On Older Equipment
Some older equipment require the use of SIMMs with parallel presence detect (more commonly called simply presence detect or PD). Some of this equipment uses non-standard PD coding, IBM computers and Hewlett-Packard LaserJet and other printers in particular. While old computers are rarely found, many old Laserjet printers are in use. Discontinued HP memory modules are officially recommended, but any 72-pin SIMM module within the capacity range supported by the printer and with the correct PD code should work. All printers will work with FPM (Fast Page Mode) memory; it is reported that some, but not all, will work with EDO memory. It is fairly easy to solder wires to the PD pins to make non-HP modules work. HP printers of this type specify that RAM with an access time of 70 ns or slower to be used; this is likely due to a limitation of the encoding in the PD data. Faster RAM can be used, but the PD encoded data should indicate a speed of 70 ns.
Read more about this topic: Serial Presence Detect
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