IBM 1620 invalid character symbol

Philippe Verdy via Unicode unicode at
Tue Sep 26 13:52:10 CDT 2017

2017-09-26 17:45 GMT+02:00 Ken Whistler via Unicode <unicode at>:

> Leo,
> Yeah, I know. My point was that by examining the physical typewriter keys
> (the striking head on the typebar, not the images on the keypads), one
> could see what could be generated *by* overstriking. I think Philippe's
> suggestion that it was simply an overstrike of "X" with an "I" is probably
> the simplest explanation for the actual operation. And the typeset manuals
> just grabbed some type that looked similar. Note that the typewriters in
> question didn't have a vertical bar or backslash, apparently.
> But adding an annotation for similar-looking symbols that could be used
> for this is, I agree, probably better than looking for a proposal to encode
> some new symbol for this oddball construction.
> If it really is an overstrike, then technically, it could probably also be
> represented as the sequence <0058, 20D2>, just to represent the data.
> --Ken

Many old computers have used the overstriking (non-advancing) X to cancel
some text. I bet that computer had the necessary printer control to lock
the paper position when striking the X just before striking the I.

On some terminals you could cancel the previous printed character by
emitting a CANCEL or DELETE control and it would overstrike an X over the
previous letter (remembering that these typewriters had only fixed-width
characters). But this xcould have been slow if the carriage was not locked
(look at the ugly metallic bar on the right which is there to avoid the
carriage to get out of rails and guide it, the right-ward force was
certainly quite strong and had to be compensated, but a emitting backspace
would have resulted in very slow printing and thre must have been a way to
keep the cariage locked to avoid advancing when striking the first letter
before the next advancing letter.

Yes there was no vertical bar on the keyboard, and only capital letters (so
this cannot be a lowercase L). But it had a distinctive asterisk and there
was certainly a need for distinction.

I also bet that the two other symbols with the vertical bar and single or
double horizontal bar (for data separators) were printed as well by the
typewriter as overstrikes (capital letter I plus equal sign, or capital
letter I and "dash",  and note that this is named "dash" in the manual, not
specifically "hyphen" or "minus").

At that time the precision of character encopding was not the goal. If that
looked similar enough it was good enough.

If there was a card puncher, and the cards were not only punched but also
printed for reading by humans, apparently it was already using with a dot
printer and the characters look a bit different (we can clearly see how the
asterisk looks like). However the cards shown may have been produced by
another device.

The encoding used (based on IBM punched cards) was also a (very incomplete)
precursor of the EBCDIC encoding used later.

It is fascinating how these machines could perform any arithmetic using a
lookup table even for basic additions (the lookup we probably necessary due
to the complex and ugly encoding and the way it was handling digits on 6
bits (4 BCD plus an additional sign bit and a flag bit used in intermediate
computing steps). But how could they even compile a Fortran program ?
Probably the programs were compiled on a more powerful machine to generate
the assembly code transfered to the machine using a card reader or paper

Anyway these machines were certainly complex to handle as the operator had
to know the numeric assembly code to perform basic functions or preparation
of the machine, or could easily corrupt the program, and had to remember
some numeric addresses.
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