The Xerox 1200 Computer Printer, introduced in the early 1970s, was a unique machine in the xerographic lineup. Unlike other Xerox copiers and duplicators of the time, which used optical exposure of full‐page images onto a photoconductive drum, the 1200 used a character generator system with a light-modulating optical drum. This key difference in how characters were written onto the drum is the reason for its distinct design‐including its reversed polarity in charging and development.

 How the Optical System Worked

• A spinning optical drum, coated with thousands of tiny lenses or reflective facets (think of it as an early version of a rotating digital display).
• A bright xenon or mercury arc lamp provided the light source.
• Character fonts were etched or imaged onto film or a mask wheel, which was synchronized with the optical drum.

 Here's how it created characters:

• Character Selection: The data stream from a mainframe or computer selected which character to print, one character at a time, across the width of the page.
• Modulated Light: As the optical drum spun, light from the lamp passed through the character mask (or was reflected by facets) at just the right moment, generating a pattern of light corresponding to the character shape.
• Image Formation: This modulated beam of light was then focused onto the photoreceptor drum, exposing it in discrete dots, essentially writing the character on the drum one line at a time, in sync with the rotating optics.

This process scanned the page in raster fashion, forming complete lines of text via moving light rather than via mirrors or full-page projection optics.

Why Charging and Development polarities were Reversed in the 1200 compared to other copiers of the time:

In standard Xerox machines (e.g., the 2400, 3600), the photoreceptor drum was:

• Positively charged, then exposed to light where the image was to be formed.
• Light, where print wasn't, discharged the drum, and toner (typically negatively charged) adhered to the more positively charged, un-exposed areas with print.

But the Xerox 1200 reversed this for two main reasons:

• Light‐as‐character source: In this system, light was used to "write" the text, not to expose a reflected image of a page. Therefore, the characters were created by exposing bright light onto the drum, while the background remained unexposed.
• Toner polarity and contrast: To ensure the toner would develop only where the characters were exposed (i.e., where light hit the drum), the charging had to be inverted:
• The drum was given a negative charge.
• The light exposure discharged it, creating a negative latent image where the characters were'nt.
• Positively charged toner then adhered to those negatively charged areas‐the text.

This reversal of polarities ensured that toner would develop only in the areas corresponding to the exposed characters, rather than in the background, resulting in sharp and clean character printing.

Summary
The Xerox 1200 was not a copier, it was a high‐speed electrophotographic printer designed for computer output.

It used optical light modulation and a spinning optical drum to expose characters directly onto the drum.

This system required opposite charging and development from standard Xerox copiers because it wrote with light instead of shadows.

The resulting print quality was very sharp for text, making it ideal for computer-generated documents, reports, and forms.