Posted By
TLC
on 2011-02-16
15:38:04
|  Re: Plus/4 C16 diagnostics and TED partial failure modes
There's probably only one "typical" partial TED failure (the one siz has already pointed out). There's a "latch", sampling the K0-K7 lines at the moment of writing $ff08. ...Under normal circumstances, you'd use that feature to select either the keys, the joysticks, or both, to be scanned. (Writing something different from $ff to $fd30 selects some part of the keyboard matrix, doing so with $ff08 selects joystick(s), either or both; you can get a picture about how the joystick port scan works if you check how the joyport select' signals are generated). You can "kill" that latch in the TED (with some shock or so), but unlike you'd guess, the keyport doesn't die then -- it'd become "transparent" instead. Ie. no sampling happens at the moment $ff08 is written to... you'd get what's on the port anytime you read $ff08.
The keyboard will still work in that case, but not the joysticks... these will obviosly stop working, unless the respective select' signal of the port (...or the joystick adapter) is hardwired to GND.
At the heart of Solders "TED protector", there's a bus driver chip which is gated by the select' line. That way, no shocks could come 'round from the joystick and harm the TED keyport. (...In fact, it does more: from that on, the digital part of the standard DB-9 joystick port provided by the adapter becomes fully "C64 compatible", in the sense that it has fixed GND and VCC... unlike the "wired" joystick adapter, whose "GND" is the select' line... So with Solders circuit, "any" C64-specific gadgets will also work with the Plus/4... one good example is the 1350 mouse).
I could only name some very rare other "typical" TED partial problems (if at all...). I have seen maybe two TEDs whose "24-rows screen" capability broke. (Ie. even though you clear bit 3 of $ff06, the top and bottom 4-4 rasterlines won't disappear ie. won't be covered with border). One TED whose "blank screen" capability (again, $ff06, bit 4) acted fuzzy... sometimes the screen "under" the blanked screen flashed in for a frame...  One single TED that had a stuck bit somewhere in its color logic (one bit of the color code of the characters appearing on the screen became "1" regardless of the contents of color ram). One TED that had serious problems addressing the screen memory, still maintaining all other functionality (screen, colors, registers, all -- but that one doesn't count, that chip has been a victim of some experimenting ie. most probably a shock :-/, not regular operation). All the rest, hopelessly dead, with no signs of life at all. Way less typical than known examples of partially dead VIC-II and SID chips.
(...Thinking that over, that's probably one of the reasons I never started designing a test cartridge myself... If you open a faulty 264-series machine, you have about 70-80% percent chances to find a dead CPU, with the rest being PLA, RAM, and TED. From that, all but the RAMs are socketed...in the worst case, you can test all of them whether they work by some swapping to-from a known-to-work machine. (...True, finding out which RAM chip from the bank of 8 chips has failed would be handy...) If there's anything more/different that you find blown, that's usually a strong sign of the machine having been a victim of some shock... shorting pins around the User Port, tape port, joystick ports, accidentally or intentionally, whatever... As a rule of thumb, ROMs, the 6529Bs and most TTLs basically never fail -- unless the component was subjected to some shock.)
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