HP-42s Memory Upgrade


I'm entertaining the idea of upgrading my 42s but find the listings of computer chips confusing. I have seen this for sale in a few places. Is this what I'm looking for:

62256-70S 32Kx8 70ns CMOS SRAM - SMD

If not, could someone post a link to the exact chip I need to get.



Hi David,

The type is ok. In general whichever XX256 memory (28 pin SMD SOIC) will fit the task.

The reponse time (-70... or something like this) is not a concern as HP-42s are slow enough for any memory currently in production to meet the specs. Even if you go thru the 2x speed-up. (See here)

However you should pay attention to the stand-by current drain in order to extend your battery life to the most.

CY62256NLL-70 (Digikey: 428-2039-5-ND) has a 0.1uA typical standby current. It has worked fine for me.

Hope this helps.


Edited: 5 Sept 2009, 9:41 a.m.


Thank you, that is exactly the info I was looking for. I went ahead and ordered a few from Digikey.

Maybe I should have ordered some crystals too, but one step at a time.



And the part number for the crystal??

- Pauli


I think this is the correct crystal, can anyone confirm?




Thanks Deigo for the memory chip order number.

I also have ordered two for my HP42s's which along with my Clonix HP-41CX make up my office duo.

This also will be a section in the pioneer chapter.

Cheers, Geoff


Hi all,

Well, crystal Digikey P/N: SER3322-ND is smaller, and is the one I've used. but previous reference will also work.

However, I'd take the "Speed-up" upgrade with a grain of salt... you more likely will lose printing capabilities.

And no, you won't get the IR printer interface back to work when you need it by "simply" writing H'03 into H'40300... it's not that easy... ;-)

IR (red eye) interface, takes the carrier frequency directly from the Xtal. so... Well, you can always install both Xtals and the required switch. Yes, I've done it, and yes it works ;-))



Edited: 6 Sept 2009, 12:58 p.m.


Well, you can always install both Xtals and the required switch. Yes, I've done it, and yes it works ;-))

This is what I was thinking of doing.

Where would the switch fit through the case? There isn't a lot of space for such...

- Pauli


A magnetic reed switch may be a good option.


Hi again,

Please note that you'll need a changeover reed, as well as a convenient place to hold the activating magnet... In HP-41's this is the way I went, using the PORT-1 cover to hold the magnet... but HP-41 only needs a simple "Open/Close" reed to switch between speeds. Actually I choose the serial capacitor approach to build my 41 speed-up.

I simply hate drilling, cutting, or otherwise "damaging" the housing of one of these bauties... so I go the "Hidden Switch" way for my 42 upgrade.

Just a paper clip tip is enough... even the edge of the IR cover can be used to move the switch up and down.

Here you are the pics:

Hope this be useful.




Thank you for taking the time to post pics. I didn't know changing crystals can effect printing.

I wonder if you can change crystals in the printer as well...?



Hi Dave,

No such things as Xtals inside the 82240A/B IR printers.

It uses a Pi shape resonant circuit (Capacitor-Inductor-Capacitor) which makes the 80C50 microcontroller run at some 3.5 ~ 4 MHz.

Nonetheless, you can replace the inductor with a Xtal. Specs for the 80C50 sets an upper limit working frequency of 6MHz; but it won't change the results. Still good printing at 42's normal speed and nothing at 2x.

Going beyond 6MHz will result in no printing at all... these micros were not very well suited for overclocking... :-(

Sorry for the bad news.



... and with a different crystal, even if the micro eventually runs at higher speed, it is very possible that the timing of outputs which control the mechanism (or the thermal printing pulse duration) will get affected and not working.

And there may be also analog components (filters) on the IR link which are tuned for some pulse-train frequency, and may not work well with a different rate.


Very useful thanks.

- Pauli

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