processor), one way to do it would be to use an FPGA like the TinyFPGA A2 and implement the SPI bus with it and then connect that to the SPI on the Atmel. That would give you a high speed interface and you could program the Arduino/Atmel to do whatever you want and control it from the Apple II like it was a native peripheral. Other microcontrollers could be used like ARM (used in Raspberry Pi, etc), ESP8266 or ESP32, or even something like PIC or Propeller.
Here is some info on the Carte Blanche II... http://www.applelogic.org/CarteBlancheII.html
This is the TinyFPGA site: https://tinyfpga.com/