HP 5961-0509 manual Memory on many HP LaserJet printers. One exception is, Run

Note	The page protection feature is available only with additional optional
	memory on many HP LaserJet printers. (One exception is the
	LaserJet 4 printer, which supports page protection for letter-size
	paper in 300 dpi mode with the standard 2 Mbytes memory.) Refer
	to the appropriate User’s Manual for specific memory requirements.
	The Page Protection feature can be used to prevent possible ERROR
	21 conditions. ERROR 21 is reported when data is too complex for
	the printer to process concurrent with actual physical printing. A
	frequent cause of ERROR 21 when printing graphics is that the
	program sends commands to print a single point many times during
	the page run.
	Page protection can be set for letter, A4, or legal sized pages. Set
	page protection for the page size most often used.

I/O

The Parallel (Centronics) I/O has higher throughput than the RS-232C serial I/O. While text processing may not benefit from a faster I/O, raster graphics processing and soft font downloads will usually benefit from increased I/O throughput.

EN Performance 24-11