A True Telnet BBS On A Casio Calculator. – EI3LH

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A True Telnet BBS On A Casio Calculator.

Written by

EI3LH

in

Calculators, Coding, Computing

Hosting a Bulletin Board Service on My Casio VX-4.

11–17 minutes

I love my Casio VX-4 so much that I made an AI-generated fake magazine advert for it! August 14, 2026.

About a eight weeks ago I had virtually no interest in calculators. Blank, dull, tedious-looking objects that remind me of long, arduous rainy afternoons in 1980’s/1990’s school maths classes.

Fast forward to August 14th, 2026 and I now own not one but two Casio Pocket Computer calculators (imported at great expense direct from Japan), have developed 4 pieces of software for them (one published to Github already, the rest will be open source for you to play with I promise!), made (AI generated) calculator fan art, and am now completely and utterly besotted with them.

So what changed my mind?

BASIC and RS232. That’s it.

Boeuf a la Mode!

It started quite simply with a post I read by Mark M5TEA on the SOTA Reflector . Mark had made mention that he had ported over a popular calculator emulator. Some sort of pocket Casio contraption that was able to write BASIC software as well as the C and Casio CASL languages.

I had a ‘curiosity click’ for the laugh, prodded a few buttons on the emulated calulator and thought little more of it. Later that day I went back for another look, curious to see how a calculator could run BASIC software, and moreto, how far could you push it?

Minutes turned to hours that turned to days. I was hooked. Starting out with the classic:

10 PRINT "HELLO WORLD"<br>20 GOTO 10<br>RUN

…and the resulting:

HELLO WORLD<br>HELLO WORLD<br>HELLO WORLD

Before long I’d had my fill of testing random quick shot programs and began to think about what might be possible with the calculator thanks to it’s programability but also the hackability of it’s two gateways. A small interface port and a 2.5mm jack which masquerades as an RS232 serial input and output port.

Boeuf a la mode!

This was the moment that hooked me. I began to think about one of my other passions, amateur radio – particularly CW (Morse Code). I don’t come from the world of software development and have very limited programming knowledge so I turned to an LLM to see could I vibe code a simple piece of software to log my radio contacts while operating portable.

This became the birth of my now published (and still very much alpha) Jamoncito FX logging software.

Shortly after I put together a small CW keying program , again written in BASIC . It’s unfinished and will require an external bridge circuit between the calculator and the radio to complete my goal, but it shows what can be done with a little thought.

My rare Casio Z-1GR pocket computer calculator. Alas in need of a repair. (Seller photo)

Aside, I have long had a habit of keeping a pocket notebook and a fountain pen (your choice of stationary and writing implement may vary). If something pops in to my mind during the day (or night) I jot it down in case it’s of use another day. I also find it helps me declutter my brain, and in comparison to taking digital notes, it keeps me focused and the mix of random thoughts betwixt notes of genuine importance do not get lost in the digital void.

Whilst continuing to work on both pieces of software I had another idea and hastily grabbed my pocket notebook and fountain pen and scribbled it down.

It quickly became apparent that if I wanted to take my ideas to the next level that I was going to need to look in to getting my hands on the actual hardware. So it was that I looked toward the East and found a Casio VX-4 that appeared to be in near mint condition.

Fully boxed, mint condition accompanying manuals and programming books, the optional 32kb RAM add-on card. This adds 32kb to the on-board 8kb – if you are feeling particularly brave you can desolder and replace the 8kb IC with a 32kb chip, taking the VX-4 up to a fully loaded 64kb beast , akin to it’s 64kb FX-870P sibling.

There was also a bonus item included with this particular VX-4 for sale. A fully built 2.5mm to USB-C FTDI RS232 cable! What a find, a fully loaded VX-4 with serial cable and for a decent price. All I had to do now was pay for it, ship it and pray that it worked on arrival to Ireland.

My treasured Casio VX-4. Seller photo as I doubt I could surpass this staging!

People Will Always Need Plates.

I began to think about communications links between the Casio VX-4 and anything that could interpret the RS232 data to and from it. And thinking about communications walked me straight up the aisle towards what was waiting for me at the technology altar.

How about a Bulletin Board Service? How original you might say. Yes, but how about a BBS hosted entirely on a calculator with 8kb of RAM ?

A genuine, true telnet BBS hosted on a calculator. Disco. I had my next project.

Technology is an ‘Ology. That makes me a scientist!

As the days passed and...