blackepyon
Veteran Member
I recently discovered that you can still get the Intel 8237A DMA chip at Jameco (or at least one of the knockoffs). I'm hoping to recreate the Tandy DMA/RAM card, either in the XT/ISA or PLUS variants (preferably both)
I'm trying to reverse engineer the schematics for the IBM 5150/5160 for the supporting logic for the DMA chip. There's plenty enough RAM cards out there, and the schematics are relatively simple. The issue I'm running into is that most of the control lines for the DMA don't exist on the XT bus. The supporting logic that would normally be on the motherboard needs to be created on the card to interface the DMA control lines with the XT bus.
I need to find out how Tandy resolved this. 74xxx series logic is jellybean parts and easy to come by, but I don't want to bring half the motherboard onto the card if I can avoid it (looks to be 8-10 ICs that are utilized for this on the Tandy cards). I can glean some information on the logic from the IBM schematics, but haven't figured enough yet to replicate the control lines from the XT bus.
If I can get detailed top-down scans of both sides of the Tandy DMA/RAM cards (laid on a flatbed scanner, preferably, with the mounting bracket removed and the capacitors standing up), I should be able to fill in what I'm missing and make a schematic to figure out how Tandy resolved this issue. I can find some pictures online of Tandy's XT versions of this card for the 1000, the 1000 SX, SL, TL, and so on, but they aren't top down, and don't include the other side of the board, so I can't see all the traces. I already have a PLUS card for the 1000 EX/HX, but I can't find any data on the custom ASIC they used (which has the DMA controller and the support logic built in), so that's a dead end.
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I'm also working on an all-in-one PLUS CompactFlash storage card based on the XT-IDE/CF-IDE for the Tandy 1000 EX/HX. I would have just gotten one of dJOS's PLUS to ISA converters and a lotech CF-IDE, but my XT multi I/O card (for serial and a second parallel port) barely fits inside the 1000 HX, and there's no room for another PLUS to ISA converter with the way I had to mount it, so I built a PLUS version of the CF-IDE onto a perfboard. dJOS suggested that people would be interested and that I layout a proper PCB. I've already got a design for this laid out supporting up to two CF cards (one inside and one out the back) for transferring files to another XT class machine, I just need to refine the design and to a first run of PCBs to test it all out (...When I have the time. Work always takes priority ). This design also has extra options for the LED. For instance, allowing a bypass to the Power LED above the keyboard on the EX/HX, so that the LED turns off to indicate activity- potentially one could swap that LED for a 2 color as well, thereby avoiding destructive case mods, or just a simple LED at the mounting bracket so you can see it from the back for diagnostics.
My intention is to be able to make proper PCBs of both these cards once I get the designs finalized and tested, and sell them at the cost of PCB manufacture and shipping for whoever wants to assemble their own. For ease of assembly, I intend to make as much of it "through hole" as possible, and the CF-IDE will include a spot for an IDE header for anybody uncomfortable with surface mount soldering of the CF header. Work takes priority though, so it may still be a while before I can complete them.
For now though, I need detailed top-down pictures of both sides of the XT/ISA versions of the Tandy DMA/RAM cards, if anybody has one and can provide them. At the very least, I want to be able to put up schematics so those so inclined can try making their own.
I'm trying to reverse engineer the schematics for the IBM 5150/5160 for the supporting logic for the DMA chip. There's plenty enough RAM cards out there, and the schematics are relatively simple. The issue I'm running into is that most of the control lines for the DMA don't exist on the XT bus. The supporting logic that would normally be on the motherboard needs to be created on the card to interface the DMA control lines with the XT bus.
I need to find out how Tandy resolved this. 74xxx series logic is jellybean parts and easy to come by, but I don't want to bring half the motherboard onto the card if I can avoid it (looks to be 8-10 ICs that are utilized for this on the Tandy cards). I can glean some information on the logic from the IBM schematics, but haven't figured enough yet to replicate the control lines from the XT bus.
If I can get detailed top-down scans of both sides of the Tandy DMA/RAM cards (laid on a flatbed scanner, preferably, with the mounting bracket removed and the capacitors standing up), I should be able to fill in what I'm missing and make a schematic to figure out how Tandy resolved this issue. I can find some pictures online of Tandy's XT versions of this card for the 1000, the 1000 SX, SL, TL, and so on, but they aren't top down, and don't include the other side of the board, so I can't see all the traces. I already have a PLUS card for the 1000 EX/HX, but I can't find any data on the custom ASIC they used (which has the DMA controller and the support logic built in), so that's a dead end.
---
I'm also working on an all-in-one PLUS CompactFlash storage card based on the XT-IDE/CF-IDE for the Tandy 1000 EX/HX. I would have just gotten one of dJOS's PLUS to ISA converters and a lotech CF-IDE, but my XT multi I/O card (for serial and a second parallel port) barely fits inside the 1000 HX, and there's no room for another PLUS to ISA converter with the way I had to mount it, so I built a PLUS version of the CF-IDE onto a perfboard. dJOS suggested that people would be interested and that I layout a proper PCB. I've already got a design for this laid out supporting up to two CF cards (one inside and one out the back) for transferring files to another XT class machine, I just need to refine the design and to a first run of PCBs to test it all out (...When I have the time. Work always takes priority ). This design also has extra options for the LED. For instance, allowing a bypass to the Power LED above the keyboard on the EX/HX, so that the LED turns off to indicate activity- potentially one could swap that LED for a 2 color as well, thereby avoiding destructive case mods, or just a simple LED at the mounting bracket so you can see it from the back for diagnostics.
My intention is to be able to make proper PCBs of both these cards once I get the designs finalized and tested, and sell them at the cost of PCB manufacture and shipping for whoever wants to assemble their own. For ease of assembly, I intend to make as much of it "through hole" as possible, and the CF-IDE will include a spot for an IDE header for anybody uncomfortable with surface mount soldering of the CF header. Work takes priority though, so it may still be a while before I can complete them.
For now though, I need detailed top-down pictures of both sides of the XT/ISA versions of the Tandy DMA/RAM cards, if anybody has one and can provide them. At the very least, I want to be able to put up schematics so those so inclined can try making their own.