Almost have a 2708 dumper working
I have an interesting... thing... with a bunch of 2708 chips in it, and I want to read them. But I'm very wary, because I don't want to fry them.
The problem is that the 2708 needs extra supply voltages of -5 and +12 volts just to read a chip, but it also needs +26V for programming. The extra cost of all the extra voltages means almost zero programmers support it without extra hardware.
A few months ago, I tried to do the simple idea, add a socket board with some pins changed so that I could try to read it on a regular EPROM programmer as a 2716, but get the voltages from a couple of wall warts. I've heard that not having the -5 volts is crossing-the-streams-bad, so I put LEDs on both voltages. I had one other 2708 chip, and its contents were already known and it could be replaced with a 2716. It didn't go well. I got a few partial reads, then the chip died.
So on to version two. Start with making a Blue Pill board read a 2716 chip. It was not a fast process, made slower because I'm trying to break away from the mbed web site compiler. I tried to get the old version of mbed working offline, but crappiness related to Python 2 vs 3, and Apple change-protecting chunks of Python 2.7's package storage made it impossible on my laptop. And the current version of mbed is super bloated, it includes all sorts of crap that makes a minimal program be like 30-50K. Eventually I not only got my own code working, I also got gdb working in text screen (ncurses) mode... more or less, but enough to be better than printf debugging through a serial port that you don't have working yet.
(To make things even more fun, a USB serial adapter I got from Amazon seems to have a bug when its internal buffer overflows before opening up the serial port on your computer, then it spews everything forever. Sometimes it even barfs out garbage with debug symbols.)
Tonight I finally got it to dump a 2716 chip. I had been poking at the various pins, and nothing would output data. So I started a new breadboard with a bunch of LEDs and DIP switches. After more poking around, finally the LEDs come on, and I recognized the bytes at 0x000 and 0x0FF. Turns out that the pin 21 (Vpp on 2716) needs to be connected to +5V, not GND. Now I just need to set up the extra voltages and cross my fingers. Maybe tomorrow.
Edited by Bruce Tomlin
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