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Dear all, this is what I came up with recently (focus is on the lower right ...): Executive Summary In this project, a 1981 TI99/4A is connected via its speech synthesizer module to Technic Control Interface A from 1986/7 using a simple bidirectional 8-bit latch parallel port interface comprised of two 74LS373 TTL ICs from the same era No address decoding required, instead, the PCB in the synthesizer is slightly modified and all required lines (D0 – D7), LR#, LW#, WE#, GND, +5V are wired to the empty upper compartment of the synthesizer module Direct access from TI BASIC (1979; mini memory module required) or TI Extended BASIC (1981; XB module required as well as a 32k memory expansion module) The interface is in full compliance with Evan Koblentz’ challenge, announced earlier here on EB as well as on his website, see below. A preliminary video showing the TI99/4A + synthesizer module in action is here: It shows my prototype board (which is now on perfboard in the TI's synthesizer module). In action means, there is speech synthesis and parallel output to 6 LEDs and 2 TTL inputs (which go directly 1:1 to #9750). I hate taking videos, as I constantly screw up, but will try to make a somewhat decent one … EDIT: Video ^^ is on Youtube: And here is the full boring story: During a trip to the US (SoCal) in May/June this year, I “accidentally” ;) googled “TI99” … … so here we go again: Motivated by Evan’s 9750 EuroBricks hacking challenge, I tried to get my 1982 dream computer TI99/4A in touch with TLG’s LEGO Interface A (#9750), what else to do with it ;) And yes, my posts in this regard may be annoying, but so are super cars. Or Cada stuff, BuWizz problems, ++$600 sets – of course >only< to me. Here is to feeling good! The TI99/4A is “beautifully weird” and a very nicely designed system, I fell totally in love with. I have these feelings after thorough studies of corresponding circuit diagrams and inspecting the main board myself. I had no idea what the story was back in the days; I did not know why TI entered the home computer frenzy ship in 1979 with the TI99/4 and abandoned it in late 1983, when they stopped manufacturing the TI99/4A, nor that 16-bit TMS9900 processors existed, and that the TI could speak … I do know now and it is truly a breathtaking story. Also the “inner logic” of the TI99 around the CPU is almost bizarre, but when diving deeper, it really is a very cool approach, descending from the concepts of making mini computers. As said, I never had one, nor saw one in reality before early summer 2024. Evan’s challenge rules are hack computers from 1992 and earlier, that were not endorsed by TLG to operate LEGO Technic Control Interface A #9750 (endorsed were: IBM PC, Apple ][, BBC Micro, Commodore C64, Philips P2000) only use contemporary hardware components up-to 1992; only use emulators on modern machines, but with no additional features other than available up-to 1992. This 1992 threshold originates from TLG’s release of their Interface B in 1993, which changed “everything”, details are on Evan’s website (https://www.brickhacks.com/2.php) For my very own purpose, I added a 4th rule: The Interface A must be operated with the BASIC language that came with the respective computer The latter is just my personal rule, as it a) makes it much easier to write programs for operating #9750 in contrast to using assembly; b) virtually all BASIC dialects have almost identical instruction sets, particularly PEEK, POKE, and or IN(), OUT, to get access to the hardware components of the system; c) assembly, which is a way more powerful language, both in terms of code execution speed and hardware accessibility, is mostly specific to CPUs due to their different internal hardware structure, as are the accompanying assemblers, linkers, and other software. I do speak Zilog Z80 assembly, but I had a hard time getting used to Intel’s 8086/8 assembly, and for Motorola’s 68000 (used in my Atari 1040 ST and others), I simply lost it. For the 99er, with its Texas Instrument TMS9900 CPU, everything is once again largely different … and I simply don’t have the time to learn all this stuff … well not yet ;) OK, enough story telling: What is the cheapest and appropriate hardware approach (cf. challenge rule 2) for getting parallel access to the 6 output and 2 input lines of #9750 in TI BASIC? Well – none! That was it, story was over. Why? Because TI BASIC (from 1979) does not have a POKE, nor an OUT or equivalent command. You can PEEK around, but no, no POKE. Back in the days, there may have been good reasons for that … But wait, there is more (there always is) … there is TI Extended BASIC, released in 1981 as ROM cartridge. And bingo, now we’ve got CALL LOAD (address, byte), which is the equivalent to POKE address, byte … but only when you have the 32k memory extension attached as well ... or if you have the TI mini memory cartridge, which also adds CALL LOAD to your commands, but without any other needs. I was lucky enough, to purchase an original TI mini memory cartridge for €40 from a >very< nice individual living in Berlin! Thank you very much again, Daniel!!! So I was all set, original hardware wise. On to hacking … it took me a while to figure out the “inner logic” of