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

  1. #161

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    Quote Originally Posted by blackepyon View Post
    Sweet!
    And Digi-Key actually keeps these in stock! Bonus!
    Not exactly cheap for a 16-pin DIP ($23 CDN) but they're new, and available!
    Adding some of that to my next order...
    That's where I got mine from - I'm going to try another source I found that was under $3 per chip, I'll report back if they are any good.
    My Retro Collection:
    CBM: C64, Amiga 500 x2, 600 & 1200
    Apple's: IIc, Mac SE, LCII, LC630 & Power Mac G3/233 Desktop
    PC's: K6-III+ 500 System + Roland MT-32 & Tandy 1000 EX 640kb, 3.5" FDD, CF-IDE 4GB HDD
    Visit my Tindie store for Tandy 1000 Adapters for EX, HX, SX, SL, TX & TL etc

  2. #162
    Join Date
    Feb 2017
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    Chilliwack, BC, Canada
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    Quote Originally Posted by Eudimorphodon View Post
    I'm still at a loss here. I've retraced everything (including checking the one thing I hadn't absolutely verified, which is that something plugged into the female connector on the bottom of the board had good connectivity; it's fine) and my conclusion is that even if my decoding logic was somehow completely b***s-up wrong the jumpering I did to dumb it down should have worked if the RAM was good. My best guess how to proceed at this point is to order another RAM chip from a reputable dealer (Digi-Key, Mouser, whatever) and pray I really did get scr3wed. (I will be hitting the board with the logic probe/scope to make sure the enables are working as expected as well, but I'm skeptical I'm going to find a smoking gun.)
    Just need to make the bloody thing work first.
    Good to know that the holes line up as I expected.

    And hey, like I said, I can go over your schematics with a fresh set of eyes if you can upload the KiCad project somewhere. I already got what I need, which is the 3-chip solution we worked out (that way I've got room for the DMA logic once I get that figured out). I'd be happy to help out in any way I can.
    My vintage systems: Tandy 1000 HX, Tandy 1100FD, Tandy 1000 RSX, and some random Pentium in a Hewitt Rand chassis...

    Some people keep a classic car in their garage. Some people keep vintage computers. The latter hobby is cheaper, usually takes less space, and is less likely to lead to a fatal accident.

  3. #163
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    May 2011
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    Outer Mongolia
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    Quote Originally Posted by blackepyon View Post
    And hey, like I said, I can go over your schematics with a fresh set of eyes if you can upload the KiCad project somewhere. I already got what I need, which is the 3-chip solution we worked out (that way I've got room for the DMA logic once I get that figured out). I'd be happy to help out in any way I can.
    It's been a while, so I figured I should probably update that I haven't given up or anything. The last couple weeks suddenly got very busy, which might be the case for a few more.

    I'll see what I can do to get the schematics posted somewhere in something like a usable form, because I'm still at a dead end. (A screenshot stuck on the forum here as a JPEG will of course suffer massive degradation.) The only work I've been able to do for the last week is start on recreating my decoding circuitry on a protoboard to quadruple check the logic does what I expect it to do with given address line inputs. About half done with that, so far everything checks out.

    Been fighting the urge to wire one of the RAM chips up to my Commodore PET to interactively check it. The PET presents all the necessary signals on a couple header connectors on the motherboard and has pre-defined chip select signals already on the connector so it wouldn't require any decoding. I'd only be able to test 4k of it at once but that would be enough to sanity check if I was given completely dead ICs...

  4. #164
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    Quote Originally Posted by Eudimorphodon View Post
    It's been a while, so I figured I should probably update that I haven't given up or anything. The last couple weeks suddenly got very busy, which might be the case for a few more.

    I'll see what I can do to get the schematics posted somewhere in something like a usable form, because I'm still at a dead end. (A screenshot stuck on the forum here as a JPEG will of course suffer massive degradation.) The only work I've been able to do for the last week is start on recreating my decoding circuitry on a protoboard to quadruple check the logic does what I expect it to do with given address line inputs. About half done with that, so far everything checks out.
    Yeah, been busy on my end too. Got a mountain of paperwork I've been lagging behind on for my "day job" (small landscaping business) that I need to get back to.

    Doesn't need to be pretty. Just drop the KiCad project folder onto Mediafire or somewhere and drop me a link. I can prototype it on a breadboard on my end to make sure that there's not some hidden difference we might have missed between the EX and HX that might be interfering (I DOUBT that would be the case, but just to make sure).
    My vintage systems: Tandy 1000 HX, Tandy 1100FD, Tandy 1000 RSX, and some random Pentium in a Hewitt Rand chassis...

    Some people keep a classic car in their garage. Some people keep vintage computers. The latter hobby is cheaper, usually takes less space, and is less likely to lead to a fatal accident.

