HP officially doesn’t support memory upgrades on the M401 series. It has the slot, but they do not sell memory modules like the p2055 and older models once allowed with a laptop SODIMM. On top of this, the module is proprietary, so HP probably uses it as a service port OR had plans and abandoned it during development. HP solders it to the formatter board on these printers; the M401dne (which I managed to secure for next to nothing with 24k pages, shove the toner DRM whre the sun don't shine HP :-) ) has 256MB onboard, as do the others like the dn, dw, and x (x=dw with paper tray). The n is only 128MB and will ALWAYS BE a 128MB printer.
It's used as an "expansion slot" for these cards:
- BarDIMM Pro, Jetmobile
- MicrDIMM Pro, Jetmobile
- TypeHaus Fonts and Barcodes Solutions, TypeHaus, Inc.
- Asian Fonts, JITCO, Ltd.
- BarDIMM® Box, Jetmobile
- BarSIMM 1.9, Jetmobile
- EIO Hard-Disk for LaserJet Series, Oberon Service SRL
- Ethernet to Token Ring Router (ETRR), Ringdale
- EuroForm Barcode 100, EuroForm A/S
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HP's help: HP LaserJet Pro 400 Printer M401n
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