The dinosaur among the 5-inch floppy disk drives was launched between August (Shugart brochure) and December (tested in BYTE magazine) 1976. According to an article by George Sollman in IEEE the SA400 was introduced in September 1976. [3]

The latest news from Silicon Valley, now confirmed from its source, is the Shugart Associates Minifloppy ["minifloppy" is a Shugart trademark] disk drive for small systems. [1]

Watch out the steprate! Modern 5" drives have steprates from about 6 or 3 ms! Older 8" drives have 10 ms.

I'm not entirely sure, but according to the technical specifications, the SA400 can only use the FM method, see above Transfer Rate and Encoding Method. The SA400L mentions both FM and MFM.

You need an old computer (floppy disk controller) to run this slow drives! For example an IBM PC 5150, N* Horizon, IMSAI or Altair. Perfectly suitable for the single-density format (FM).

Stepper - Head positioning

As with the BASF 6106, a spiral turntable is also used here as the stepper for head positioning. You can hear this stepper immediately because this system makes a very characteristic noise.

The SA400 drive exhibits 'gear lash' in its head positioner - the head will be off-track slightly toward the outside diameter when a track is approached from the outside (stepping toward a higher track), and will be off-track slightly toward the inside diameter when a track is approached from the inside (stepping toward a lower track). [4]

Use the program NODEXER.COM by Martin Eberhard in combination with a scope or a alignment tester and exerciser (ex. Lynx 470) to check your SA400.

My SA400 was manufactured in December 1979 at the earliest, because the newest IC has the date code (7912); red circle. However, it is probably from 1980. The oldest IC has the date code 7347; yellow circle.

And here is the Shugart SA400 in the North * Star Micro disk system.

Serial A43708

It is hard to believe that the rotation speed is exactly 300 RPM; measured with a 50 Hz stroboscope, the test pattern stands still.

I connected the drive to my N* Horizon with the FM Controller MDC-A4, started it up and it boots perfectly, what more could you want. So the drive is 43 years old and it works flawlessly.

Serial 026116

Today (12/09/2023) I tested another SA400 which was installed in the MDS shown above. It was a bit dusty but otherwise fine.

This SA400 was manufactured in week 38, 1977 at the earliest. The oldest IC has the date code 7341.

After a few initial difficulties - who can give you full performance after such a long deep sleep - it works (partially) again. However, the VERIFY test (COPY.COM) was still not perfect. But after two or three adjustments to the potentiometer R12, the rotation speed was adjusted exactly to 300 RPM (see below) and lo and behold, it now works perfectly with my MDC-A4.

Spindle Rotation

Actually, I always test the rotational speed of a floppy disk drive with the speed pattern and my strobe on the iPhone. That works very well. But today I tried the program NODEXER.ASM (North Star Disk Exerciser for CP/M, Version 1.02, Single Density Controller) by Martin Eberhard. I needed two floppy disk drives for this, because only about 85 KB fit on one floppy disk. I did the transfer from NODEXER.ASM to drive B: as usual with PGGET from Mike Douglas.

Trial 1

B>dir
B: NODEXER  ASM
B>a:asm nodexer
CP/M ASSEMBLER - VER 2.0
OUTPUT FILE WRITE ERROR
END OF ASSEMBLY

B>dir
B: NODEXER  ASM : NODEXER  PRN : NODEXER  HEX
B>a:stat b:

Bytes Remaining On B: 0k

B>a:stat *.*

 Recs  Bytes  Ext Acc
  276    35k    3 R/W B:NODEXER.ASM
   30     4k    1 R/W B:NODEXER.HEX
  312    39k    3 R/W B:NODEXER.PRN
Bytes Remaining On B: 0k

The first attempt to compile the program failed. What was the reason? Too little space on drive B! As the assembler always writes the PRN file as well as the HEX file by default, 85 KB is not enough.

Trial 2

I had to do a bit of research before I found the solution. And it is in the imaginary file name extension .bbz. Original .shp: source-hex-print

The first "b" loads the ASM file from drive B:. The second "b" writes the HEX file to drive B: and the "z" suppresses the writing of the PRN file. [5]

B>a:asm nodexer.bbz
CP/M ASSEMBLER - VER 2.0
08EA
00AH USE FACTOR
END OF ASSEMBLY

B>dir
B: NODEXER  ASM : NODEXER  HEX
B>a:load nodexer

FIRST ADDRESS 0100
LAST  ADDRESS 0895
BYTES READ    0796
RECORDS WRITTEN 10

B>nodexer

North Star Single Density Disk Exerciser Vers. 1.02
      by Martin Eberhard

Type ? for help

+?
BO         Boot from A:
LH         Load/unload head (drive select/deselect)
MS         Measure spindle rotation
RE         Restore
SK [tt]    Seek track tt (0-39)
...
...
+MS
Measuring CPU Speed: 3.9943 MHz Z80 CPU detected
Measuring spindle revolutions. Q to quit.
200.67 mSec/rev
200.63 mSec/rev
200.65 mSec/rev
200.66 mSec/rev
200.59 mSec/rev
200.61 mSec/rev
200.67 mSec/rev
...

short calculation:
300 rev/min
5 rev/sec
0,2 sec/rev
200 msec/rev
-> looks very good
-> looks very good

The N* Horizon reacts very sensitively to the rotational speed. This must fit.

• Shugart brochure, 1976
• Shugart OEM manual, 1978
• Shugart service manual, 1979
• Shugart service manual, 1981

• c't Magazine: Das Floppy-Laufwerk in Theorie und Praxis, Dec 1983, part 1
• c't Magazine: Das Floppy-Laufwerk in Theorie und Praxis, Jan 1984, part 2
• c't Magazine: Das Floppy-Laufwerk in Theorie und Praxis, Feb 1984, part 3

• CHM - Computer History Museum: Alan Shugart - About the Floppy Disk.
• CHM - Computer History Museum: Oral History Panel on 5.25 and 3.5 inch Floppy Drives.

1. (↑) BYTE, December 1976, page 86
2. (↑) Shugart SA 400 Brochure, August 1976
3. (↑) IEEE Transactions on Magnetics, volume: 14, issue: 4, July 1978, page 160
4. (↑) Martin Eberhard & Mike Douglas, North Star Disk Exerciser for CP/M, Version 1.02 (Single Density Controller)
5. (↑) Mitchell Waite & John Angermeyer, cp/m bible, Howard W. Sams & Co., Inc, 1st Edition, page 222, ISBN: 0-672-24007-6