  5. #165

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    Wow, this is really cool! I’ve got an EX that needs more memory, and am very interested in this. I wonder what the reason of not building a DMA controller into the machine was?
    Last edited by compaqportableplus; July 14th, 2019 at 05:37 AM. Reason: Typo
    Compaq - “It simply works better”

  6. #166
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    Mar 2011
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    Atlanta, GA, USA
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    Quote Originally Posted by compaqportableplus View Post
    I wonder what the reason of not building a DMA controller into the machine was?
    $$$

    Tandy bet the farm (and won) on the Tandy 1000 as a low cost home PC-compatible entry. The EX & HX were even lower cost reduced versions of the 1000 line.
    "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

  7. #167
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    Quote Originally Posted by compaqportableplus View Post
    Wow, this is really cool! I’ve got an EX that needs more memory, and am very interested in this. I wonder what the reason of not building a DMA controller into the machine was?
    That's the idea. I was fortunate that when my dad bought the HX originally, he got it with the DMA/RAM card (though the RS232 card would have been nice too). If you're desperate, and feeling so inclined, we figured out a quick and dirty method on page 3 or 4 to get extra RAM installed using only a single 512KB static-RAM chip (such as the AS6C4008 ) and some male-male jumpers. It overlaps a bit with the onboard memory, but as long as the onboard RAM is good, It shouldn't complain.

    Pay attention to the fact that the A and B sides of the PLUS header are opposite of what an ISA slot would be. Power/ground are on the side of the floppy drives, address/data on the side of the PSU (lookup an EX or HX technical manual for details). Vcc from one of the 5v pins, Address 0-18 and the Data lines go straight to the chip, Address 19 is the Chip-Enable. Memory-Write and Memory-Read go to Write-Enable and Read/Output-Enable respectively. Then some stickytack or tape or something to keep the chip from flopping around. Not the most elegant solution, but like I said, quick and dirty, and it should get you functional until one of us can get a proper card out.
    Last edited by blackepyon; July 14th, 2019 at 02:52 PM.
    My vintage systems: Tandy 1000 HX, Tandy 1100FD, Tandy 1000 RSX, and some random Pentium in a Hewitt Rand chassis...

    Some people keep a classic car in their garage. Some people keep vintage computers. The latter hobby is cheaper, usually takes less space, and is less likely to lead to a fatal accident.

  8. #168
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    Quote Originally Posted by eeguru View Post
    $$$

    Tandy bet the farm (and won) on the Tandy 1000 as a low cost home PC-compatible entry. The EX & HX were even lower cost reduced versions of the 1000 line.
    Despite being based on the PC-Jr, it was much less expensive, and far easier to upgrade. That seems to have made all the difference. I guess IBM was counting on their reputation in the business market to sell the PC-Jr at such a ridiculous price to home users.
    My vintage systems: Tandy 1000 HX, Tandy 1100FD, Tandy 1000 RSX, and some random Pentium in a Hewitt Rand chassis...

    Some people keep a classic car in their garage. Some people keep vintage computers. The latter hobby is cheaper, usually takes less space, and is less likely to lead to a fatal accident.

  9. #169

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    Quote Originally Posted by blackepyon View Post
    That's the idea. I was fortunate that when my dad bought the HX originally, he got it with the DMA/RAM card (though the RS232 card would have been nice too). If you're desperate, and feeling so inclined, we figured out a quick and dirty method on page 3 or 4 to get extra RAM installed using only a single 512KB static-RAM chip (such as the AS6C4008 ) and some male-male jumpers. It overlaps a bit with the onboard memory, but as long as the onboard RAM is good, It shouldn't complain.

    Pay attention to the fact that the A and B sides of the PLUS header are opposite of what an ISA slot would be. Power/ground are on the side of the floppy drives, address/data on the side of the PSU (lookup an EX or HX technical manual for details). Vcc from one of the 5v pins, Address 0-18 and the Data lines go straight to the chip, Address 19 is the Chip-Enable. Memory-Write and Memory-Read go to Write-Enable and Read/Output-Enable respectively. Then some stickytack or tape or something to keep the chip from flopping around. Not the most elegant solution, but like I said, quick and dirty, and it should get you functional until one of us can get a proper card out.
    Cool! I’ll look into that. It also appears there is a riser being made that would let me use a standard ISA RAM expansion, so I’ll look into that as well.

    Still very interested in this DMA memory board though!
    Compaq - “It simply works better”

  10. #170

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    Quote Originally Posted by compaqportableplus View Post
    It also appears there is a riser being made that would let me use a standard ISA RAM expansion, so I’ll look into that as well.
    I've sold a lot of the EX / HX PLUS-ISA Riser Cards to folk who have used them to install the Lo-Tech 1MB RAM board and Lo-tech XT-IDE CF boards and from all reports they work very well.

    I'd like to combine a 512kb expansion with the Riser card one day (if someone doesn't beat me to it) but I'm a bit time poor atm (more senior job) and haven't had the time to put into it lately.
    My Retro Collection:
    CBM: C64, Amiga 500 x2, 600 & 1200
    Apple's: IIc, Mac SE, LCII, LC630 & Power Mac G3/233 Desktop
    PC's: K6-III+ 500 System + Roland MT-32 & Tandy 1000 EX 640kb, 3.5" FDD, CF-IDE 4GB HDD
    Visit my Tindie store for Tandy 1000 Adapters for EX, HX, SX, SL, TX & TL etc

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