Big Mess o' Wires
Apple II

Disk Drive Testing

July 26, 2021 Big Mess o' Wires

While I await the new Yellowstone v2.1 PCBs, I’ve been spending more time on testing. With my hacked-up v2.0 PCB, most things are working pretty well, especially with DOS3.3, ProDOS, or game disks. I can connect any odd combination of drives to the Yellowstone card, and expect that it will …

Big Mess o' Wires
Apple II

Back-Powering and Phantom Power From JTAG

July 23, 2021 Big Mess o' Wires

Throughout Yellowstone’s development, I’ve struggled on and off with mysterious problems related to the JTAG programmer. When the programmer is connected to the Yellowstone card, and I turn the computer off and on again, strange things sometimes happen. The Apple IIGS will boot to a screen of green and pink …

Big Mess o' Wires
Apple II

Back-Powering and Phantom Power From JTAG

July 23, 2021 Big Mess o' Wires

Throughout Yellowstone’s development, I’ve struggled on and off with mysterious problems related to the JTAG programmer. When the programmer is connected to the Yellowstone card, and I turn the computer off and on again, strange things sometimes happen. The Apple IIGS will boot to a screen of green and pink …

Apple II

Four Years of Yellowstone

July 22, 2021 Big Mess o' Wires

July 21, 2017 – I began the first work on what would become Yellowstone, with some attempts at designing a Verilog model of Apple’s IWM chip. Four years have passed since then, and I’m still working on this project! Today version 2.1 of the Yellowstone PCB is nearly ready …

Big Mess o' Wires
Apple II

Yellowstone Ground Noise and Bypass Capacitors

July 21, 2021 Big Mess o' Wires

I’m working on what I hope will be the final hardware changes for the Yellowstone 2.1 PCB. Among these are changes to my capacitor usage and placement strategy. In my previous posts I discussed intermittent problems that looked to be caused by ground bounce, VCC sag, or both. Apple II …

Big Mess o' Wires
Apple II

Current Limiting, Part 3

July 18, 2021 Big Mess o' Wires

I’ve been searching for a simple and reliable way to prevent a short-circuit on Yellowstone’s -12V supply when a particular type of Macintosh disk drive is connected. Connecting this drive results in a +5V to -12V short circuit, but if the short is removed, the drive otherwise works just fine. …

Apple II

Current Limiting Strategies (Again)

July 14, 2021 Big Mess o' Wires

I’ve returned to the question of how best to limit the current from Yellowstone’s -12V supply. I discussed this earlier here and here, and had more-or-less settled on the idea of using a current mirror circuit. But now I’m taking a second look at possibly using JFETs or a purpose-designed …

Apple II

FPGA In-System Programming: Beginnings

July 9, 2021 Big Mess o' Wires

I’ve written several times about Yellowstone’s need for in-system FPGA reprogramming. Once the Yellowstone card is in the user’s hands, the user needs a way to update the FPGA firmware when new versions are released, without requiring a JTAG programmer or some kind of USB interface built into the …

Big Mess o' Wires
Apple II

Yellowstone Option Configuration Design

July 8, 2021 Big Mess o' Wires

The Yellowstone disk controller’s whole reason for existence is its ability to control any type of Apple II disk drive, but for a few Apple II programs, this universality causes problems. Yellowstone handles standard 5.25 inch drives just fine, but some software gets confused when it doesn’t find the ROM …

Apple II

Transistor Puzzles

July 2, 2021 Big Mess o' Wires

I’ve fallen into a deep hole involving current-limiting circuits and current sources, in an attempt to solve a minor problem with the Yellowstone disk controller. In the big scheme of things, this isn’t a very important problem, but it has me intrigued. I’ve created a simulation of a circuit …