TI 99/4A Weather Station

If you still have a drawer full of slap bracelets from the 1990s because, you know, they might come back, then you’ll appreciate [Vorticon’s] latest project. Sure, we see lots of weather stations, but this one is controlled by a TI 99/4A computer. This home computer from the 1980s was actually ahead of its time with a 16-bit processor.

The sensors use Xbee modules and an Arduino Uno. Of course, the Uno has more power than the TI computer, but that’s not really the point, right? He’s made a series of videos detailing the construction (you can see the first one below, but there are five, so far).

Normally, talking serial to the Arduino would require assembly language on the TI computer. However, TI hacker [Rich Gilbertson] already fixed that. He created Rich Extended BASIC (RxB), which has a CALL IO statement that was perfect for [Vorticon’s] needs.

The TMS9900 CPU had a novel feature where subroutine calls caused the registers to shift so that each subroutine had some registers in common with their caller and some that were private. This sounds good on paper, but the registers residing in main memory was a death sentence as processor speeds increased (the 9900 in this computer only ran at 3 MHz). It also had a sprite-based graphics processor that was used to good effect in games.

No matter how impractical, we love these old retro projects. Even though a new owner of a TI 99/4A back in the 1980s would have been shocked at how much computing power you can buy for the cost of a good meal now, the TI was still capable of some impressive output for its day. While it might not be able to play Doom or Call of Duty, it can handle Flappy Bird.


21 thoughts on “TI 99/4A Weather Station

  1. At the time, it was a ripper, and the big 64 pin DIP was impressive. The TMS9900 instruction set also included a register-programmable single-instruction 16 bit rotate, using a barrel-shifter. That was very nifty for a compact and fast implementation of the CORDIC algorithm for generating sin and cos. And being able to save all 16 registers in one instruction, by simply pointing to another set of 16 registers in RAM (a “context switch”), was also a speed enhancer.

    A FPGA reproduction with fast on-chip RAM would make the design fly again, especially if it were made Harvard, i.e. separate data and program spaces, for simultaneous fetches.

    1. There were versions of the TMS9900 that had a Harvard Architecture for 64K for program and 64K for data. There was even one that could have three 64K banks: program, data, and macrostore. The macrostore allowed adding custom instructions to CPU.

      1. And of course, it’s a microprocessor version of TI’s minicomputer.

        A friend had a TI development board for the TMS9900, so the TI 99/4 wasn’t the only “home computer” that used the microprocessor (actually, I think a third party used it too). The development board had a keyboard and readout encased in a TI calculator housing. Familiar, yet different.


        1. There was the MBX, Milton-Bradley Expansion for the 99/4A. It offered a large kyepad, a headset with speaker and microphone, better joysticks and some other stuff. There were only a dozen or fewer game cartridges made for it and IIRC only 2 or 3 of them actually required the MBX to work. The rest would work on the computer without the MBX, and without whatever features of the MBX they could use.

    2. Thanks for the CORDIC reference! Fascinating reading.

      Loved my 99/4A .. really seemed head and hands above the C64. I vaguely remember buying it. There was a massive push that year around XMas, and Commodore​ and TI each had reps in the store, touting their hardware. I think the TI was $99 .. hard to beat.

      Somehow, TI seems to have a knack for making slightly better mousetraps in the consumer market and then losing the sector. The 99/4, the TI-PC, the Beagle*** stuff etc. Speak and Spell and their various calculators (locked in through teaching materials) seem to be their only consumer successes.

      1. The Texas Instruments Professional Computer was an IBM PC (IIRC, PC not PC/XT) clone with an enhanced video card they called “Three Planes”. Very little software was written to use the fancy video capabilities, and much standard DOS software had issues with it. Replacing the TI card with any standard compliant text, Hercules, CGA or EGA card converted the TI Professional into an ordinary PC clone.

        The people who started Compaq, and who went on to beat IBM to market with the first 80386 computer, all came from TI.

        For years, Texas Instruments made laptops, and they were nearly always rated #1 by the major PC computer magazines, even beating IBM’s ThinkPad line. But then for some reason, TI decided to chuck all that in the bin, ceding the #1 spot for top line business laptops to IBM, selling their laptop line to Acer in 1998. The TravelMate name is still used by Acer.

  2. Atmospheric pressure is usually displayed as inHg, not KPa! Especially when you are using MPH for the windspeed. :P

    Nice project. I want to make a weather display, but am too lazy.

  3. The sprites were a very nice and advanced feature for the time. Compared to the hoops you had to jump through to get something similar going on a ZX Spectrum (which I owned).

  4. I love the 1990’s interface. Who would ever know the wind speed was requested when the result only says “wind speed = “.

    A 1990 version of novice programmer pretty-printing in BASIC. Great touch :-)

  5. An enhanced FPGA replacement with VGA output for the TMS 9918A, 9919A and 9929A. http://codehackcreate.com

    9918A is composite NTSC out, 9919A is composite PAL out (seems to have been very rarely used), 9929A is PAL component out- used in European 99/4A computers. If there was a 9928A component NTSC out version, I know of nothing that used it.

    The F18A replaces them all, original video output format doesn’t matter because it only outputs VGA. Who wouldn’t want VGA out on their MSX-1 computer or Colecovision?

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