IBM 2741 -- ASCII or EBCDIC?

Tom Watson sdc695 at yahoo.com
Thu Dec 28 04:46:45 CST 2006


Richard <legalize at xmission.com> said:
> The subject pretty much says it all... which character code did the
> 2741 typewriter/terminal use?

Short answer:  Neither.
Long answer:
2741's came in two flavors, "correspondance" and "BCD" (someting close to
PTTC/8).  The serial code transmitted is 6 bits and parity (total of 7 bits)
with start and stop bits.  In PC parlance: 6E1.  The transmission speed is
134.5 bps.  Most baud rate generators actually have this speed.

The tricky part of this is that the 2741 communicates in HALF duplex.  It has
character codes that turn the line around.  On the terminal proper the <return>
key and the <attn> key.  If you actually have a 2741, it might have manuals in
its back which describe some of this stuff (cryptically as I remember it).  If
you want to release the typing mechanism, you can QUICKLY cycle the power and
it will release.

The difference between the codes is only the position on the ball and the
encoding of the keyboard.  If you feel up to it, you can use the other type of
ball on the foreign machine.  You need to send it the right codes.  The
keyboard can't be changed, so if you have the wrong code, it types "wrong".

Why two codes?  History.
Correspondance code was what they used when they coded the original typeball on
the selectrics (before they were used for computer I/O).  They made the ball
that way to minimize thavel for frequently used codes and to not have pairs of
large letters (like M and W) next to each other.  The BCD codes were used when
they adapted the units to computer use.  IBM changed the keyboard coding on the
BCD units (there is some documentation on bitsavers) to correspond (bad word in
this context) to  what they changed the typeball to.

Simple facts:  The typeball has 88 positions.  Two shifts, 44 in each shift. 
There are 4 tilt positions and 11 rotate positions (neutral and 5 each way left
and right).  At least that is the way on Selectric 1's and 2's.  Selectric 3's
have 96 positions.  Inside the typing unit, it takes 6 bits to make something
happen:  Tilt-1, Tilt-2, Rotate-1, Rotate-2, Rotate-2A, and Rotate-5(minus). 
The 2741 has codes (that don't type) much like an old 5-level teletype to do
shifts (upper case/lower case) and space. backspace, index (we call it line
feed) and return (CR and LF combined). 

Just so you know:  There is a symbol B134 in /usr/include/bits/termios.h on the
Linux box I'm typing this on.  The speed is actually 134.5 bps.  See the man
pages

Another wierd fact:  Normal baud rate generators (and uarts) use a 16x clock. 
For 134.5 bps this is 2152 cps.  This is very close to a modem tone used in 103
type modem (2025Hz and 2225Hz).  It can really screw up accoustic couplers (I
found out the "hard way"!).

Wierd fact:  When IBM contracted with Qume for the units in wheelwriters, they
used correspondance code on their printwheels.  Qume had an awful time since
some of the tines on the wheel had adjacent "high density" codes.  Wierdness
compounded.

We now return you to the regularly scheduled mailing list for even more
trivia...



-- 
Tom Watson
tsw at johana.com

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