Imaging TRS-80/III single-sided disks

Fred Cisin cisin at
Thu Jun 25 18:26:02 CDT 2015

On Thu, 25 Jun 2015, Alexandre Souza wrote:
>   Dear sirs,
you'll have to settle for us.

>   Imagedisk is my savior, and I image all kind of disks I know with it :)
>   But now I got a pair of TRS-80 model III single-sided disks. How do I 
> image it using imagedisk?
Same as every other disk that you have imaged.  However tell Imagedisk 
that you only want side A.  That shouldn't be a big deal, but many people 
with single sided drives reFORMATed, without erasing what was on the 
second side.  Therefore, it is possible, if they were using used disks, 
that side B might still contain side B from some previous use, such as PC 
or Apple.  And, some people using single sided drives punched extra holes 
in the jacket to be able to flip the disk over ("FLIPPY disks") to use the 
other side as if it were another disk.  so, if you see an extra write 
enable notch on the wrong edge of the disk, and/or amateur holes made to 
make index hole access symmetrical, then the disk may have been flipped 
over and written with a single sided drive.  "Berkely MicroComputer 
Flip-Jig" was the best jig ever made for marking diskettes for making 

The model 3 TRSDOS was 256 bytes per sector, with 18 sectors per track.
HOWEVER, besides TRSDOS, there were other operating systems that could be 
run on it, and some of the CP/M conversions used other sector sizes.

> Can I use a double-sided floppy drive to image it? 

Although there have existed some drives where SS and DS were actually 
incompatible, such as the difference in index hole placements on 8", Side 
A of a Single Sided 5.25" disk is the same as Side A of a DS disk.
360K drives will work just fine.

NOTE: If you use a 1.2M drive, then you will encounter exactly the same 
issues as when you use the 1.2M on a 360K PC disk.

MORE IMPORTANT NOTE:  TRS-80 used a Western Digital FDC; IBM uses NEC. 
(TYPE of FDC, disunirregardless of BRAND used)
TRS-80 starts the first sectors of the tracks earlier on the track than 
IBM.  You will sometimes encounter TRS-80 disks on which the first sector 
of each track can't be found/read by the PC!  That can actually be worked 
around fairly easily.  If you interrupt the index signal, then a disk can 
still be read or written, but can not be formatted (and any errors will no 
longer be properly understood by the BIOS).
You can interrupt the signal by making a special floppy cable without that 
wire, or with a toggle switch on that wire, or maybe putting a tiny piece 
of electrical tape on the card edge, so that that pin of the cable doesn't 
connect.   Will you remember to set things right afterwards?
Or, you could clip the wire of the index sensor (in a reversible way!!!). 
OR, you can try just putting a small piece of opague tape (such as a write 
protect tab!) over the index hole access hole of the disk jacket.  On some 
drives that are too smart for their own good, such as the otherwise nice 
Teac 55, that will be misinterpreted as drive not ready!  But it works 
great on the original full height Tandons used by TRS-80 and by PC/5150.
Do not let your tape fall off into the drive.
Sometimes it may work to slow down the motor of the drive, and run it at 
too slow a speed.

> Or do I need to put the single-sided drive on my PC? Please, help! :)
In fact 160K/180K PC drives (the SS version used before DOs 2.0 provided 
double sided capability) were often the SAME BRAND, SAME MODEL drives as 
TRS-80 model 3.
The 360K DS drives (since DOS 2.0) were still the same, with the addition 
of a second side.
Later, IBM switched to other brands and models of drives, such as "half 
height", but they remained completely compatible.

WARNING: If you DO put the TRS-80 drive into a PC, you will need to change 
some jumpers!  The original normal/standard configuration for such drives 
("SA400 interface") uses pin 10 for drive select of drive A, pin 12 for 
drive select of drive B, pin 14 for drive select of drive C, and did not 
provide for a fourth drive.

Radio Shack did not consider their customers, NOR THEIR STORE PERSONNEL, 
to be competent to change jumpers of the drives for TRS-80 model 1.  So, 
they put in ALL of those jumpers, including using another "unused" pin for 
drive D (which later drives used for SIDE SELECT!, and then they removed 
the pins in the connector cable for each position so that each drive 
connector of the cable only had one drive select pin connected.  Then, the 
drive would automagically be whichever drive that position of the cable 
was for.   Therefore, connecting a TRS-80 drive to a PC requires 
changing the jumpers.

IBM did not consider their customers, NOR THEIR STORE PERSONNEL, to be 
competent to change jumpers of the drives for PC/5150.  So, they jumpered 
every drive as if it were drive B (pin 12), and then they put a twist in 
the cable so that pin 10 of the FDC board connected to pin 12 of drive A, 
and pin 12 of the FDC board connected to pin 12 of drive B. That twisted 
cable also made changes in how the motor was turned on.  Therefore, 
connecting a TRS-80 drive to a PC requires changing the jumpers.

Both TRS-80 and PC/5150 have provision for FOUR drives, but that's not 
your current problem (except for the TRS-80 model 1 having screwed around 
with what would later become SIDE SELECT)

Grumpy Ol' Fred     		cisin at

More information about the cctalk mailing list