I’ve finally made some visible progress in turning this Tranz 330 point-of-sale terminal into a general purpose Z-80 computer. It prints my name on the display! I may faint from the excitement.

OK, it’s not much to look at yet, but getting this far took a substantial amount of work in reverse-engineering the Tranz 330 hardware. Through tracing the circuit paths on the logic board, and examining the point-of-sale program in the terminal’s ROM, I slowly pieced together a map of where the peripheral devices are in I/O space, and how to communicate with them. Then after digging through the datasheets for the Z80 CPU, PIO, CTC, and the VFD display controller chip (all well-documented fortunately), I was ready to write a new program. I whipped up a tiny “hello world” program using a Z-80 assembler, burned it to my EEPROM, swapped out the original terminal ROM, and voila! There’s my name, in beautiful aqua-colored vacuum fluorescent glory.


I’ll provide more technical details in a bigger “how to” post once I have a more full-featured example. Until then, here are some of the minor problems I ran into, in addition to the job of reversing the hardware design:

The original terminal ROM is a 32K UV-erasable 27C256. I’m sure not going to wait around to UV-erase my ROM each time I want to try a program modification, so I got a more modern 28C256 EEPROM as a replacement. The trouble is (and I didn’t notice this at first) the 28C256 is not exactly pin-compatible to the 27C256. Two pins are swapped. Where the Tranz logic board supplies +5 and A14, the 28C256 expects A14 and /WE. What the heck? It turns out I probably should have gotten a 27SF512 instead, which is a 64K part, but is drop-in compatible as long as you put the 32K image in the upper half.

I could almost fix my pin mismatch by bending some pins and adding some jumper wires, but modifying the 28C256 would prevent me from reprogramming it later. In the final result, I sacrificed a nice ZIF socket, cut off the /WE pin, jumpered /WE to VCC, and moved my program data to the upper half of the ROM where A14 = 1.

The original purpose of the ZIF was to avoid needing to socket and re-pull the ROM every time I wanted to test a change to my program. I put the ZIF in the motherboard’s socket, and the ROM in the ZIF. Unfortunately the ZIF fit the socket pretty loosely, and even more loosely after I cut it up, so it doesn’t work unless I press down on it with my thumb while it’s running. Ah, quality. I’m not sure what a better long-term solution is.

One last headache: my EasyPro 90B EPROM programmer apparently doesn’t work under Windows 7 64-bit. After searching for a while, I concluded there’s no appropriate driver, and I have to boot into XP whenever I want to program a ROM, which is a pain in the butt. I know nobody cares, but I can’t resist a chance to complain! If you’ve got a recommendation for a decent and not too expensive EPROM programmer that works under Win 7, please let me know.

Next step: some demos involving the keypad and speaker. Maybe a game of battleship?

I found an old credit card terminal at an electronics surplus store recently, and bought it to tinker with. $5 at Weird Stuff in Sunnyvale! This Verifone Tranz 330 was used to process credit card transactions in the 80’s and 90’s, and is probably still used today some places. I wasn’t really interested in the credit card aspect though, but hoped to take it apart, learn how it works, and see if I could do anything interesting with it. Inside is a simple, self-contained 8-bit computer with a lot of hacking potential.

The Tranz 330 has a 16 key keypad, 16 character vacuum flourescent display (each charachter with 16 alpha-numeric segments), and a serial port with an 8 pin DIN connector. It also has an internal modem, a pair of telephone jacks, and a specialized 6 pin DIN serial port intended for use with peripherals. Lots of interesting interfaces to experiment with here.

Inside are two stacked circuit boards. The logic board contains a Z-80 CPU running at 4MHz, with a 32K socketed ROM, and 32K SRAM. The display board contains the VFD controller and keypad interface, as well as a piezo speaker.

So what do I do with this thing? It’s a full-on Z-80 based computer capable of lots of mischief, cramped only by the limited keypad and display. In theory, I could play Zork on this box. The socketed ROM would make it easy to replace the credit card terminal program with something of my own.

The challenge lies in determining the memory and I/O mapping used. How do you read the keypad? How do you write a character to the display? How do you work with the serial port or drive the speaker? I could dump the contents of the ROM and reverse-engineer the credit card program to find the answers. That’s probably the best way to go, but it sounds like a pain in the butt. I could also attach probes and examine the busses with my logic analyzer while the terminal is running. The logic and display boards are joined by a single 20 pin connector, so there’s not that much to analyze.

Meanwhile, let’s take a closer look at the components on the circuit boards, and see what’s there. Examining the CPU board first, in the center there’s the Z-80 CPU, PIO (parallel ports, probably used for the keypad and display), and dual UART (presumably used for the serial ports). Across the bottom row from left to right, there’s the Z-80 counter/timer, 32K SRAM, 32K EPROM, real-time clock chip, battery, and the single-chip modem. There are a few smaller chips at the top– I couldn’t learn anything about them, but by their placement I’m guessing they’re related to the phone interface.

On the display board, examine the chips going counter-clockwise beginning with the largest chip. The large chip is the VFD display controller, a Micrel MIC10937. Following that is a dual-D flip-flop chip, a quad op-amp chip, and a second quad op-amp chip of a different type. I’m not sure what the eight op-amps are needed for– powering the VFD perhaps? Driving the speaker? Decoding the keypad key presses?

Post your best ideas for interesting hacks using this little box!