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HP42S with non-volatile ram
06-11-2020, 04:40 AM (This post was last modified: 06-11-2020 04:43 AM by GreyUser.)
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HP42S with non-volatile ram
Sorry for the narrative wall of text but I don’t see anything here on the subject so here goes:

A while back, I stumbled upon ferroelectric ram while tinkering with the Arduino platform. I got to wondering if they made parallel bus devices rather than the serial SPI part I was using and sure enough, Cypress makes a 32Kx8 part as the FM18W08. It is in the industry standard 28 pin soic package so I thought for $14 and shipping from DigiKey, my 42S is getting some surgery...

I found that it worked but had some issues, primarily high current consumption due to the fact that ferro ram needs the CS/ pin to cycle for every access due to the destructive read which requires another internal write. The HP42S didn’t support this and I the shelved the unit, figuring I’d put the standard ram back at some point, once I had fully admitted defeat.

With covid in full swing, my routine changed and I needed my beater 42S returned to service. So, I found the regular 32k memory I had and set about replacing the ferroram. I couldn’t remember if the jumpers needed to change so I thought, I’ll just Google it and it will lead me here for Paul Brogger’s old article on the topic. So, the Google search was “HP42S 32KB”. I thought the Museum would be the first link. Surprise. That’s not what happened. This is what I found:

Takayuki Hosoda figured it out! The Saturn processor has an unused address select signal that properly gated with the CS/ pin, provides the required signal to make ferro ram work at normal current levels!

I ordered the parts and did the mod and I’ve now got my 42S with non-volatile memory! Wahoo! Take the batteries out, short the terminals, leave it for two days, put the batteries back in and all you get is a Reset message and your programs are there. The only way I’ve been able to clear memory is to do the usual Pioneer three finger salute.

I then did the double speed mod to one unit and the capacitor swap but I don’t see that as necessary. It doubles the run, standby, sleep and deep sleep current levels. Since my solver programs are not long, the speed increase doesn’t matter as much to me as battery life. Granted, there’s very low danger of losing programs due to memory corruption but I find that batteries go dead at the most inconvenient time and I don’t (but should) carry spares in my bag these days.

Ultimately, I found some traps/issues with the method Takayuki Hosoda illustrates on his website. So, if anyone is interested in doing this mod, I’ll write up my process and post some photos. I honestly don’t want to bother if this is old ground or there is no interest. This post was enough effort for the moment!
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Messages In This Thread
HP42S with non-volatile ram - GreyUser - 06-11-2020 04:40 AM
RE: HP42S with non-volatile ram - mfleming - 06-11-2020, 02:26 PM
RE: HP42S with non-volatile ram - lyuka - 09-26-2020, 12:12 PM
RE: HP42S with non-volatile ram - GreyUser - 09-28-2020, 02:41 AM
RE: HP42S with non-volatile ram - lyuka - 09-28-2020, 01:13 PM
RE: HP42S with non-volatile ram - GreyUser - 09-29-2020, 01:41 AM
RE: HP42S with non-volatile ram - lyuka - 09-29-2020, 09:48 AM
RE: HP42S with non-volatile ram - lyuka - 11-17-2020, 06:11 PM

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