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Thread: I wish to create a new DMA/RAM expansion card for the Tandy 1000 line.

  1. #11
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    The email exchange was Aug 2017 which post-dates your post by a lot. However I had no idea it existed till now.
    "Good engineers keep thick authoritative books on their shelf. Not for their own reference, but to throw at people who ask stupid questions; hoping a small fragment of knowledge will osmotically transfer with each cranial impact." - Me

  2. #12
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    How will you get around the two PLAs used by the ISA card? You will need the truth tables from those PLAs to generate the necessary DMA signals. I do not believe Tandy published those truth tables anywhere.

    Oh, I believe the DMA + 512KB RAM ISA Expansion Board from Tandy used the same DMA ASIC that the EX and HX Plus Cards used.

    I think that DMA chips are speed rated and use a clock, so an 8237-5 may not be a good fit for a system that runs at 7.16MHz by default.

    There has been a recent hoembrew attempt at a Sound Blaster clone : https://www.vogons.org/viewtopic.php?f=62&t=61098
    My Retro Computing and Vintage Gaming Blog : http://nerdlypleasures.blogspot.com/

  3. #13
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    Quote Originally Posted by Great Hierophant View Post
    How will you get around the two PLAs used by the ISA card? You will need the truth tables from those PLAs to generate the necessary DMA signals. I do not believe Tandy published those truth tables anywhere.
    At least part of them are in the T1K tech ref. I'm not sure if they are complete.

    Code:
    RAS0B = /MCF1./MCF0./19./18./17./16./REFRESH.(MEMRB+MEMWB)
          + /MCF1. MCF0./19./18.        /REFRESH.(MEMRB+MEMWB)
          +  MCF1./MCF0./19./18.        /REFRESH.(MEMRB+MEMWB)
          +  MCF1. MCF0./19./18./17./16./REFRESH.(MEMRB+MEMWB)
          + REFRESH.MEMRB
    
    RAS1B = /MCF1./MCF0./19./18./17. 16./REFRESH.(MEMRB+MEMWB)
          +  MCF1./MCF0./19. 18.        /REFRESH.(MEMRB+MEMWB)
          +  MCF1. MCF0./19./18./17. 16./REFRESH.(MEMRB+MEMWB)
          + REFRESH.MEMRB
    
    RAS3B = MCFl. MCF0./19./18. 17 /REFRESH.(MEMRB+MEMWB)
          + MCFl. MCF0./19. 18./17 /REFRESH.(MEMRB+MEMWB)
          + REFRESH.MEMR
    
    DMACSB = /A7./A6./A5./A4./AEN.(IORB + IOWB)
    
    WPRCSB = A7./A6./A5./A4./A4./AEN.IOWB
    
    DBDIR = DMACSB.IORB
          + /MCF1./MCFO./19./18./17./REFRESH.MEMRB
          + /MCF1. MCFO./19./18.    /REFRESH.MEMRB
          +  MCFl./MCF0./19.        /REFRESH.MEMRB
          +  MCFl. MCF0./19.(/18+18./17)./REFRESH.MEMRB
    
    DBENB = DMACSB + WPRCSB
          + /MCFl./MCFO./19./18./17./REFRESH.MEM-B
          + /MCFl. MCF0./19./18.    /REFRESH.MEM-B
          +  MCFl./MCF0./19.        /REFRESH.MEM-B
          +  MCFl. MCF0./19.(/18+18./17)./REFRESH.MEM-B
    Quote Originally Posted by Great Hierophant View Post
    Oh, I believe the DMA + 512KB RAM ISA Expansion Board from Tandy used the same DMA ASIC that the EX and HX Plus Cards used.
    Maybe there are two. Tandy did market using the PLUS board with the ISA conversion bracket. But I do have a completely ISA based board from Tandy somewhere. I went down to the dungeon to find it, but couldn't. I remember it having a genuine 8237A. I think it's in my 1000. A little too much work to get to atm.

    Quote Originally Posted by Great Hierophant View Post
    There has been a recent homebrew attempt at a Sound Blaster clone : https://www.vogons.org/viewtopic.php?f=62&t=61098
    I get really critical when I see MCUs bit-banging ISA signals (this case DMA). I get even more critical when people try to etch/drill complex boards instead of a week at OSHpark et al. But it all seems to work well. And the choice of song at the end of the video redeemed the effort a great deal. Ryan is one of my favorite artists then and now. Maybe #2 in the #trax scene to Andy. Ah.. those were the days.
    "Good engineers keep thick authoritative books on their shelf. Not for their own reference, but to throw at people who ask stupid questions; hoping a small fragment of knowledge will osmotically transfer with each cranial impact." - Me

  4. #14
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    Well, I tried Front Panel Express's free designer program to get an idea of price, and managed to whittle a bracket down from $29 to $23 dollars using only 1 tool change. (I'm assuming this is USD). I save 76 cents a hole if I tap it myself (#6-32). How much did yours cost?

  5. #15
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    Quote Originally Posted by Great Hierophant View Post
    How will you get around the two PLAs used by the ISA card? You will need the truth tables from those PLAs to generate the necessary DMA signals. I do not believe Tandy published those truth tables anywhere.'
    The PLAs you're referring to are the 82S153's, U5 and U9? I'm not certain that those are necessary. U5 seems to deal with the memory, and I'm not sure what U9 is for (it's 1 AM right now). I managed to snag a knockoff brand of that DMA/RAM ISA card from ebay that doesn't appear to have them (it's the ATD T512CLK). Once it arrives, I'll do a schematic of that and compare it.

