HP 7550A Plotter

What the HP 7550A was

The HP 7550A is an eight-pen desktop graphics plotter that Hewlett-Packard introduced in 1984. Like HP's other plotters of the period, it is a vector output device rather than a raster printer: instead of building an image out of dots, it draws line art by moving physical pens across the page under program control. The Computer History Museum records it as a Hewlett-Packard plotter manufactured in 1984 and notes that HP introduced it on page 6 of the July/August 1984 issue of HP Measurement Computation News. Within Hewlett-Packard's early-1980s plotter range it sat above the two-pen 7470A and the six-pen 7475A as the larger, faster, eight-pen model. The HP Computer Museum records that it was built by HP's San Diego Division and carried a US list price of $3,900.

Eight pens and media handling

The plotter held eight pens in a carousel and, according to Hewlett-Packard's specifications, changed pens automatically and capped them when idle to keep the ink from drying. HP offered interchangeable carousels loaded with fiber-tip transparency pens, fiber-tip paper pens, roller-ball pens, or drafting pens, so a single drawing could combine up to eight colors or line weights without manual pen swaps. It was not limited to plain paper: HP's Technical Data sheet lists standard paper, vellum, double-matte polyester film, and transparency film - with the IBM/OSE service reference giving a media thickness of 0.05 to 0.1 mm - and the HP Computer Museum likewise notes paper, transparency film, vellum, and polyester film. Media handling combined a Letter/A4 loading tray holding up to 150 sheets with a built-in single-sheet feeder for 11 x 17 in and A3 sheets. The HP Computer Museum credits the 7550A as the first plotter to include a sheet feeder, which allowed unattended plotting.

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

The 7550A drove the paper on grit wheels: grit-coated wheels grip the edges of the sheet and roll it back and forth along one axis while the pen carriage moves along the other, so a drawing is produced by the combined motion of paper and pen. This is the same moving-paper approach HP used on the smaller 7470A and 7475A, and an IBM Technical Support Services (OSE) service reference for the 7550A - repackaged from HP's operating documentation - describes cleaning these grit wheels and the pen carousel. Drawings were described in HP-GL (Hewlett-Packard Graphics Language), HP's compact vector command set, and the plotter carried 20 built-in character sets for lettering. HP's Technical Data sheet gives the 7550A a plotting speed of 80 cm/s and a smallest addressable move of 0.025 mm; the later 7550B ran at 120 cm/s, and the HP Computer Museum adds that the plotter accelerated its pen at 6 g, calling it "one of the fastest plotters ever."

Connecting to a computer: HP-IB and RS-232-C

Each 7550A carried two host interfaces built in - an HP-IB (IEEE-488) parallel bus and an RS-232-C serial port - which distinguished it from the 7470A and 7475A, whose buyers had to choose one interface or the other. Both were configured from the plotter's LCD control panel: the panel set the HP-IB bus address (0 to 30) or the serial parameters such as baud rate, parity, duplex, and handshake mode, storing them in nonvolatile memory. The plotter buffered incoming HP-GL, with a documented plot buffer of 9,954 bytes by default and 12,750 bytes maximum, and it offered a replot function that could redraw a stored plot up to 99 times without re-sending it from the host.

Reception, place in HP's line, and known limitations

The HP Computer Museum characterizes the 7550A in strong terms, calling it "the most advanced small plotter ever built" and stating that the product "never had a peer in the marketplace." The museum also credits the 7550 line with a ten-year product life - a characterization of the product family (the 7550A together with its successor, the 7550 Plus) rather than of the 7550A's own roughly 1984-to-1990 market run. It also records the design's weaknesses candidly: the replot buffer was too small to be very useful, the unit was physically large, and its cooling fan was noisy. These characterizations are the museum's own. The plotter's $3,900 list price placed it well above the six-pen 7475A that HP sold alongside it, which the same museum lists at $1,895 - reflecting the 7550A's position as the top of HP's small-plotter range rather than a direct replacement for the cheaper models.

Discontinuation, preservation, and what the record does not show

The Computer History Museum records that Hewlett-Packard replaced the 7550A with the 7550 Plus, priced at roughly $4,000-$4,200, around August 1990. Because HP-GL is simple and thoroughly documented, surviving 7550A units remain usable by vintage-computing enthusiasts, and the model is preserved in institutional collections - the Computer History Museum's example was donated by Lawrence Livermore National Laboratory. This page records only specifications that can be traced to Hewlett-Packard's documentation, museum records, or period service references; any figure that cannot be sourced to one of them is omitted here rather than estimated.