There’s something about Reverse Polish Notation (RPN) and the calculators that use it. It calls to mind a time when a calculator was a serious tool, and not just a throwaway toy. Created in the legacy of such calculators by HP and Texas Instruments, [Simon Boak] shows off his SB116, sporting an Arduino Nano under the hood. It’s a fully custom design, with a hand-built metal case, a custom PCB for the keyboard, and a tiny OLED display for maximum retro green goodness.
The impetus for this build was to replace a particular calculator, a well-used TI Programmer, that’s useful for working with 6502 assembly. The SB116 supports binary, octal, decimal, and hex; and boasts some downright useful functions — AND, NOT, OR, XOR, and bitshifts. The source code is available, but you’re on your own for the case and keyboard. And for maximized retro faux-nostalgia, [Simon] designed a box that would have looked right at home on an 80s store shelf.
[Ziggurat29] had been playing around with infrared protocols, and realized he had a spare point-of-sale printer kicking around in his junk box. So he decided to whip up his own calculator infrared printer by bolting on an STM32 Blue Pill module and an IR receiver. [Ziggurat29] initially thought such a homemade printer would be cheaper than a commercial HP 82240 IR printer, even a used one. In hindsight, these point-of-sale printers can be pricey. If you don’t have one laying around, it may be cheaper to buy one, but not as fun as building it yourself.
It used to be commonplace for calculators to have a printing mechanism — even entirely mechanical adding machines often had them. As electronic calculators became the norm, the printer began to fade away. Back in 1987, HP introduced a portable calculator printer, the HP 82240A (see HP Journal Oct 1987). The calculator could print using a one-way infrared protocol which came to be known as Redeye. This made good sense, since not every one needs a printing calculator. As well, if you had one of these printers, it could be used with multiple calculators. Later in 1991, HP added a bi-directional infrared link called SIR beginning with the HP 48SX calculator (see HP Journal Jun 1991), allowing calculators to communicate with each other or with an IR-equipped PC. Finally HP and other companies teamed up in 1995 to create the IrDA standards you are probably more familiar with. But a bunch of Redeye and SIR devices are still floating around, and even some modern calculators like SwissMicros offerings can still output to these printers.
It was the fall of 1965 and Jack Kilby and Patrick Haggerty of Texas Instruments sat on a flight as Haggerty explained his idea for a calculator that could fit in the palm of a hand. This was a huge challenge since at that time calculators were the size of typewriters and plugged into wall sockets for their power. Kilby, who’d co-invented the integrated circuit just seven years earlier while at TI, lived to solve problems.
Fig. 2 from US 3,819,921 Miniature electronic calculator
By the time they landed, Kilby had decided they should come up with a calculator that could fit in your pocket, cost less than $100, and could add, subtract, multiply, divide and maybe do square roots. He chose the code name, Project Cal Tech, for this endeavor, which seemed logical as TI had previously had a Project MIT.
Rather than study how existing calculators worked, they decided to start from scratch. The task was broken into five parts: the keyboard, the memory, the processor, the power supply, and some form of output. The processing portion came down to a four-chip design, one more than was initially hoped for. The output was also tricky for the time. CRTs were out of the question, neon lights required too high a voltage and LEDs were still not bright enough. In the end, they developed a thermal printer that burned images into heat-sensitive paper.
Just over twelve months later, with the parts all spread out on a table, it quietly spat out correct answers. A patent application was filed resulting in US patent 3,819,921, Miniature electronic calculator, which outlined the basic design for all the calculators to follow. This, idea borne of a discussion on an airplane, was a pivotal moment that changed the way we teach every student, and brought the power of solid-state computing technology into everyday life.
When writing a recent piece about Reverse Polish Notation, or RPN, as a hook for my writing I retrieved my Sinclair Scientific calculator from storage. This was an important model in the genesis of the scientific calculator, not for being either a trailblazer or even for being especially good, but for the interesting manner of its operation and that it was one of the first scientific calculators at an affordable price.
I bought the calculator in a 1980s rummage sale, bodged its broken battery clip to bring it to life, and had it on my bench for a few years. Even in the early 1990s (and even if you didn’t use it), having a retro calculator on your bench gave you a bit of street cred. But then as life moved around me it went into that storage box, and until the RPN article that’s where it stayed. Finding it was a significant task, to locate something about the size of a candy bar in the storage box it had inhabited for two decades, among a slightly chaotic brace of shelves full of similar boxes.
The Sinclair’s clean design still looks good four decades later.
Looking at it though as an adult, it becomes obvious that this is an interesting machine in its own right, and one that deserves a closer examination. What follows will not be the only teardown of a Sinclair Scientific on the web, after all nobody could match [Ken Shirriff]’s examination of the internals of its chip, but it should provide an insight into the calculator’s construction, and plenty of satisfying pictures for lovers of 1970s consumer electronics.
The Sinclair is protected by a rigid black plastic case, meaning that it has survived the decades well. On the inside of the case is a crib sheet for its RPN syntax and scientific functions, an invaluable aid when it comes to performing any calculations.
It shares the same external design as the earlier Sinclair Cambridge, a more humble arithmetic calculator, but where the Cambridge’s plastic is black, on the Scientific it is white. The LED display sits behind a purple-tinted window, and the blue-and-black keyboard occupies the lower two-thirds of the front panel. At 50 x 111 x 16 mm it is a true pocket calculator, with an elegance many of its contemporaries failed to achieve and which is certainly not matched by most recent calculators. Good industrial design does not age, and while the Sinclair’s design makes it visibly a product of the early 1970s space-age aesthetic it is nevertheless an attractive item in its own right.