    Oh, I believe the DMA + 512KB RAM ISA Expansion Board from Tandy used the same DMA ASIC that the EX and HX Plus Cards used.
    All the ISA DMA/RAM cards I've seen pictures of have discrete logic built around the 8235A-5, rather than an ASIC, but I think the card in the 1000 tech reference manual only had 256K on it. It wouldn't surprise me if they did come out with a later version once they had that ASIC in production. Is that what you're talking about?

    I think that DMA chips are speed rated and use a clock, so an 8237A-5 may not be a good fit for a system that runs at 7.16MHz by default.
    As far as I can tell, the ASIC in the PLUS card of my HX uses the 8237A-5, and according to the HX refrence manual in the DMA section, '
    Code:
    TIMING GENERATOR
    The input clock is OSC (= 14.31818 MHZ).
    1.) It is divided by three to recreate the 4.77 MHz
    system processor clock which is used as the clock for
    the 8237.
    2.) It is used to delay the memory access strobe MEM-B
    twice to create the timing for RAS-f MUX, and CAS.'
    That divider would have to be part of the integrated support logic in the PLUS card's ASIC. The t1000's card wouldn't have needed it.

    There has been a recent hoembrew attempt at a Sound Blaster clone : https://www.vogons.org/viewtopic.php?f=62&t=61098
    THAT is cool. Complete kludge. I prefer perfboards myself, but hey, it works! I'm impressed.

  6. #16
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    Tandy did market using the PLUS board with the ISA conversion bracket.
    I haven't seen the relevant kit, but I think that was to use a PLUS card on top of a ISA DMA/RAM card (like the one below).
    t1000hd5.jpg
    I can't imagine PLUS cards having been too popular outside of the EX/HX. The only incentive I can see to use them on an ISA system is if you already had the cards and didn't want to use them in the EX/HX any more.

    Somewhat related: I don't know if you're familiar with this, but there was apparently a company back when that made accessories for the EX/HX http://www.mediafire.com/file/h14b1n..._1992.pdf/file

  7. #17
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    Tandy marketed three memory boards for the Tandy 1000/1000A.

    The first board was the 25-1004 added the DMA chip and 128KB of 256KB of RAM. The 25-1009 was the second board and can add an additional 128KB or 256KB of RAM. The only difference between the 25-1004 and 25-1009 was the presence/absence of the DMA chip. With these boards you will take up two slots to maximize the RAM inside the Tandy 1000 or 1000A.

    The third and final board for the Tandy 1000, which was included in the Tandy 1000HD was the 25-1011, which appears to be pictured in the previous post. That board comes with the DMA chip, allows up to 512KB of RAM and has the 62-pin header to install a PLUS card. The idea is that you could mount the serial, modem or calendar/mouse board on that card and save yourself a slot. The Tandy 1000, 1000A and 1000HD only have three ISA expansion slots.

    All three boards have the two PLA chips on them, I believe the one of the PLA chips mainly generates memory address signals and the other deals with the DMA.
    My Retro Computing and Vintage Gaming Blog : http://nerdlypleasures.blogspot.com/

  8. #18
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    RAM must have come down in price considerably between the two (three) cards. 4x 8-chip banks at 8KB/chip on the first two cards, and 2x 8 banks of 32KB/chip on the latter card. Was it really cheaper to have two large boards with double the chips (minus one 8237A), than to have a quarter the RAM chips, but more expensive ones on a single board? I'm not sure exactly when the two systems were released.

    All three boards have the two PLA chips on them, I believe the one of the PLA chips mainly generates memory address signals and the other deals with the DMA.
    That's more or less what I've gotten so far from the schematics. U5 for memory bank selecting, but also has a pin going to U9 called "MemAdd." Not sure what MemAdd does. Might be related to the jumper pins that are referenced high when open? U9 might have another 5-input NOR gate programmed into it like the two in U8, but I'm not certain yet. I've found the verbal description of the function within the tech reference manual, but haven't located the truth table eeguru pasted yet (I think I'm reading the same manual?). Between the two I should be able to get something, when I have another moment to dedicate to it.

    ta6.jpgThe ATD T512CLK that I snagged from eBay (supposed to be Tandy compatible) doesn't appear to have those PLA chips, just 74xx series logic supporting the 8237A. Once it arrives, I'll reverse engineer that and compare the two. This board could also take a RTC chip, which must have been convenient. I had to perform "surgery" on the Smartwatch chip on my HX when it's batteries finally died. It had been slowly loosing time over the past 5 years or so (that I had noticed anyways), but almost 30 years of service before the batteries died completely is pretty impressive for a device that was only guaranteed to work reliably for 10!

  9. #19
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    A thought occurred to me... Absent the truth table, it should be possible to reverse engineer a PLA (8 inputs and 10 outputs in this case) by feeding a binary count (0-255) into it and recording on a logic analyzer what comes out, would it not?

  10. #20
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    Quote Originally Posted by blackepyon View Post
    A thought occurred to me... Absent the truth table, it should be possible to reverse engineer a PLA (8 inputs and 10 outputs in this case) by feeding a binary count (0-255) into it and recording on a logic analyzer what comes out, would it not?
    As long as it's combinatorial only 'L' variant. If its an 'R' or 'V' registered variant (containing flip-flops), it becomes more difficult. If the security bits are not set, you could just read the fuse data out with a programmer.
    "Good engineers keep thick authoritative books on their shelf. Not for their own reference, but to throw at people who ask stupid questions; hoping a small fragment of knowledge will osmotically transfer with each cranial impact." - Me

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