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Thread: Kaypro FD terminating resistor questions

  1. #11

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    In my earlier days, it would take a week to order a r-pack and then there was the minimum order thing. I used to buy a bunch of resistors from RS and solder them together to make the needed r-pack. Your drawing is not correct, usually one end is common but sometimes both ends are. As Chuck says, the common lead is connected to +5V.
    Although, you may have problems, I've seen short runs run without the terminator resistors. These are just using the TTL input pulling up. Also, for a floppy drive on one, the last, floppy drive should have the terminator installed.
    Dwight

  2. #12
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    "Hope this answers the question. "

    It does, thank you. Open collector, totem pole and wired-or were terms I had run across but didn't understand well. Unfortunately with my level of understanding I resemble an inquisitive three-year old at times. I apologize for having to ask and appreciate both of you taking time to respond.

    "Still, 470 ohm would be kinder. "

    As I said, I've ordered the 470 Ohm arrays and will substitute them. It may be that the use of the lower resistance was one of those MicroCornucopia - inspired "tweaks" that all the insiders adopted. Considering the power supply in the 4/84 and the wiring at hand, maybe lower current is a good thing. We'll see.

    Thank you again for all your help and guidance.

    -CH-

  3. #13

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    FYI, I think the current issues are more about the 7406 chip and PCB voltages rather than power supply. With too high a current (and driving all outputs at once) the 7406 might not be able to force the signals low enough, and could possible cause some spikes on the 5V lines on the mainboard. Those are just guesses based on what I recall others saying. If one were using a drive with 220/330 termination you'd have a 220 ohm resistor to Vcc to drive, so 150 is only a little more. One reason "they" may have gone to 150 was to get faster rise times on the cable, or it could have been something as random as price/availability of parts. I've not seen any photos of original-issue drives, and the ones I had are now gone, so I can't say what was used back in 1984. I suspect most drives have been refurbished, and about half of them would have been without resistors, so the people refurbishing them may have just stuffed a new resistor pack in each one - so that the drive was ready to go in any position.

  4. #14
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    Quote Originally Posted by Dwight Elvey View Post
    In my earlier days, it would take a week to order a r-pack and then there was the minimum order thing.Dwight
    Yeah, but these days you order $7 worth of components and pay $8 worth of "shipping".

    Quote Originally Posted by Dwight Elvey View Post
    I used to buy a bunch of resistors from RS and solder them together to make the needed r-pack. Dwight
    A comment about California guys liking to roll their own occurs to me but I will suppress it.

    Quote Originally Posted by Dwight Elvey View Post
    Your drawing is not correct, usually one end is common but sometimes both ends are. As Chuck says, the common lead is connected to +5V.Dwight
    Thanks for pointing that out.

    Quote Originally Posted by Dwight Elvey View Post
    Although, you may have problems, I've seen short runs run without the terminator resistors. These are just using the TTL input pulling up.Dwight
    I'm amazed that those ribbon cables carry a signal at all, especially inside the case and next to the video electronics.

    Quote Originally Posted by Dwight Elvey View Post
    Also, for a floppy drive on one, the last, floppy drive should have the terminator installed.Dwight
    Got it. Been barking my shins on SCSI for the last year: terminate the last one at either end of the chain.

    Thanks for your help, good to hear from you again. Hope all is well and you are having a happy start to the new year.

    -CH-

  5. #15
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    Quote Originally Posted by durgadas311 View Post
    FYI, I think the current issues are more about the 7406 chip and PCB voltages rather than power supply. With too high a current (and driving all outputs at once) the 7406 might not be able to force the signals low enough, and could possible cause some spikes on the 5V lines on the mainboard. Those are just guesses based on what I recall others saying. If one were using a drive with 220/330 termination you'd have a 220 ohm resistor to Vcc to drive, so 150 is only a little more. One reason "they" may have gone to 150 was to get faster rise times on the cable, or it could have been something as random as price/availability of parts. I've not seen any photos of original-issue drives, and the ones I had are now gone, so I can't say what was used back in 1984. I suspect most drives have been refurbished, and about half of them would have been without resistors, so the people refurbishing them may have just stuffed a new resistor pack in each one - so that the drive was ready to go in any position.
    I can't find much on these old 5.25" drives; I've got Epsons, Panasonics, TEACs, Tandons and maybe half of them work reliably. You search for Epson SD 521 and nothing comes up.

    -CH-

  6. #16

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    Actually, the problem with termination on flat ribbon is reflections and not outside noise. You'll notice that on a floppy cable, adjacent wires are all grounds. The cables have an impedance of around 100 ohms, as I recall, but they are driven and terminated with less than optimum impedances. The drives are more sensitive to stray magnetic field. I was surprised that the reads are quite immune but the writes are more sensitive. I found this out after some experimenting. I had a typical flat table top PC. I'd place the display on top of it. The top most drive started having issues. Some experimenting found it to be a write failure. I put two old phone books under the display. Thank goodness for the inverse square law.
    Dwight

  7. #17
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    At Durango, our initial prototypes used a monochrome monitor sitting off to one side, with things scattered around the workbench as was usual--card cage here; power supply there, disk drives over there...

    When the enclosure finally came together, we mounted the monitor above the disk drives. Not only did the monitor interfere with the floppy signal processing, but the floppy motors returned the favor by causing the CRT image to wobble as soon as the disk drive came on.

    Eventually, we solved the problems with a 14 gauge mild steel shield that went completely around both floppy drives. Still, if you looked closely, you could see a bit of "wobble" in the CRT image.

  8. #18
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    Quote Originally Posted by Chuck(G) View Post
    At Durango, our initial prototypes used a monochrome monitor sitting off to one side, with things scattered around the workbench as was usual--card cage here; power supply there, disk drives over there...

    When the enclosure finally came together, we mounted the monitor above the disk drives. Not only did the monitor interfere with the floppy signal processing, but the floppy motors returned the favor by causing the CRT image to wobble as soon as the disk drive came on.

    Eventually, we solved the problems with a 14 gauge mild steel shield that went completely around both floppy drives. Still, if you looked closely, you could see a bit of "wobble" in the CRT image.
    http://www.old-computers.com/museum/...asp?st=1&c=939.

    "I was involved in the development of the operating system; I wrote the ISAM file manager and the floppy disk drivers. I was in charge of the Star Basic language and was responsible for its design and implementation. I also designed the bank-switching mechanism for the add-on memory. I also did the CBIOS adaptation for CP/M 2.2."

    This explains a lot. Very impressive. Some day I'd love to see your curriculum vitae.

    -CH-

  9. #19
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    FWIW: Took another look at the Tandon Service manual (for T-100 and T-200). At the end of my PDF copy there is a hand-drawn sketch and note citing 150 Ohm resistor arrays for termination.

    -CH-

  10. #20
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    Quote Originally Posted by clh333;554227This explains a lot. Very impressive. Some day I'd love to see your [I
    curriculum vitae.[/I]
    Some of it's probably still classified.

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