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Thread: Model III DSDD Drive Q&A

  1. #1
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    Default Model III DSDD Drive Q&A

    Short background: Acquired a bare-bones 16K "Cassette" Model III and realized that if I wanted to upgrade it the most economical path was to cannibalize another Model III. The "non-working" Model III that I bought for that purpose turned out to be in better shape than the seller realized. Somewhere along the line I decided to refurbish the "donor" as well, which brings us to today:

    Although it had been well-used and somewhat neglected, the only thing really wrong with the donor III, apart from being dirty and having its video controls maladjusted, was a drive :0 that would not step. The drive would spin and read track 0 but nothing more because the stepper motor spindle was frozen. I cleaned, lubed and adjusted both TexPer drives. After servicing drive :1 was smooth, drive :0 a little tight in the step motor spindle. It would move but its action was stiffer. When I reassembled I put :1 in the :0 position and the former :0 on top.

    The machine booted but subsequent R/W tests revealed that the "new" drive :1 was still not working correctly. My supposition is that the stepper is still not performing correctly. But as I had a Tandon T100-2A sitting around I put that in the top slot, configured it as D1 and booted the machine once again. The machine not only booted, it both wrote to and read from the Tandon.

    Since the Tandon is a DSDD drive it could in theory respond to a call to write to the "other" side of the disk. However, the RS cable has pin 32 (side select) removed from both drive connectors, so the Tandon will never receive that signal as long as the RS OEM cable is in use.

    Question 1 then is: In this set up is the Tandon the functional equivalent of a SSDD drive, like the TexPer it replaced, or is there more to the story? Could the Tandon create an unreadable disk, for example?
    Question 2 is: If the RS FDC uses pins 14 and 32 to signal external drive selects :3 and :4 (which is why these pins are missing on the internal cable, I'm told) then how does the Model 4D accommodate DSDD drives? Different FDC or cable?

    Thanks for your replies,

    -CH-

    PS - Another question, unrelated to the drives: The Model III boots to TRSDOS but not to LSDOS 6.x.x. What is the recommended DOS to use with a Model III?

  2. #2

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    Back in the 80s I ordered an aftermarket kit for my Model III and have always used DSDD Tandon drives.
    Normal straight through drive cable. Drive # is configured on the drive itself by the jumper pad traces. NO terminating resistor on the internal drives.
    Normal straight through cable used for external drive(s) but a terminating resistor is needed on the last drive.

    TRSDOS works fine and uses them as SSDD drives. LDOS 5.3.1 is for the Model III and it takes advantage of the DSDD drives but can still format single sided if you want.

    My controller card is made by J&M, if you have a stock Radio Shack card it may work differently?

  3. #3
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    Quote Originally Posted by clh333 View Post
    Question 1 then is: In this set up is the Tandon the functional equivalent of a SSDD drive, like the TexPer it replaced, or is there more to the story? Could the Tandon create an unreadable disk, for example?
    Yes, it's fine to use a double-sided drive in place of a single.

    Question 2 is: If the RS FDC uses pins 14 and 32 to signal external drive selects :3 and :4 (which is why these pins are missing on the internal cable, I'm told) then how does the Model 4D accommodate DSDD drives? Different FDC or cable?
    On a TRS-80 Model III or 4 only drive selects 1 and 2 (or 0 and 1 depending on how you count) are used; those machines accommodate 4 drives but the external and internal connectors each only support 2. The Model I used what was later assigned the side-select pin (32) for the fourth drive and for compatibility reasons Radio Shack's 2 drive cables for single-sided drives all had pin 32 yanked out. But the III and 4's controllers were wired so they could use pin 32 for side select, so if you want to use the second side of your double sided drive all you need to do is:

    1: Replace the cable with one that has all pins present, and

    2: Modify the settings on the stock drive so it no longer responds to all drive select pins. (You may be familiar with how IBM computers used that "twisted cable" convention so service people installing drives didn't have to change jumpers, all drives are jumpered as "B". The "pulled teeth" thing was Radio Shack's equivalent to that.)

    PS - Another question, unrelated to the drives: The Model III boots to TRSDOS but not to LSDOS 6.x.x. What is the recommended DOS to use with a Model III?
    As noted above, LDOS is the Model III equivalent of TRS-DOS 6.x.x. Most commercial software for the Model III expects to be run on TRS-DOS 1.3, but alternative DOSes have their advantages.
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

  4. #4
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    Thank you both for your information. As you can probably deduce I am not very experienced with the Model 4 and even less with the Model III. But I cut my (computer) teeth on a Z80 Kaypro II, also SSDD and I have to admit to a growing admiration for most things Tandy.

