Post Reply 
HP42S with non-volatile ram
06-11-2020, 04:40 AM (This post was last modified: 06-11-2020 04:43 AM by GreyUser.)
Post: #1
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:

http://www.finetune.co.jp/~lyuka/interes.../hp42s/#nv

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!
Find all posts by this user
Quote this message in a reply
06-11-2020, 02:26 PM
Post: #2
RE: HP42S with non-volatile ram
(06-11-2020 04:40 AM)GreyUser Wrote:  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!

Congratulations on the successful upgrade! Having lost some programs because of dead batteries, I'd love to have a similar mod. Alas, no spare 42S to experiment on.

Any writeup, even without photos, that can supplement Takayuki's excellent instructions and help others accomplish the same modification would be appreciated by many. Anything that can add to the sum of community knowledge!

~Mark

Remember kids, "In a democracy, you get the government you deserve."
Find all posts by this user
Quote this message in a reply
06-11-2020, 03:58 PM
Post: #3
RE: HP42S with non-volatile ram
(06-11-2020 04:40 AM)GreyUser Wrote:  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...

If I wasn't happy with the RAM or speed of the 42S, I'd just get a SwissMicros DM42. It has the capability of the 42s plus many extra features, including the features you're looking for.

Tom L
Cui bono?
Find all posts by this user
Quote this message in a reply
09-26-2020, 12:12 PM
Post: #4
RE: HP42S with non-volatile ram
(06-11-2020 04:40 AM)GreyUser Wrote:  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.

Thank you for your interest in the 42s non-volatile mod. article. By the way, what was the issue / trap?
Find all posts by this user
Quote this message in a reply
09-28-2020, 02:41 AM
Post: #5
RE: HP42S with non-volatile ram
The schematic pinouts are incorrect, he indexed the wrong corner of the tab package as pin 1 so go by the pictures, not the schematic.

The other issue is where he mounted the SN74LVC1G97. When mounted on standard thickness proto boards, the chip will be pressed into the metal esd plate attached to the case back. This can lead to fractured solder connections with the chip in compression.

On my installs, I thinned (sanded) the proto board down to about 0.5mm and mounted in the area where the second 17B rom would live so that the chip fits within a recessed area in the esd plate. That results in no pressure on the solder connections. It also provides easy access to VCC and gnd connections, they’re right there on the unused pins 14 & 28.
Find all posts by this user
Quote this message in a reply
09-28-2020, 01:13 PM
Post: #6
RE: HP42S with non-volatile ram
Regarding pinouts, I couldn't find any official documentation about the pinouts of the processor used in the 42s, so I assigned the upper left corner as pin 1 when the logo in the TAB package was made to look upright. Besides, I couldn't find any other schematic of the 42s until I drew that partial schematic.
If you say "wrong", please show me the official documentation for the correct pin assignment.
If it found, I'll modify the schematic parts library to fix the processor pinouts in the schematic.

I'm sorry, I never thought that someone trying to modify a calculator wouldn't care about using such a thick board or insulation.

It seems easy to use the location of option ROMs, but at the point of poor power wiring for the parts at the time, there is a bit of anxiety about making strobe signals with the current high speed parts. So I implemented the logic IC in the part of the photo that seems to be better.
The series termination resistor of 100 Ohm for *CE is also inserted for that reason.
Find all posts by this user
Quote this message in a reply
09-29-2020, 01:41 AM (This post was last modified: 09-29-2020 01:42 AM by GreyUser.)
Post: #7
RE: HP42S with non-volatile ram
As is typical for HP pc board layouts, all chips have their pin 1 facing in the same orientation.

The cpu pin 1 indicator is obvious with the protective plastic frame removed from the tab package. With the frame in place, it is approximately 50% covered. It is the round, hollow circle in tinned copper on the kapton film of the chip carrier, in the lower right hand corner. It is above pin 14 of the ram chip, partiality obscured by the protection frame. Above 1/2 of the diameter can been seen with the plastic frame in place.

I have completed a pinout of the LCD, only hand drawn at this time. I would be happy to share with you if wanted, send me a PM. I intend to make a formal drawing and add it to the archives here on the museum when time permits. I am the author of the Pioneer keyboard schematic that you referenced that is in the archives here.

Thank you for sharing all your excellent work with the 42S!
Find all posts by this user
Quote this message in a reply
09-29-2020, 09:48 AM
Post: #8
RE: HP42S with non-volatile ram
Thank you, I didn't know that HP's convention for parts placement. I searched for the 1-pin mark in and around the protective frame and die, but couldn't find it.
Also, your keypad connection diagram helped me verify the schematic I drew. Thank you again.

If you can give me a connection diagram of the LCD part omitted in my schematic, I think it's a good idea to merge it and contribute here.
Find all posts by this user
Quote this message in a reply
11-17-2020, 06:11 PM
Post: #9
RE: HP42S with non-volatile ram
Hi guys, (and thanks to GreyUser)

Pin assignment of the L1R2 processor in the partial schematic corrected.
[Image: hp42s_nv2_sch-0.20-s.png]
Double speed HP 42S with 32KB of nonvolatile RAM and protection diode

Based on the case of failure in HP15C that the processor survived because the solid tantalum capacitor of the power line broke in short circuit mode when the battery was inserted backwards.
A protective Schottky barrier diode was added in parallel to the MLCC that was replaced from the tantalum capacitor on the power line of the 42S.
This diode may protect the processor and memory from ESD and reverse battery insertion.

Cheers,
Lyuka
Find all posts by this user
Quote this message in a reply
Post Reply 




User(s) browsing this thread: