HP 7475A Plotter

What the HP 7475A was

The HP 7475A is a six-pen desktop plotter made by Hewlett-Packard. It is a vector output device rather than a raster printer: instead of building an image out of dots, it drew line art by moving physical pens across the page under program control. The System Source Computer Museum describes it as "a drafting and graphic plotter that provides accurate engineering drawings, charts, and graphs." Within Hewlett-Packard's early-1980s plotter range it occupied the middle ground, above the smaller two-pen HP 7470A and below the faster, eight-pen HP 7550A.

The six-pen carousel and media

Where single- and dual-pen models forced the operator to swap pens by hand, the 7475A held six pens in a rotating carousel, so one drawing could combine up to six colors or line weights automatically. The pens were interchangeable, and HP offered fiber-tip, roller-ball, and drafting pen tips in a range of colors and widths. The plotter was not limited to ordinary paper: the museum record notes it could also plot on vellum and film. It accepted media up to 11 × 17 inches, the ANSI B (tabloid) size.

How it drew: the grit-wheel drive and HP-GL

Rather than a flatbed, the 7475A used Hewlett-Packard's grit-wheel paper drive. Small grit-coated wheels gripped the edges of the sheet and rolled it back and forth along one axis while the pen carriage moved along the other, so a drawing was produced by the combined motion of paper and pen. This design kept the plotter compact relative to the drawing size it could produce. Drawings themselves were described in HP-GL (Hewlett-Packard Graphics Language), HP's compact vector command set, which host software generated and streamed to the plotter.

Connecting to a computer

Each 7475A shipped with one of two host interfaces — an RS-232-C serial port or an HP-IB (IEEE-488) parallel bus — but not both, so buyers chose the version matching their computer. Hewlett-Packard's own connection guidance covers attaching the HP-IB variant to a standalone PC. Both versions understood the same HP-GL command language, so software written for one interface produced the same drawings on the other.

Place in HP's line and commercial success

The 7475A was a strong commercial performer for Hewlett-Packard. According to the HP Computer Museum, when HP introduced the eight-pen ColorPro in 1985 the division kept the 7475A in the catalog and held its price at US$1,895 rather than cutting it, and "sales of the 7475A continued at almost the same pace after the ColorPro as they did before." The model remained in HP documentation into the late 1980s; the HP 7475A service manual carries a 1987 date.

Legacy and use today

Because HP-GL is simple and thoroughly documented, the 7475A endures as a favorite among vintage-computing and pen-plotter-art enthusiasts, who still drive surviving units from modern computers over serial adapters. Its combination of a small footprint, six-color output, and B-size drawing capacity made it a common fixture on 1980s engineering and business desks, and it is frequently preserved in computing museum collections.