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Thread: Newb Question: ISA address decoding logic levels

  1. #41
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    Going through schematics and transcribing things, I ran across a little bit of a head scratcher. On XT-CF cards, why is D7 tied to ground via a 10k resistor? I apparently lack the google fu.

  2. #42
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    Never mind, I found a reference to why here.

    http://www.unige.ch/medecine/nouspikel/ti99/ide2.htm

  3. #43
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    Quote Originally Posted by Eudimorphodon View Post
    My self-imposed challenge is to try to fit this into a 100x100 mm board, because one of the PCB mills I was looking at has an extra cheap price on small quantity boards smaller than that. Maybe I'll put in a header to connect on off-board battery instead of a CR20xx holder..
    Gotta be careful with some of those deals. I had some boards made from PCBWay (in China), and by the time I got them, I'd paid just as much in duty as I did for the boards themselves. Need to find a good PCB fab in Canada.
    My vintage systems: Tandy 1000 HX, 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.

  4. #44
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    If you're in the US, it's hard to do better with small quantity fab than OSH Park. After DHL shipping from China (insisted upon by most Chinese vendors), the difference in price is negligible.

  5. #45

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    Quote Originally Posted by Chuck(G) View Post
    If you're in the US, it's hard to do better with small quantity fab than OSH Park. After DHL shipping from China (insisted upon by most Chinese vendors), the difference in price is negligible.
    JLCPCB is 100% the way to go now, they offer $5.00 shipping to the US and $2 prototype runs

  6. #46
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    Quote Originally Posted by maxtherabbit View Post
    JLCPCB is 100% the way to go now, they offer $5.00 shipping to the US and $2 prototype runs
    I was looking at PCBWay, but this definitely looks like like a possibility. Because of the steep uptick in price I've been keeping my designs inside of a 100x100mm square, I wonder how badly they hit you for going bigger...

    So, going way back to the beginning of this thread, I ran into a snag, and said snag has made me wonder what the absolute minimum decoding scheme to test a certain configuration might be. Full ugly story follows, skip the next paragraph for TL;DR readers:

    The main board I want to build is a RAM card for a Tandy 1000 EX, and from the start I've assumed that the memory map of a 256k EX is the same as a 256k PC/XT, IE, that 256k is welded in place starting at 00000h, ends at 3FFFFh, and therefore the backfill card's decoding should start at 40000h. While reviewing everything an alarm went off in the back of my head that made me second-guess that assumption, and now, after reading the tech/service manuals for almost every Tandy 1000 model from the original 1000 through the TX I am approximately 85% sure that Tandy 1000 expansion RAM needs to start at 00000h, which the BIOS tests for, and after determining where the end of any expansion RAM is it programs a mapping register that places the built-in RAM at the end of it all. (This is apparently a fundamental difference from the PCjr, as my understanding of that machine is it leaves the source pages for its video buffer in the first 128k even if you have expansion memory hanging off it.) However I'm having trouble finding any posts on the web from people building Tandy RAM cards or using generic cards like the lo-tech offerings which confirm or deny my new suspicion. So...

    Question:

    Can anyone confirm or deny my new suspicions about the expansion RAM addressing scheme of a Tandy 1000? (IE, it should start at 0h) Wondering if anyone here might have dealt with one of these animals at a sufficiently low level to know the truth back in the day.

    I'm thinking I might need to bite the bullet and breadboard this when my RAM chips arrive on the slow boat from China. Been pondering the most minimal circuit I can use to verify the behavior. I'd rather not do such crude experimentation but I'm worried I might not be able to get a definitive answer otherwise.

  7. #47
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    Quote Originally Posted by Eudimorphodon View Post
    Can anyone confirm or deny my new suspicions about the expansion RAM addressing scheme of a Tandy 1000? (IE, it should start at 0h) Wondering if anyone here might have dealt with one of these animals at a sufficiently low level to know the truth back in the day.

    I'm thinking I might need to bite the bullet and breadboard this when my RAM chips arrive on the slow boat from China. Been pondering the most minimal circuit I can use to verify the behavior. I'd rather not do such crude experimentation but I'm worried I might not be able to get a definitive answer otherwise.
    I replied in the other thread as well, but for anybody here, LGR did just that for his 1000A. https://youtu.be/malgCK7qHQA?t=531
    My vintage systems: Tandy 1000 HX, 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. #48
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    Also replied on the other thread, but the TL;DR is that according to the evidence of how he has his switches set he has expansion RAM starting from 00000h, not 20000h. Technically I suppose that's not absolute proof that his card isn't conflicting with the RAM behind Big Blue and the Tandy's just fine with it, but it's totally a data point.

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