the address decoding within the console, particularly because the TMS9900 processor is a true 16-bit CPU! When interacting with peripherals though, the 8-bit world is back, by doing … things, I still don’t understand. All I know for sure is: The TI99 uses an 8-bit wide data bus to talk to its speech synthesizer box, an add-on that came with my machine. And that CALL LOAD and CALL PEEK have direct access to this thing, i.e., its memory mapped address space: One can read data from the synthesizer at address 0x9000 and write to it at address 0x9400. So the documents say. With some further study of the circuit diagrams of the console as well as the speech module and most importantly, this website (http://www.unige.ch/medecine/nouspikel/ti99/titech.htm), which is an invaluable resource, I finally came up with these ideas: Use the speech synthesizer box as enclosure for my “TI 9750 interface” in the way, TI originally planned it: Slide-in hardware speech extensions into the upper compartment. Well, the upper compartment with that little door is empty, nor are there any sockets; in other words, TI never released anything that may slide in there. The circuit board with the synthesizer’s electronics is located in the lower compartment. I read that TI planned to extend the speech capabilities of the synthesizer with additional “speech ROM modules”, but before they actually made that happen, they found out a way more flexible approach using software … so no one wanted to book that room up there in the synthesizer, but me. As the full I/O bus including all address, data, and control lines of the TI console are fed through (and also into) the speech synthesizer’s PCB, everything is available within the synthesizer box; no reason to hack the console. When you attach the speech module to the console, the I/O bus is replicated on its other side, and more peripherals can be attached (parallel/serial interfaces, disk controllers, modems, etc. pp.). There is one exception: the +5V line is not, as the power consumption of the different additional original TI hardware extensions was simply too high for the console’s power supply. However, when you only want to power a modern 32k memory extension, just solder a wire from one side of the synthesizer PCB to the other. I did that. So, here’s the plan: Circuit diagram of some of the address decoding sections of the TI console and synthesizer module (blue boxes), and my added two 8-bit transparent D-latches (74LS373) interfacing to #9750, along with a 4 x NOR gate 74LS02 TTL chip. Note that only one NOR gate is used. Here are the console and synthesizer circuit diagrams I used for my project. The former is actually a large collection of diagrams of the TI console and some peripherals. The weird CON notation is just describing my wires going from the PCB into the upper (empty) compartment of the synthesizer module. Address decoding in the TI99/synthesizer system (useful for hacking …) The logic of the IBM PC/#9771 combo has been described before, and is summarized (again ) in the hidden section below, where the corresponding TMS9900 CPU lines are used for the description. The thing is: The TMS9900 has no IOREQ# equivalent; all I/O access is via memory mapping (which may be wrong: I simply have no clue, how this A15/CRU stuff works, and most importantly how I would invoke/apply/use it from BASIC. Furthermore, there is no RAMCS# or ROMCS# input as on the ZX computers, which is really useful on systems with unresolved address space. The TI99 is no exception in this regard – out of the box, its address space is not fully resolved, see below. With reference to the above circuit diagram, I figured out a rather simple route to make use of the present address decoding hardware in the console and synthesizer module for additional memory mapped hardware, without screwing up the system – and without using assembly. In essence, I am hacking the synthesizer module, there are no changes to the console. The minor hardware change, without compromising the functionality of the synthesizer(!) is: A cut of one trace on the synthesizer’s PCB: Remove GND from pin 3 (C input) of U3 = 3-to-8 decoder 74LS138. And then connect this pin to address line A14[A1]: Note on usage of weird notation A14[A1]: For some reason, the TI engineers back in the days decided to use a different notation for their address and data bus lines: In contrast to every other vendor, TI denotes the most significant address bit as A0, and the least significant bit as A15. This is exactly the other way around in all other computer systems known to me. The same holds true for the data bus. I am thus using the notation A15[A0], where the bracketed bit denoting what it is outside of TI universe. Here is how the address decoding for the synthesizer works in the original configuration (you can read much more about that here): The speech synthesizer chip TMS5220 has, among many other things, two inputs, RS# (L = CPU reads data from synthesizer) and WS# (L = CPU writes data to synthesizer). Both cannot be L at the same time; thus there is a dedicated read address and a write address, respectively. These signals are generated by U3 = 74LS138 (3-to-8 decoder) in the synthesizer – this chip selects only two outputs: Q2# and Q3#. No other Q# outputs