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Thread: Model 4 3.5" FDD Conversion.

  1. #1

    Default Model 4 3.5" FDD Conversion.

    So, forgive me if this has been asked before, but I've looked and I've seen it done, but I have never seen HOW they did it.

    I have a TRS-80 Model 4, and I'm tired of having a tower dedicated to making 5.25 disks. Which is more like 2 computers since the floppy computer is too old for the internet.

    So... I'm looking to convert to 3.5 FDD's on my Model 4. Something along the lines of two 3.5s in the lower bay, and one full height 5.25 in the top. Just like TRS-Ian did in his video.

    Now, to the questions. What do I need to do this conversion? Will any particular drives work? What cable do I need? Do I need any adaptors to power the drives?

    Many thanks, and I look forward to your replys.

    These are mine:
    A TRS-80 Model 4, iMac G3 SE, Gateway Tower--My first computer. From circa 2002, Compaq Armada 1550DMT. Oh, and a Heathkit HERO Jr. Robot--Your friend, companion and security guard!

  2. #2
    Join Date
    Feb 2011
    Eugene, OR
    Blog Entries


    I’m currently working on my own TRS-80 Model 4. Besides installing a 64K memory kit, I’m swapping out the Model 4s two 180K floppy disk drives for a pair of full height 360K floppy drives I’ve sourced out of other computers. That will effectively double the amount of floppy disk space. The only thing I really have to change when I install the 360K drives is to also swap out the current floppy cable, because Tandy chose to use floppy cables with some of the pins removed. By swapping the existing cable with one that still has all of the pins still intact, the Model 4 will accept the larger 360K, and other formats of floppy disks. However, I dont know how to add a 3rd floppy. Disc drive. When I’m done though, I’ll have the computer outfitted with two full height 360K floppy disc drives, and 128K of memory.

    Later, I’ll be swapping out the B&W monitor with either a green, or amber (preferably an amber one) display. However, because I know very little about working with or around the voltages inherent with CRT monitors, so instead of attempting to go it alone, I’ll probably take the computer repair shop that’s only a couple blocks away.

    That’s my 2 cents worth. I hope anything I’ve mentioned will be of some help.

    Good luck...
    Wanted: - Complete SERIES ONE TRS-80, IBM 5170 AT, Original authentic IBM 5100 Portable, IBM 5151 Monitor. Also looking for Packard Bell 75Mhz computer, Commodore Pet w. Built-in cassette drive, amber 12 CRT, Mac SE-30 hard disk to replace the bad drive currently installed, TRS-80 Coco series 1 upgrades including memory, IBM PC 5160 compatible CD ROM.

  3. #3


    Ok. So I opened up the Model 4. And took a look at the Motherboard. Above the external floppy cardedge, there's a 34 pin male connector. Is this for floppy drives? Also, I made an adapter from the 4 pin Molex to 3.5 FDD power plug. Am I correct in thinking that the TRS-80 Molex colors/power are Red=5V, Orange=12V, Brown=0V/Ground?

    So would my adapter be like this:

    Red. To. Red
    Yellow to. Orange
    Black. To. Brown
    These are mine:
    A TRS-80 Model 4, iMac G3 SE, Gateway Tower--My first computer. From circa 2002, Compaq Armada 1550DMT. Oh, and a Heathkit HERO Jr. Robot--Your friend, companion and security guard!

  4. #4
    Join Date
    Jun 2015
    Sioux Falls SD


    3.5" floppy drives only use +5V and ground, but the commercial adapters I have use all four wires. Red is +5.

    Frank Durda IV wrote a great reference to floppy use in TRS-80s

    If you have 3.5" drives with jumpers, then you can use a straight cable to use 80 tracks and 2 sides (720K). A list of those I've found is at the end of the post. I use Chinon FZ-357 drives that have jumpers.

    If you have 3.5" drives with no jumpers, then you need an adapter. @ldkraemer made some and they work great. He also has written some info on hardware. You might search the forum for it.

    I have 4Ps so not familiar with floppy controller on the 4.

    3.5" floppy drives with jumpers according to documentation (not an exhaustive list)
    CHINON FZ-357
    PANASONIC JU-257-03T
    PANASONIC JU-257-03P
    PANASONIC JU-257A-083P
    PANASONIC JU-257A-103P
    PANASONIC JU-257A-293P
    PANASONIC JU-257A-624P
    TEAC FD235HF-3xxx
    TEAC FD235HF-63xx
    TEAC FD235HF-65xx
    TEAC FD235HF-75xx
    Y-E DATA YD-702B-6039B
    Y-E DATA YD-702B-6037B
    Y-E DATA YD-702D-6037D

    No jumpers
    EPSON SMD1040-418
    EPSON SMD1040-321
    EPSON SMD1040-419
    MITSUMI D359T3
    PANASONIC JU-257A-081P
    PANASONIC JU-257A-101P
    PANASONIC JU-257A-104P
    PANASONIC JU-257A-294P
    PANASONIC JU-257A 746P
    SONY MPF 420-1
    TEAC FD235HF-4xxx
    TEAC FD235HF-Axxx
    Y-E DATA YD-702D-6537D

  5. #5


    Ok ok, I know this sounds stupid, but humour me.

    How is it that you guys can use a regular cardedge FDD cable on the external drive connector? It doesn't have a slot for the keyed notch on the connector?
    These are mine:
    A TRS-80 Model 4, iMac G3 SE, Gateway Tower--My first computer. From circa 2002, Compaq Armada 1550DMT. Oh, and a Heathkit HERO Jr. Robot--Your friend, companion and security guard!

  6. #6
    Join Date
    Sep 2011
    Melbourne, Australia


    Yah its a pita how they did that. (The same problem also precludes connecting Coco FD50x drives to a Model I easily). Either buy cable connectors which don't have the keyway or remove the motherboard and carefully cut a notch with a hacksaw. I have done it both ways, depends on what cable I have on hand or if I am making new cables.

    *There is never a charge for Tech Support even if you don't purchase from me - We are Enthusiasts Helping Other Enthusiast and that is just the way it is

  7. #7


    Cant you just grab the key with a pair of pliers and pop it out? Although I like having the key there, I have removed them before when I encountered the problem.

  8. #8


    On the most connectors the key is fixed. You won't get it out without damage to the contacts nearby.

    If you're lucky you can snatch a card edge connector with a removable key but these are hard to find nowadays. These connectors have slots between the contacts to insert the key anywhere you like.

    card edge connector.jpg


  9. #9


    Wow I have been very lucky, I got 30 of them from unicorn with slots in them, needed to meet a minimum order. I guess the name of the company kind of says it all though.


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