I've made some progress on the TI-990 :3
tl;dr is that the PSU works fine, I tested the front panel and some cards, and I'm just waiting for some supplies to get delivered so that I can start dumping the PROMs
so, first thing's first, I checked the voltage rails on the backplane and everything seems nominal! I also accidentally shorted one of the +5V rails to ground when testing, which caused it to go from reading 5.02V to a dead 0, which means some form of safety circuitry in the PSU is working too!!
anyway, I also got to cataloguing all the PROMs present in the system, which totals to 38 different chips, as well as a couple ROMs for ASCII character generation in the VDT911 interface board and at least one FPGA in the DS10 controller. that is a lot of irreplaceable chips!! (well, the ASCII ROMs were standard TI parts, so replacements wouldn't be totally impossible, but everything else is very much not standard)
annoyingly, most of the PROMs (26 I think) are soldered directly to their PCBs, so I'm gonna have to get some more desoldering practice in lol
fortunately, the 4 PROMs from the SMI/M ("System and Memory Interface with Mapping") board which implement its firmware are all socketed, so I went ahead and pulled them out (after labelling them with my Dymo), and here they are, with a closeup of the one with some corrosion
doing this cataloguing, I figured out that both of the half-width cards (the TTY/EIA interface and the PROM programmer controller) don't actually have any, and the PROM programmer controller actually has a broken connector on the front of it (not like I can use it anyway, since the programmer that it communicates to didn't come with my system), so I decided to use it to check whether the PSU is completely okay by plugging it into the backplane and powering it on lol
it didn't explode and the voltages coming into it seemed good, so I'm pretty confident that the PSU is good :3
anyway, now that I know the PSU is good, and that the SMI/M board has all its irreplaceable chips removed from it, what if? I put them together??
the answer is this!!!
blinkenlights!!! that don't actually blinken (yet) lol
except! when you press the HALT/SIE button, the RUN light switches off!!! (i forgor to get a photo of it and can't be bothered to lmao)
when I saw the LEDs light up for the first time I was jumping around with excitement lol I was even more excited when I saw that it can actually respond to an input (though I took these images later, since the first ones were when it was super bright outside)
as you can see in one of the photos, I've also hooked up my 'scope to check the clock signal, which looks like this:
the settings on the 'scope for this shot was: 2V/div and .2µs/div, so it seems the clock is running at around 2.5MHz with a peak of about 3.8V. plus or minus a massive margin of error. I'm not even 100% sure I hooked the probe up to the right leg of the chip, cos I did just that when I was first checking it, and I wouldn't be surprised if I did it again
anyway, I'm now waiting for some supplies to arrive so that I can put together a reader for the PROMs, and make images of them, and then I can start testing for actual functionality of the machine and I'll update once I make more progress :D