are used – in other words they are free! The output enable dependencies for U3 are: A15[A0] = H; RESET# = H (which is always the case when there is no trouble). The inputs A and B (C is bolted to GND = L) select the active output Q#: Possible combinations in the original configuration are: Table 1: “-“ = not connected/no function; SBE = speech block enable; RS#: read from speech chip; WS# write to speech chip. Note, that A5 = [A10] in the non-TI world. That’s it; Q#4 – Q#7 cannot be addressed, as input C is always L. And here comes the hack: Remove input C from GND = L, and hook it up to A14[A1]. This is all and changes the output selection to: Table 2: LR# = LEGO 9750 read; LR# = LEGO 9750 write, my fancy addition of synonyms … Now we need to have a look at the address decoding inside the TI99 console; on pin 2 of the I/O port, the above mentioned SBE signal is provided, which indicates CPU wants to address/access the synthesizer module. The NAND gate U506b in the console “combines” the outputs Q4# and Q5# of U505 (74LS138 3-to-8 decoder) generating the SBE signal: When either one of the output is selected (= L), SBE = H. When none are selected (= H), SBE = L; and finally both cannot be selected. Regarding the enable inputs of U505: Q4# of U504 (again a 74LS138) needs to be selected = L and A15[A0] needs to be L. The third enable input is active H; the NAND gate U506a combines A5[A10] and DBIN in the following manner (DBIN = H indicates a read data request from the CPU): When the CPU has issued such a request, DBIN = H and thus A5[A10] needs to be L, otherwise the outputs of U505 are disabled (are all = H). When DBIN = L, A5[A10] needs to be H for a write data request. The outputs Q4# or Q5# are selected, when A3[A12] = H, A4[A11] = L, and A5[A10] = L for read and H for write, see above. Finally, for selection of Q4# = L on U504, MEMEN# (memory accesses enable) needs to be L, A0[A15] = H, A1[A14] = L, and A2[A13] = L. All this results in the following table for read/write access to my interface connecting the TI99 to #9750, preserving full access to the speech synthesizer: Table 3: X = L or H, thus the addresses are mirrored (i.e., are not fully decoded) every 4 times in the address range shown in the last column. Using this approach, we now have two (+ all mirror!) new addresses: 0x9002 for reading data from the LEGO #9750 interface and 0x9402 for writing data to the interface. As DBIN is already tied into the address decoding logic, C and OC# of the corresponding 74LS373 input latch (IC1 in the circuit diagram above) are both connected to Q6# of U3 = LR#. For the 74LS373 output latch (IC2), we need a positive signal on the C input, which is “composed” using Q7# of U3 = LW# AND WE# (write enable) from the I/O bus of the TI99. This is done with the 4 x NOR gate 74LS02 chip, but only IC3a is used. NOR gates behave as AND gates in negative logic, so LW# NOR WE# = H, that’s exactly what we need: Only when both inputs are L, the gate output is H, in all other cases L. The 74LS373s are wired as in the #9771 card for the IBM and many other simple 8-bit I/O solutions from back in the days. I used it for my ZX and Amstrad interfaces as well. Lastly, PEEKing and POKEing in TI BASIC is again a little different than for the computer systems I played with: In the address range 0x0000 – 0x7FFF (0 – 32767) it works as “everywhere”, e.g.: CALL PEEK (32767,variable). Above that address range (and the speech synthesizer’s addresses are located above 32767), one needs to subtract 0x10000 (65536) from the address you wish to PEEK/POKE: CALL LOAD (-27646,0) is equivalent to POKE 37890,0 (which is 0x9402, our write address for #9750). If we want to read data from #9750, we need address 0x9002, which is 36866; subtracting 65536 yields -28670 and thus CALL PEEK (-28670,variable) is used in TI and Extended TI BASIC programs to do that. If you like to read more about this approach, here is one source. And finally, here are some pictures: Above: Disassembled synthesizer module Above: CON lines soldered to freely accessible soldering spots. Yellow wires = D0-D7, white = WE#, yellow/white = LR#, yellow/green = LW#, whatever color (I am color blind) = A14[A1], red = +5V, black = GND. Missing: WE# - I simply forgot that line on first try and almost gave up, because nothing worked as intended. Then, when walking our dog, it just occured to me ... when you want to write data, you need to tell your chippies so ... Above: Perfbord attached to the connector, the top white connector was glued to the 12 others; this is the missing WE# line The left 10 x 2 pins attach to the Interface A with the original cable. Above: Synthesizer module. Bottom connector: Replicated I/O port of the console - this is where my 32k expansion attaches to and is also powered from. Top left: Connector for Interface A and 74LS02 trying to figure out, what's going on ... Above: Synthesizer module attached to console, with interface for #9750 extension under the hood and original LEGO cable for connecting to #9750 attached ... In summary, we can add another computer system from the 1980’s, which was not endorsed by TLG, to @evank's challenge (and https://www.brickhacks.com/2.php); The TI99/4A. All the best, Thorsten P.S.: Will try to make a video proof soon!!! Done
