[GreenKeys] Re: High Speed Printer

[Douglas W. Jones]

The original message in this thread described an adding machine
printer where the type bars were raised to the correct height and
slammed into the page.  Some of IBM's electromechanical tabulating
machinery from the first half of the 20th century had these
mechanisms, I think, but I don't know that they were used as
computer printers.

High-speed computer printers (before the ink-jet era) were typically
of one of two kinds:

1) Drum printers.  The type was on a rotating drum, spinning at fixed
page, with the entire alphabet repeated around the drum once per
column on the paper.  The drum spanned the entire width of the line.
The hammers struck the paper from behind, slamming it into the drum
just as the right character went by on the drum.  IBM used these in
their first vacuum tube era computer printers (I have an IBM 701
manual from 1953 that explains how they work).

2) Chain printers.  Each character of type was part of one link of a
chain.  The alphabet was typically repeated several times around the
chain.  The chain ran in a track across the line to be printed, at
high speed.  Again, print hammers struck the paper from behind,
slamming it into the type just as the correct character went by.

In both types of printer, the entire alphabet passes by each spot on
the page many times per second.  In a 300 line-per-minute printer,
the entire alphabet must go by 300 times per second.  In both types,
the ribbon was as wide as the paper and was rolled slowly back and
forth between reels above and below the print mechanism, which was
mounted horizontally.

When drum printers were badly maintained, they had vertical
registration problems, with individual letters in a line jostled up
and down.  Chain printers tended to have horizontal registration
problems, which were a bit easier on the eye.

Chain printers were frequently made so that each print hammer spanned
two or three columns, and the letters on the chain were spaced
correspondingly far apart.  This had several advangages:  Fewer
links in the chain, fewer hammers, and a lower peak power
requirement.

The peak power requirement of these printers was determined by the
likelihood that all the hammers would be fired at once.  In the
crudest case, imagine a drum printer where all the characters were
aligned identically around the drum.  Then print this line:

--------------------------

All the hammers must fire at once.  You'd typically blow out the
power supply when you did this.  So, they skewed the characters on
the drum.  Of course, if you knew the skew pattern, you could cause
trouble by printing it out:

ABCDEFGHIJKLMNOPQRSTUVWXYZ

The worst case on a chain printer with characters spaced one per
column was to attempt to print an image of the chain.

The GE Terminet terminals had chain printer mechanisms that could
easily handle a communications line at 1200 baud (7-bit ASCII).

		Doug Jones
		jones at cs.uiowa.edu