Jump to content

TI99/4A Motherboard Replacement (In Progress)


Recommended Posts

  • 1 month later...

Just thought I'd give this a nudge and see if there's anything new going on? Hope all is well over there?


I've been playing around with Sprint Layout a lot lately and making gerbers of some rare arcade related stuff and I'm starting to get tempted to do one of the motherboard and make some mods of my own, mainly just making the board in two sections so it'll fit in smaller cases and maybe changing some of the connectors, though I don't know if I'm up to the task of incorporating upgrades like internal 32k and such. Is there a gerber for the original motherboard available anywhere?

Link to comment
Share on other sites

  • 1 month later...
On 11/4/2021 at 7:06 AM, Musicdad said:

How about doing a SMC version. This would clear upnALOT of space for built in upgrades on board. I am little excited to try your new board. When will you have Gerber files available? 

I am still needing to sort out the DIP version.. baby steps ;)

So, I finally found a bit of time in that magical center of the Venn Diagram of (Space on my workbench) and (Motivation to work on the TI) this morning.  It looks like there is an issue in the system GROM area, as GREADY is constantly being pulled low.  The GROMs themselves work fine in another system and the 74LS04 inverter chip tests fine, but I did swap it for good measure with no change. I ended up getting called into work at this point, but diagnostic progress has been made.

I need to dig out my other power supply so I can power both the good original PCB and the non-working new PCB at the same time and do some proper side-by-side measurements.

I am thinking about cleaning up and publishing the schematic portion of the KiCAD project up on my Github, so that I can get some more eyeballs on it.  I am thinking I am just missing something really small here.   Since the PCB layout is not working, I will not publish that part quite yet.

Edited by rkrenicki
  • Like 3
Link to comment
Share on other sites

I did stick the schematics and the base project file up on my Github here:  https://github.com/rkrenicki/TI99-Motherboard


It is in KiCAD 6.0 format, so you will need to be on at least that version to open it.   The layout has not been cleaned up.. about half of the hierarchical sheets are going to go away in the end, but I wanted to focus on getting the errors figured out before fixing the layout.   If anyone wants to peruse the schematics and sees anything amiss, let me know!

  • Like 4
Link to comment
Share on other sites

17 hours ago, rkrenicki said:

If anyone wants to peruse the schematics and sees anything amiss, let me know!

Hi rkrenicki I took you up on your request to peruse the schematics.  I compared them against the MainByte Copy of the TI-99/4A Console Technical Data:

  • CPU - looks good
  • Address Decoding - looks good
  • 8/16 Bit Switch - looks good
  • ROM / RAM / Sound - looks good
  • Power - didn’t check
  • GROM Logic - Looks good
  • Video - checked it over some, looks good although I skimmed some of the power feed
  • Keyboard - didn’t check
  • GROM IO Port - checked a little bit of this, it’s spread out on the schematics a trickier to validate than the others


Tiny thing on the 8/16 Bit Switch, U615 is missing a bypass capacitor on VCC.  I'm more or less putting this here to convince you I actually did look at the schematics ?




Also a question, I noticed this arrow on the main byte schematics, south of the grom's and east of the memory address decode.  Probably just a trivial artifact.  Wonder why that cute little arrow is there ?




I’m still a newcomer, but my eyeballs agree with you that the schematics look pretty solid, and I'm still a little nervous because fabrice has stated outright that there are lots of errors in the official documentation, so I assume you will have to work through those, and I'm wondering how many of the "errors" are small things like component values, and how many are larger leaps like whole components and /or connections wrong or missing! :? 


Good luck and god speed, if you have any other specific areas you would like eyeballs on please let us know.


  • Like 3
Link to comment
Share on other sites

U615 does not have its "own" bypass cap on the original documents, however C608 (0.1uF), C601 (1nF), and C609 (0.1uF) are right there between the CPU and U614-616, acting as bypass capacitors for all of those chips.  These are all powered by the flood-fill on an inner layer, so they are all bussed together.


410639155_ScreenShot2022-02-19at11_15_40AM.thumb.png.7036cb95da44ef9dbbc85b4362b7f7a8.png.  853418972_ScreenShot2022-02-19at11_16_30AM.thumb.png.1333e79f54b636ccc4f7ec9e707b6a29.png


I am not entirely sure of the significance of the Arrow there either.

As for the Power sheet, you will note that I have removed a bunch of ferrites and coils from the power input.  I did try both with and without those extra filters and I found no significant difference on power delivery, so I chose to omit them.  I am not nearly as afraid of EMI compliance as TI was back in the day.   The Video sheet will look a little different as well since I tried to incorporate both NTSC and PAL options.

As for the rest, I did catch a few errors.  It was about a year ago since I was working them, but an incorrect voltage reference comes to mind, a couple of value issues as well, but nothing too out of the ordinary.   

Edited by rkrenicki
  • Like 6
Link to comment
Share on other sites

  • 9 months later...
  • 9 months later...
  • 1 month later...

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Recently Browsing   0 members

    • No registered users viewing this page.
  • Create New...