Apple LaserWriter 12/640 PS - Manual (Page 158)

The LaserWriter 12/640 PS has several user-installable printer options.
You can add a duplex printing unit, a 500-sheet cassette and feeder, an
envelope cassette (the envelope cassette fits into the 500-sheet feeder), and
a face-up output tray. You can also increase the printer’s random-access
memory (RAM).
This chapter provides instructions for installing your paper-handling
options and increasing RAM.
WARNING When installing any of the options described in this chapter,
except for the face-up output tray, be sure the printer is turned off.
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Installing Options