Swapped ROM Revives Ailing HP-48S Calculator

Buying broken gear for cheap is time honored hacker tradition, and while we might not always be successful in reviving it, rarely do we come away empty handed. There’s always parts to salvage, and you can’t put a price on the knowledge to be gained when poking around inside an interesting piece of hardware. So we’re not surprised at all to hear that [Tomas Pavlovic] jumped at the chance to grab this faulty HP-48S calculator for a couple bucks.

Luckily for us, the story doesn’t end at the bottom of his parts bin. When he got the HP-48S back home, he immediately set out to see if it could be repaired. After changing out a few choice components and not seeing any result in the device’s behavior, he became suspicious that the problem may be with the firmware; specifically, the soldered-on chip that holds it.

Dumping the original ROM.

After carefully lifting the NEC uPD23C2000GC from its resting place for the last 30 years or so, he wired up an adapter that let him connect the chip to his programmer so its contents could be dumped. Rather than trying to find another ROM chip, he decided to wire in a socket and found a re-writable SST39SF040 that could stand in as a replacement. Flashing a fresh copy of the firmware to the new socketed chip got the calculator up and running again, with the added bonus of allowing [Tomas] to pull the chip and flash a different firmware version should he care to experiment a bit.

Now, we know what you’re thinking. Where was the fix? What exactly brought this piece of 1990s gear back to life? That part, unfortunately, isn’t very clear. You’d think if the original ROM chip was somehow faulty, [Tomas] wouldn’t have been able to so easily pull a valid firmware image from it. That leaves us with some pretty mundane possibilities, such as a bad solder joint on the chip’s pins. If that was indeed the case, this fix could have been as simple as running a hot iron over the pins…but of course, where’s the fun in that?

Update: We heard back from [Tomas], and it turns out that when compared to a known good copy, the dumped firmware did have a few swapped bits. His theory is that the NEC chip is in some weird failure mode where the calculator wouldn’t run, but it was still functional enough to get most of the content off of it. What do you think? Let us know in the comments.

10 thoughts on “Swapped ROM Revives Ailing HP-48S Calculator

  1. HP has made the firmwares for its calculators available for many years. I’ve used a variety of hp48 emulators over the years (from x48 in the early 90s to Emu48 on Android today) and one of the startup steps was always to download current fimware from HP. Back in the day it was a bit of an exercise, but today it’s so fast you never even notice it the first time you run the app. Thank you, HP. I still love my (now dead) hp48S. Maybe I should see if I can get it working again.

  2. I’m a huge fan of HP Calculators glad this 48s got repaired. Got my first HP-41CV in College and moved onto 42S, Free42, and the DM 42 from Swiss Micro. I have a 48gx but never used it as much as the 42.

    I wish someone would make a calculator with a keyboard like HP used too.

  3. I like the HP48 because it was my calculator at student times and it feels still better than a new HP50G, but in these days it looks outdated because of the low contrast of it old graphic LCD. So I still thinking from time to time to replace it by an OLED. However it is not so easy you might think because of it unusual SPI-word-length. Hm..perhaps next winter time. :-)

    Oh…and I have many of them because they were sold cheap at flea-market 15-20years ago…


    1. +1, FACT.

      it is sometimes possible to recover data by adjusting drive/scan/address power or increasing (and filtering) load/bus impeadence…

      the data FADES,. as in: logic levels are no longer valid, but often vary along with the original state.
      IE 0v out (programmed) becomes 2v out, and 5v out (NONprogrammed) stays 5v out.
      so with TTL levels it appears blank (all 1’s), but with CMOS it might still appear working, when read extra-slow.

      the original device was maybe pre-CMOS and logic transition point is more towards the low end (30% not 50%), so it is possible, without any special kind of reader, to take an unreadable chip and read with off-the-shelf reader with high-current CMOS-output address lines (full 5v @ >1mA like found on modern MCUs) and CMOS data input lines (like found on modern MCUs).

  4. There are some very good emulators for HP calcs. But nothing has the feel of those buttons. The HP machines were far more expensive than the TI ones, but the build quality was far superior. I have an HP-25 I bought in grad school and an HP-41 I bought after graduation. But my favourtie machine is the V41 emulator I run under WINE on my Linux machine: takes up no space on my desk, never needs to be recharged, but the downside is…no button feel.

    HPCalc.org is worth a visit if you’re interested. Also the Museum of HP Calculators.

    1. And don’t forget the special expensive HP-smell! When they are new they smelled very special. I have an HP Logicdart, that is a Logicanalyzer made by the HP48-Team, because it looks like an HP48, in a condition better than new. Even this part smelled the old 90-smell of the HP-parts.


  5. Did this person open the 48S without destroying it? Even Ben Heck couldn’t do it (see his right-to-repair video). I can’t seem to post a message on his blog to ask. If anyone has contact information, I’d appreciate it.

    I have a 48S that, like many, has a bad keyboard connection. I suspect it would be easy to fix if you could actually get into it.

Leave a Reply

Please be kind and respectful to help make the comments section excellent. (Comment Policy)

This site uses Akismet to reduce spam. Learn how your comment data is processed.