    The 16K - which I plan to keep - may look like Frankenstein when I'm through with it but this one will be box-stock and sold on as soon as I can finish it off.

    Thanks again,

    -CH-

  5. #5
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    Quote Originally Posted by Torch View Post
    My controller card is made by J&M, if you have a stock Radio Shack card it may work differently?
    The stock Radio Shack Model III card works fine with DSDD drives. I use it with a floppy emulator with 80 track DSDD images (under LDOS of course because TRSDOS doesn't support that configuration out of the box).

  6. #6
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    I've used double-sided drives externally on my non-GA Model IV, which uses the same controller board as a Model III and, yeah, it's no problem.

    (For that matter I used to use double-sided drives on my Model I; the limitation there was you could only use three drives total because of the pin 32 repurposing, but if you have a double-density board a single 40 track DSDD drive gives you as much online storage as all four SSSD's in a stock configuration, so it's a no-brainer tradeoff.)
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

  7. #7

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    make a new cable reuse the connector of the edge card (FDC side) and replace the 2 floppy side, (they are the only ones with missing pins) connectors cost about 2-3$ /each and cable will cost you about 2$/ft
    if you cannot source either of these, contact me, and Ill make you a new cable you can buy from me say for .. 15$.

    the RS floppy controller handle SSDD, DSDD, 35, 40 and 80 tracks without ANY modifications to it, just need proper cable. internal and external.

    PS: I suggest put the work DSDD drive in 0 and slap a gotek in drive 1, that way you can have as many 720K data diskettes with games, utils and word processing your heart can handle. about 10,000 720K floppies (1024*720*10,000 = 7,372,800,000Kb) on 8GB usb stick
    Last edited by Patrick Bureau; April 10th, 2020 at 06:43 PM.
    Gaming: i7-8700K, 16GB, 1TB SSD, RTX 2060, Dual 27" LG IPS Monitors.
    Vintage: TRS-80 MODEL 4, 128K RAM, 360K FDD, Gotek USB FDD, FreHD HDD, RS232-WIFI MODEM, DWP-510.
    Programmer: Dell Inspiron 660S i3-3220 with 16GB Ram, running a BK-844USB & EP-2A-88.
    Web Site: https://texastandyrestoration.com/
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  8. #8
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    Thanks again for your responses. In no particular order of importance:

    I can make or adapt a new cable; I have the pieces, but thank you for the offer.

    I want to "restore" this MIII so I will use another SSDD to replace the Texas Peripherals that - even after disassembling the stepper motor and freeing its action - still will not R/W correctly. (Problem with the circuit board, it now seems.) (Note to self: Think twice before you try that again. PITA)

    The 16K will get the upgrades and a Gotek seems like a good addition. The front face has two blanks and one could be adapted to accept the Gotek.

    I had planned to rob this MIII of its drive bay but after looking at it I think it would be possible to reproduce one without too much trouble. Anyway, I traced patterns.

    From the discussion of the RS FDC it seems that it can address two internal and two external DSDD drives at the same time. Do the external drives need to be "jumpered" as 3 and 4 (or 2 and 3), or are they addressed as 1 and 2 (0 and 1) on a separate channel. In other words, are the top and bottom FDC card edge connectors on the same bus?

    And thanks for suggesting LDOS; I have found it on Tim Mann's site.

    -CH-

  9. #9
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    Quote Originally Posted by clh333 View Post
    From the discussion of the RS FDC it seems that it can address two internal and two external DSDD drives at the same time. Do the external drives need to be "jumpered" as 3 and 4 (or 2 and 3), or are they addressed as 1 and 2 (0 and 1) on a separate channel. In other words, are the top and bottom FDC card edge connectors on the same bus?
    They're separate buses.

    By the way, as an alternative to putting drives/Gotek on the internal bus, you could consider putting them on the external bus and using the solution in the following thread to swap the internal and external buses:
    http://www.vcfed.org/forum/showthrea...on-a-model-III

  10. #10

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    the internal and external drives are addressed in the OS as 0,1,2,3 but hardware wise thy are 0,1 internally and 0,1 externally
    Gaming: i7-8700K, 16GB, 1TB SSD, RTX 2060, Dual 27" LG IPS Monitors.
    Vintage: TRS-80 MODEL 4, 128K RAM, 360K FDD, Gotek USB FDD, FreHD HDD, RS232-WIFI MODEM, DWP-510.
    Programmer: Dell Inspiron 660S i3-3220 with 16GB Ram, running a BK-844USB & EP-2A-88.
    Web Site: https://texastandyrestoration.com/
    Facebook Page: https://www.facebook.com/TexasTandyRestorations/

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