A True Telnet BBS on a Casio Calculator

Authored by EI3LH Categories: Calculators, Coding, Computing

I am so enamored with my Casio VX-4 that I even used AI to craft a vintage-style magazine advertisement for it!

The Unexpected Obsession

I used to find calculators incredibly boring. Roughly two months ago, I had zero interest in these devices. To me, they were just blank, tedious tools that evoked memories of grueling, rainy math lessons during the 80s and 90s.

Fast forward to August 14th, 2026, and my perspective has shifted entirely. I am now completely besotted with them. My current achievements include:

• Importing two Casio Pocket Computers from Japan (at a significant cost).
• Developing four distinct software applications (one is already on GitHub; the others will be open-source soon).
• Creating AI-generated fan art of the hardware.

The Catalyst

This obsession began with a post by Mark M5TEA on the SOTA Reflector. Mark mentioned he had ported a calculator emulator—a pocket Casio device capable of running software written in C, Casio CASL, and BASIC.

Initially, I clicked the link just for a laugh. I poked around the emulator, but by the end of the day, I was hooked. I wanted to know how a simple calculator could execute BASIC and, more importantly, how far I could push the hardware.

The First Experiment I started with the absolute classic: 10 PRINT "HELLO WORLD" 20 GOTO 10 RUN

The result was a screen filled with: HELLO WORLD HELLO WORLD HELLO WORLD

Expanding the Horizon

Once I moved past simple tests, I began investigating the device's "hackability." The hardware features two primary gateways:

1. A small interface port.
2. A 2.5mm jack acting as an RS232 serial I/O port.

This sparked a connection to my other passion: Amateur Radio, specifically CW (Morse Code). Since I am not a professional developer and have limited coding experience, I used an LLM to "vibe code" a solution for logging radio contacts while operating in the field.

Software Milestones

• Jamoncito FX: A portable logging software (currently in alpha and published).
• CW Keying Program: A BASIC program designed to key a radio.
  • Note: This is still unfinished and requires an external bridge circuit to function.

The Hardware Quest

I have a lifelong habit of using a fountain pen and pocket notebook to declutter my mind and capture random thoughts before they vanish into the "digital void." It was in one of these notebooks that I scribbled the idea to take my projects to the next level. To do that, I needed the actual physical hardware.

I searched the East and found a Casio VX-4 in nearly mint condition, complete with:

• Original box and manuals.
• Programming guides.
• An optional 32kb RAM expansion card.

Memory Specifications

The base VX-4 comes with 8kb of on-board RAM. With the expansion card, the math looks like this: $8\text{kb (Internal)} + 32\text{kb (Card)} = 40\text{kb Total}$

Pro Tip: If you are brave enough to desolder the internal 8kb IC and replace it with a 32kb chip, you can create a $64\text{kb}$ beast, matching its sibling, the FX-870P.

The "Junk" and the Dream

Along with the VX-4, I purchased a Z-1GR. Unfortunately, it arrived as "junk." Upon arrival in Ireland, it showed no signs of life, even under a multimeter.

Recovery Plan for the Z-1GR:

• Attempt to power via DC instead of batteries.
• Test the LCD display.
• Replace the tired LCD polarizer using specialized film.

Building the World's First Calculator BBS

With the VX-4 in hand, connected via a USB-C to RS232 cable in COM mode, I set out to achieve the impossible: Hosting a Telnet BBS entirely on a calculator.

While I have the 32kb expansion, my goal is to ensure the BBS works on all VX-4 models, meaning it must fit within the base 8kb of RAM.

Current Status

I won't dive into the specific code here just yet. However, once the software is polished and "ship shape," I will upload it to GitHub. I invite the community to tear it apart, laugh at my novice skills, and hopefully improve the design.

Visuals of the BBS in action:

Additional Gallery: