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Thread: Where to start?

  1. #31
    Join Date
    Feb 2015
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    Cleveland, OH, USA
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    While waiting for DigiKey parts delivery I have been investigating the Model 4 further, and have more questions. Thanks to Larry and Patrick for their explanations - I'm just slow.

    -there is such a thing as an amber CRT on a Model 4, as I have seen on the "Show us your Tandy" thread. (Nice work, guys!)
    Was this a production model or a post-mod? Is this tube still available? I always admired amber monitors, never had one.

    -the 5.25 FD system is different from the IBM PC "standard", this much I know.
    But I'm not clear on how they differ.

    -apparently you can swap out the SSDD drives for DSDD drives without changes to the FDC, but can't substitute HD for DD.
    Are any BIOS changes necessary as a result? Where would they be made, software or firmware?

    -my SSDD drives - which have a sticker labeled "Texas Peripherals", another mfr. I've never heard of - do not appear to have jumpers for drive designation (0,1,2, etc.) although one source I read suggested they do.
    Does (only) the position on the cable determine which is A and which is B? In other words, are they interchangeable? One appears to be working better than the other.

    -there is no twist in the cable, which in a PC allows both drives to be set up as B / 1 and one of them to be addressed as A / 0 by the FDC.
    I can find the pinout for a PC cable, but I haven't found one for an M4. Could someone please point me in the right direction?

    -in some TRS cables there is voltage on what would be ground pins on a PC cable.
    Is this true for the M4 or only T1000 and later.?

    -there is no terminal resistor on the internal drives but one is required at the end of the external bus.
    Why is this not necessary?

    Thanks for any info,

    -CH-

  2. #32

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    Smiles, Ill try to answer your questions as clearly as possible, but let it be known (and the crowd gasps in fear) I dont have all the answers.

    Q) there is such a thing as an amber CRT on a Model 4?
    A) yes, I have one, and good luck finding one, they are rare... they where 3rd party, Blue and red are also available at the time. Langley St-clair was a big producer for TRS-80 and other compatible models here is an example of their ads *see attached below*
    7YePIYc.jpg

    Q) How he 5.25 FD system is different from the IBM PC "standard"?
    A) the PC standard did not exist, there were little standards in those days, and TRS-80 are no exception. TRS-80 cable select was done by removing pin from cable edge connectors and removing the selector of drive.

    Q) but can't substitute HD for DD?
    A) yes you can use a double sided floppy drive because the FDC can handle DSDD 35,40 or 80 tracks (160K, 180K, 360K, 720K) because it was a MFM Standard at the time.
    the HD (1.22 or 1.44MB FDD) cannot be use because it was a new standard and need a different FDC (PC standard) that never was designed for the TRS-80 series computer

    Q) my SSDD drives - which have a sticker labeled "Texas Peripherals", another mfr. I've never heard of - do not appear to have jumpers for drive designation (0,1,2, etc.) although one source I read suggested they do.
    Does (only) the position on the cable determine which is A and which is B? In other words, are they interchangeable? One appears to be working better than the other.?
    A) Texas peripherals OEM's for TRS-80 and Kaypro computers and those drives where "rigged" the way the Manufacturer wanted them OEM's and because of it a Drive 0 and a drive 1 are interchangeable because the drive selection is done via the removed pin on the edge connector of the cable.

    Q) I can find the pinout for a PC cable, but I haven't found one for an M4. Could someone please point me in the right direction?
    A) I already shown you the picture of the OEM R/S cable in previous page they are the same for the models 3, 4, 4D and 4P.
    if your looking for data-line information here it is found on the technical manual :https://archive.org/details/Model_4_...ference+manual
    the information you seek starts at page 59 " Floppy disk interface" and disk drives information starts at page

    Q) in some TRS cables there is voltage on what would be ground pins on a PC cable. ?
    if your referring to the non standard "pc cable" used by Tandy in their later 1000-2000-3000 models, this doesn't apply here.

    I hope I have answered "some" of your question clearly.
    Last edited by Patrick Bureau; January 21st, 2020 at 06:40 AM. Reason: typos and various adjustements
    Vintage: TRS-80 MODEL 4, 128K RAM, 360K & 720K FDD, Gotek, FreHD, GURU RS232-WIFI MODEM, DWP-510.
    Web Site: https://texastandyrestoration.com/
    Programmer Inspiron 660s, I5-3330s, 8GB Ram, 1TB HDD, BK-844USB & EP-2A-88.
    Workstation: Optiplex 7010, I5-3470s, 8GB Ram, 1TB HDD.
    Gaming: i7-8700K, 16GB, 1TB SSD, RTX 2060, Dual 27" LG IPS Monitors.

  3. #33
    Join Date
    Mar 2013
    Location
    Chaffee, MO
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    -there is no terminal resistor on the internal drives but one is required at the end of the external bus.
    Why is this not necessary?
    Because Tandy (Radio Shack) Engineers installed those PULLUP Resistors on the FDC PCB, versus
    using the ones available on the Floppy Drives.

    In the Photo below E is External and I is Internal for the FDC Signals.

