origins of IBM 3740 diskette format - test-drb@ccmp.vtda.org

2 Mar 2005

On Fri, 25 Feb 2005 07:12:41 +0100, Nico de Jong <nico at farumdata.dk> wrote:
...

 Yes. The discussion is not about bytes, but about card images.
 A card image (apart from some special ones like 96 bytes and "stubs" of
 about 30 bytes) is 80 bytes. Furthermore, the lowest sectorsize that could
 be formatted on a 3740, was 128 bytes. As there are 73 "user adressable"
 tracks with each 26 sectors, there are only 1898 sectors. And as the
 discussion is about card images, there is only room for 1898 cards, unless a
 technique called spanning is used. 
The original diskette design was for  a microcode load device for an
IBM mainframe. I recall there was an IBM Journal of R&D article about
it, I'll see if I can find it.

3740 DE (Data Entry) format actually had the option of 80 or 128 
characters / "columns" per record. The basic 3741/3742 data entry
station only did 80 characters per record, although the diskette
sectors were always 128 data bytes. You had to order the optional
feature (a plug-in ROM card) to support 128 characters per record.

Track 0 was housekeeping and index. Tracks 1 to 73 ware data. I still
have my 3741 alignment diskette. It has eccentric tracks, and there
was a built-in ROM routine on the 3741/2 station to read a full track
and display the sector addresses in binary. You adjusted the head
alignment to get two areas of good IDs and two areas of bad IDs.  The
binary display was unusual, it  actually displayed  what looked like a
capital "H" with additional horizontal bars for each  1 bit. "00" hex
looked like "H", "FF" hex looked like a solid block,, "18"
hex looked
like a square-edged digit "8" etc.

...

  IBM's 3740 data entry station put its stamp
of approval on the
 floppy. The 3740 format is still the de facto interchange medium
 within the industry.  This is worth a whole new topic, as I happen to know a bit
about that (I'm
 running a service bureau for media conversion)

  ...
 IBM's design for the 3740 was very conservative. At the time, IBM
 believed that floppies would be used for the batch entry of data.  That is correct;
the 3740 family came to be rather big, eventually ending up
 with the 3749(?) data entry station, where one could use DSDD floppies with
 a sector length of 1024. IIRC, it could accommodate 6 segments per track,
 ending up with 8 x 1024 x 73 x 2 = 1.1196.032 bytes

  The full-sized floppy was designed to hold the
same amount of
 information that 3,000 punch cards would hold - the maximum of
 what a single keypunch operator could do in a day.  See discussion on
"spanning"

 Nico