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TRS 80 Model 4 floppy drives

abruno17

Experienced Member
Joined
Mar 10, 2020
Messages
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4104 Longworth Loop Kissimme, FL 34744
I successfully converted my Model 4 to accept 360k floppy drives. But my original plan to use the floppy drives from a Model 4D parts machine didn't go well. The drives were ceased so luckily, I had some backup model T-100-2A Tandon full height drives from an old IBM computer. After many hours of fighting with the computer I got the TRS 80 to boot from the top drive. The problem was that no matter how many times I tried to get LDOS to read the second drive to load games nothing! The drive would even bother to access the second drive. I would only access the top drive. No matter how many times I typed in DIR:1 it would access the first drive! The computer thinks that there is only one drive so DIR: 0 and DIR: 1 net the same results! I know the second drive was getting power because I could see the motor moving. I don't know what to do with these Tandon T-100-2A drives. There are no jumpers. And I know there is a chip that is supposed to tell the second drive it's the b drive but that seemed to confuse the computer and it wouldn't boot from disc whatsoever! Then after some twiddling I finally got it to boot but it still won't access the second drive! What am I doing wrong?
 
Somewhere in the PC age someone came up with the idea to wire all drives as drive one and then use a cable with two drive connectors with a twist between them to swap the drive select wires. Try one of these:
 
The TRS-80 Model 3&4 have the following Pinout from the FDC.
REF: TRS-M4.PNG

The Tandon Drives have the following Connections to the FDC.
REF: TRS-M4.pdf

And somehow you are interconnecting with a crossed cable for conductors 10 thru 16
to follow the Newer IBM Standard???

Larry
 

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Also be advised you CAN NOT use the internal Tandy cable to use the Tandon drives. You must make a new cable. The tandy cable has missing pins for drive select. So you need to make a new
one that is fully pinned and let the drives do the drive select.
 
I successfully converted my Model 4 to accept 360k floppy drives. But my original plan to use the floppy drives from a Model 4D parts machine didn't go well. The drives were ceased so luckily, I had some backup model T-100-2A Tandon full height drives from an old IBM computer. After many hours of fighting with the computer I got the TRS 80 to boot from the top drive. The problem was that no matter how many times I tried to get LDOS to read the second drive to load games nothing! The drive would even bother to access the second drive. I would only access the top drive. No matter how many times I typed in DIR:1 it would access the first drive! The computer thinks that there is only one drive so DIR: 0 and DIR: 1 net the same results! I know the second drive was getting power because I could see the motor moving. I don't know what to do with these Tandon T-100-2A drives. There are no jumpers. And I know there is a chip that is supposed to tell the second drive it's the b drive but that seemed to confuse the computer and it wouldn't boot from disc whatsoever! Then after some twiddling I finally got it to boot but it still won't access the second drive! What am I doing wrong?

My guess is the cable you are using isn't compatible. Here is a nice writeup someone did connecting modern drives to their Model III. Maybe it might help you out:

 
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