A $15 Thrift-Store TRANZ 330 Credit Card Terminal Now Uses Its Z80 and a GPS to Work Out Where the Planets Are, Then Prints It Like a Receipt

The terminal is the box you swiped a card through at a gas station in the ’90s: a Z80, 32K of RAM, 32K of ROM, a 16-button keypad, a magnetic stripe reader and a glowing vacuum fluorescent display. “I found mine at a thrift store for $15.00, incl. a printer,” CharlesD writes on his project page, posted October 26.

The numbers are computed on the spot. “The ephemeris data is calculated and not just from a lookup table,” he writes. His GitHub README says the Z80 runs “the low-precision Keplerian orbital calculations” from the date, the time, each planet’s orbital elements and where you’re standing, following Paul Schlyter’s tutorial on computing planetary positions, a method Schlyter aims at “an error of no more than 1-2 arc minutes.”

Getting there took a detour. The first math routine came from ChatGPT, and “the data didn’t match what I could get from the Heavens Above website,” he writes, so he debugged it against Heavens-Above until it did. He also admits, “I have no books on the Z80.” His progress notes read like a diary: “Baby steps, lots of baby steps,” then “Bigger baby steps.”

The wiring is the part that will make a hardware person grin. “But wait, these ports are RS232 not TTL,” he writes, so a MAX3232 level-shifter board sits between the GPS and the terminal. The GPS needs 5 volts the serial port doesn’t offer. He found pin 8 on the terminal’s DIN8 port unused, with no trace to it, and ran a wire from a nearby pin to make it a power line. The GPS clock also makes the terminal’s own real-time clock unnecessary.

The output goes to a Verifone Printer 250, which the README calls “a thermal printer that can print in two colors (RED and BLACK).” Each ticket carries the planet’s name, whether it’s up, the UTC date and time, azimuth and elevation. The code was burned to a UV-erasable 27C256 EPROM and tested first in a Z80 simulator.

He is the first to keep expectations level: “By no means is this an accurate astronomy device,” the README says. The two write-ups even disagree on the clock speed, about 4 MHz on the project page and 8 MHz in the README. In the comments, he’s already pointing people at the faster TRANZ 380, with “more ROM.” Whether anyone turns that extra ROM into the Moon is the open question.

Expect more of this. Every dead category of single-purpose hardware, from receipt printers to lottery terminals, has a keypad, a display and a printer someone already paid to engineer. The people who can read a schematic get those parts for the price of lunch, and the receipt printer turns any calculation into something you can hand to another person.