    Larry

    M4-Pullup.png

  4. #34

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    I knew someone would catch that bit, that I only knew it wasn't needed, but not the why. thanks Larry
    Vintage: TRS-80 MODEL 4, 128K RAM, 360K & 720K FDD, Gotek, FreHD, GURU RS232-WIFI MODEM, DWP-510.
    Web Site: https://texastandyrestoration.com/
    Programmer Inspiron 660s, I5-3330s, 8GB Ram, 1TB HDD, BK-844USB & EP-2A-88.
    Workstation: Optiplex 7010, I5-3470s, 8GB Ram, 1TB HDD.
    Gaming: i7-8700K, 16GB, 1TB SSD, RTX 2060, Dual 27" LG IPS Monitors.

  5. #35
    Join Date
    Feb 2015
    Location
    Cleveland, OH, USA
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    Parts arrived from DigiKey, replaced the defective caps, reassembled drive tower and power supply. Waiting for memory from Australia.

    -CH-

    Pics:

    Rebuilt1.JPG Rebuilt2.JPG

  6. #36
    Join Date
    Oct 2019
    Location
    Richland, WA (USA)
    Posts
    27

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    > -the 5.25 FD system is different from the IBM PC "standard", this much I know.
    > But I'm not clear on how they differ.

    I've been working on a Model III drive situation and recently posted some notes to the (currently) last page of this thread: http://www.vcfed.org/forum/showthrea...-floppy-drives [That post has been stuck in new-member moderation for a few days but I'm assuming it will make it through before this one does anyway…]

    I won't repeat all the details I posted there or some of the other responses you've gotten but the basic difference is that these are "Shugart" style drives rather than "Common PC" pinouts. I found the tables and discussion at https://old.pinouts.ru/HD/InternalDisk_pinout.shtml to be helpful.

    The drives in my Model III were permajumpered to respond on all four drive selects (the motor signal is shared — key difference from IBM drives) and the cable edge connector is missing 3 of these for each slot. In my testing I found some thin slices of electric tape worked in lieu of the "pulled pins" of the original cable. You can see some pictures and notes in my postings at https://twitter.com/natevw/status/1220216344899010560

    Finally, you asked about using other drives. I found the notes on http://www.vcfed.org/forum/showthrea...344#post157344 to be helpful — the 180K/360K drives were actually a diversion running at 300rpm whereas the original 8", HD 5.25", and 3.5" drives all ran at 360rpm. That said because of the ubiquity of the 360K drives a lot of 5.25" HD drives could run at that speed and therefore might also be useable. With that background, you might then find http://nemesis.lonestar.org/computer...floppyfix.html to be a great reference as to replacing a Shugart drive in any of the early TRS-80 machines with a newer 5.25" or even 3.5" one.

  7. #37
    Join Date
    Oct 2019
    Location
    Richland, WA (USA)
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    Forum/moderators ate my post from a few days ago, but I'll try more briefly re-share some of the tips:

    > -the 5.25 FD system is different from the IBM PC "standard", this much I know.
    > But I'm not clear on how they differ.

    The main thing is that these are "Shugart" style drives. Furthermore, the drives in my Model III were permajumpered to respond on all four drive selects and the cable itself uses left-out contacts ("pulled pins") to do the selection. I found a nice reference for the pin differences, PM me if you want details I guess, or browse on the Pinboard bookmark site for links tagged "floppy" by my same username there, since I suspect it's the links that got my earlier post dropped which is unfortunate :-/

    You also asked about using other drives. This is possible if you take into consideration the Shugart vs. PC pinout caveats as well as some RPM and tpi differences. Same story on the links here though if you search these forums for a "Teac FD 55FR 511U is a 720KB drive" thread there's some helpful background on drive differences/history.

  8. #38

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    as shown in the picture of your PSU, I would pull it off the frame and re-float all the connectors points, why? because with age in my experience a lot all have cracked solders and ref-floating them take only a few minutes and secures them back in position, especially the video board connector (2 pin connector on the picture), this is what I suggest to do while you wait
    Vintage: TRS-80 MODEL 4, 128K RAM, 360K & 720K FDD, Gotek, FreHD, GURU RS232-WIFI MODEM, DWP-510.
    Web Site: https://texastandyrestoration.com/
    Programmer Inspiron 660s, I5-3330s, 8GB Ram, 1TB HDD, BK-844USB & EP-2A-88.
    Workstation: Optiplex 7010, I5-3470s, 8GB Ram, 1TB HDD.
    Gaming: i7-8700K, 16GB, 1TB SSD, RTX 2060, Dual 27" LG IPS Monitors.

  9. #39
    Join Date
    Feb 2015
    Location
    Cleveland, OH, USA
    Posts
    1,048

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    I was hoping for some witty quote as a response. Got nothing. Good idea.

    -CH-

  10. #40

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    slips you a witty quote , you can borrow it, but you have 30 days to return it or you will be charge late fees:
    "A computer would deserve to be called intelligent if it could deceive a human into believing that it was human."- Alan Turing
    Vintage: TRS-80 MODEL 4, 128K RAM, 360K & 720K FDD, Gotek, FreHD, GURU RS232-WIFI MODEM, DWP-510.
    Web Site: https://texastandyrestoration.com/
    Programmer Inspiron 660s, I5-3330s, 8GB Ram, 1TB HDD, BK-844USB & EP-2A-88.
    Workstation: Optiplex 7010, I5-3470s, 8GB Ram, 1TB HDD.
    Gaming: i7-8700K, 16GB, 1TB SSD, RTX 2060, Dual 27" LG IPS Monitors.

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