On 12/5/19 12:39 PM, Eric Smith via cctalk wrote:
Last year I read most of the PALs and HALs of a TRS-80 Model 16B main
board (same as Model 12 main board). I was surprised that none of the
PALs were protected, and even more surprised that some (but not all)
of the HALs could be read as PALs.
Eric, first, I want to thank you for the excellent M2 ROM disassembly
you did; while I was helping debug the MAME trs80m2 emulator your
listing was extremely helpful in tracing down the problem which
single-stepping in the MAME debugger (it was a bug in the WD1791
emulation of deleted data address marks).
Second, there is a published European patent on the Model 4 circuitry;
see http://www.freepatentsonline.com/EP0126305.pdf for the details. The
PAL equations and pinouts begin on PDF page 31.
The 12/16B/6000 mainboard's PAL equations are printed in the service
manuals for those machines; the 6000 service manual is available on
archive.org. But it is a bit surprising that the security fuse wasn't
blown; Jerry Ballard and Frank Durda were both unsure (years ago when I
asked them both) if the M4's PALs had the security fuse blown or not,
and at that time I didn't have the equipment to try to read them.
