PET startup sequence?? (was LF: Commodore PET schematics, troubleshooting info)

On Wed, Aug 11, 2004 at 08:04:28AM -0400, Dave Dunfield wrote: You are welcome. The video controller is only present on later models. The earlier video is TTL and non-programmable. Specifically, you should check documents related to the 4016 (16K 40 cols), 4032 (32K 40 cols), and 8032 (32K 80 cols). These all use the same board, but with different ROMs and different jumper settings. I do not happen to know that. I have run across kernel disassemblies on funet. You shouldn't have to do it all I just fired up 'xpet' (part of VICE) and got the following: 0x20 space 0x30-0x39 0-9 0x41-0x46 A-F So I guess it's reasonable ASCII-like. I just made the caution because the strangeness of the PET character set(s) throws people if all they've ever worked with is genuine ASCII. Standalone? No. AFAIK, there's never been a case where someone ripped out the C= kernel and replaced it with 100% home-grown code. There _is_ a monitor program included with all PET ROMs except the first version (from old 4K/8K SRAM PETs). The BRK vector points to it. If you execute a BRK instruction (0x00), that's where you end up. If you do code your own ROM for the $F000-$FFFF block, remember that there's a hardware interrupt at 60Hz on the IRQ line. You'll need an ISR to catch that. In the real ROMs, that's what scans the keyboard matrix, updates the real-time clock, and blinks the cursor (it's not hardware, like on an Apple II). To use it, there has to be enough of the machine working that the keyboard is scanned, and the kernel and editor ROMs ($Fxxx and $Exxx) are present and functional. I had a BASIC ROM die on my 2001-N and it dropped into the monitor at power on. That reminds me, there is a 'diagnostic pin' on the user port (it's documented in most of the user-level docs of the day)... if you reset the processor with the diagnostic pin shorted to ground, it aborts much of the POST logic and jumps to the TIM monitor program. If BASIC is FUBAR, but you have RAM and upper ROM, that should work for you. Something else to check, while I'm thinking about it, is the reset circuit itself - you could wire a manual reset button onto the CPU and give it a kick after powering on. I have lost the message with your exact symptoms, but if it acts totally dead, the processor might not be getting its reset line pulled down briefly at power on. It's probably just a simple RC network hanging off of a 555 timer, acting as a one-shot. PETs, BTW, unlike VIC-20s and C-64s have no provision for user-installed firmware from taking over the machine on power-up (like a game cartridge). You can install 8K-12K of your own stuff, but you have to manually jump to it when the machine finishes POST and gives you the "READY." prompt. That would be a reason to install patched kernel ROMs, but I don't recall it being common in the old days... instructions for that sort of stuff were to turn on the machine and type "SYS X" to jump to the dedicated application. I can't do a lot from here, but recently I got a pod for my Fluke 9010 for the 6502... that would be really handy for checking things out, testing RAM, checksumming ROMs, etc. It's a shame the RAM isn't socketed. That would be an easy thing to test, then (and easy enough to replace with a 62256 SRAM wired into the sockets since ISTR your expansion bus has something on it already). Fortunately, screen RAM is completely separate from the 32K of DRAM. On the old 40 col boards, I think it's a pair of 2114s. Not sure about the 40/80 col selectable boards. I never had to debug video RAM. The machine would still work, but the visible output would be funny depending on how many bits were bad. One other thing to do might be to build a NO-OP tester - that's a 6502 with all 8 data pins bent up so they don't sit in the socket, then certain bits are pulled up and certain bits are pulled down to force an 0xEA on the chip for all read accesses (a NOP instruction). The processor free-runs starting at address $EAEA, then cycles through all addresses so you can watch various select lines on I/O chips and see what pulses and what does not. It won't help diagnose dead RAM chips, but it will help test all address select logic. -ethan -- Ethan Dicks, A-130-S Current South Pole Weather at 12-Aug-2004 08:10 Z South Pole Station PSC 468 Box 400 Temp -66.4 F (-54.7 C) Windchill -101.9 F (-74.40 C) APO AP 96598 Wind 11 kts Grid 088 Barometer 685.2 mb (10432. ft) Ethan.Dicks(a)amanda.spole.gov http://penguincentral.com/penguincentral.html