HP DeskJet 1200C printer architecture - includes related articles on use of solid modeling CAD system in design, on how reusable packaging and rugged design can minimize environmental impact, and on a new model for product development - HP DeskJet 1200C and HP DeskJet 1200C/PS ink-jet printers - Technical

Hewlett-Packard Journal, Feb, 1994 by Kevin M. Bockman, Anton Tabar, Erol Erturk, Robert R. Giles, William H. Schweibert

The product architecture of the HP Deskjet 1200C printer--mechanical, electrical, and firmware--played a key role in addressing the technical challenges demanded by the office color printer market.

In office printers, not only are speed and low cost paramount, but also print quality and the emerging use of color. The HP Deskjet 1200C printer was conceived to address these issues. This article discusses the printer's architecture and its development.

The primary applications for this printer are laser-quality black text, spot color, full color, and transparencies. Challenges for the design team were achieving print quality and speed goals for these applications, becoming LaserJet compatible, achieving small size and ease of use for the office, and designing for high-volume production.

The major performance features of the HP Deskjet 1200C Printer include:

* LaserJet-equivalent print quality

* 300-dpi color

* Outstanding color quality

* Six-page-per-minute text speed

* Four-page-per-minute high-quality mode

* One-to-two-minute-per-page color speed

* Industry leading color cost per copy.

The major usability features of the HP Deskjet 1200C printer include:

* Full range of media supported, including plain paper (copier to bond), special paper, overhead film, glossy paper

* Dual media path, with label stock and envelopes manually feed through the rear path

* LaserJet IV compatibility

* PCL 5C color language

* Four-print-cartridge operation

* Upgradability to PostScript[TM] Level 2 language (standard in the Deskjet 1200C/PS)

* Expandability using HP MIO (modular IO) boards

* LaserJet font cartridge slot

* 45 internal scalable fonts.

The Deskjet 1200C carriage holds four print cartridges: black and three primary colors (cyan, magenta, and yellow). Printing is accomplished by traversing the cartridges back and forth and using a linear encoder to fire drops of ink onto the page. Black text is printed with the black print cartridge and color is created using a combination of dots fired from the color print cartridges (e.g., red consists of magenta and yellow). All of the colors of the rainbow can be created using a dithering process. The cartridge carriage also holds an optical sensor which is used for paper size sensing and to differentiate between transparency film and other media.

A print cartridge service station caps the inkjet cartridges to prevent the ink from drying out when the printer is not in use. The service station includes four separate wipers to help maintain a clean nozzle plate for optimum print quality. Also included is a manually selectable and manually activated reprimer that is used to clear nonfunctioning nozzles.

The dual media path is designed to support a range of media types and sizes. Typical A-size, metric A4, and legal-size media can be fed automatically through an input tray in the front. Contmon LaserJet labels, overhead transparencies, and glossy media can also be fed automatically. Single sheets, card stock, and a range of envelopes can be fed manually through the rear path.

The printer can connect to a wide range of systems. A Centronics interface is standard and an HP MIO slot allows connection to a range of other interfaces including networks. The base unit is expandable from the standard 2M bytes of memory up to 26M bytes by inserting industry-standard SIMMs through a slot in the rear. The printer is upgradable to a Postscript Level 2 machine or it can be purchased that way.

Technical Challenges

Print Qualy. To compete in the office market, the Deskjet 1200C had to have excellent print quality on all plain paper media. This includes standard copier paper as well as bonds in the 16-to-30-pound range. Capillary action between the ink and the media, which can result in ink wicking along the paper fibers and causing fuzzy characters, is overcome with a solution that includes new inks and a heated paper path. Because jagged off-axis characters are perceived as low-quality, the scan axis resolution was increased to 600 dpi and HP Resolution Enhancement technology is employed, greatly improving the appearance of the characters' edges.

To gain media independence and good media advance accuracy, a heated paper path is provided. The purpose of the heater is to aid the ink drying process without overcooking or browning the media. Where typical printers have used large rubber rollers to advance the media, for the heated path a small, accurate, and heat resistant system needed to be developed. Adhesive-bonded grit particles on small cylindrical drive wheels have long been used on many Hewlett-Packard products as a media drive system. However, the adhesive would not stand up to the temperatures in the heated media path, so a tungsten carbide grit was developed to meet all the requirements for accuracy, beat resistance, and traction capability.

Speed. Current users of black and white lasers are accustomed to print speeds of four to eight pages per minute. Current thermal inkjet printers are throughput-limited by several factors including firing frequency, retrace velocity, acceleration, and sheet-feed time. The scanning technology requires the carriage, which weighs over 1.5 pounds, to accelerate and stabilize accurately and quickly before it is ready to print. High-performance servos are required to accelerate, scan, decelerate, and return the carriage to produce each line of text. The paper axis motor's advance time needs to match the scan axis turnaround time in both length and accuracy. Adding a motor to the sheet feeder reduces the waiting required between printing successive sheets. To control these functions, a fast hardware system and optimized firmware are required.