Before we move on, let's take another look at the Jacquard loom. We want to
do this because the Jacquard loom is a mechanical analog of a typical video
subsystem. Most historians consider the Jacquard loom the earliest automated
machine that works on modern computer principles. Describing its operation can
help demystify how a computer video subsystem works.
A
Jacquard loom
The Jacquard loom used a series of heavy paper cards with a pattern of holes
punched into them.
The
cards of a Jacquard loom.
The loom had a matrix of dowels or pins (called needles) against which the
cards were pressed sequentially. Each pin was connected to a hook such that if
the pin did not pass through a hole in the card, the pin pushed the hook out of
the path of a lifting bar (the knife).
A
frame from a video about the Jacquard loom produced for the Macclesfield
Silk Museum (youtu.be/pzYucg3Tmho).
The presenter is pressing one of the needles, showing the associated
hook pushed out of the knife's path.
In the next step, the lifting bar lifted the unpushed hooks. Each lifted hook
pulled a warp thread. Therefore, each hole in a card resulted in a lifted warp
thread. The shuttle, carrying the weft thread, was then passed between the lifted
and unlifted warp threads.
Passing the shuttle between the warp threads on a simple loom.
As the process was repeated, a pattern, predetermined by the pattern of holes
in the sequence of cards, was woven into the cloth.
Finished cloth from a Jacquard loom.
Let's compare the Jacquard loom to the video terminal described above.
Instead of holes punched in cards, we have information stored in ROM memory.
Let's say this ROM presents stored information at TTL levels[1] of either 0 volts
or +5 volts. While scanning the character bitmap in ROM, if the hardware
encounters 0 volts, it turns the electron beam off, and if it encounters +5
volts, it turns the beam on. This is much like the Jacquard loom lifting a weft
thread when a pin encounters a hole in the punched card and not lifting the
thread when the card pushes the pin into the receiver.
Let's now replace the ROM with RAM, where we can store any arbitrary pattern
of binary numbers. Instead of using a text buffer to find what character to
display next and then finding the bitmap for that character in ROM to draw the
display, we can go directly to the pattern stored in RAM and display any pattern
of on and off spots we want. We now have a graphics terminal instead of a
text-only video teleprinter. The paper cards with holes are much like the RAM
frame buffer (except the cards are more like ROM than RAM), and the lifted or
not lifted warp threads are much like the electron beam being turned on and off
as it scans the screen.
We can think of the Jacquard as the first modern digital video subsystem. It
even had a color display.
