pindari.com / signetics 2650 Projects of Interest

Hardware build & Python emulator

Signetics 2650
8-bit Microprocessor

From a 1978 magazine build to a full software emulation — revisiting the joys of 8-bit programming

The hardware build

The original hardware was built in 1978, based on a series of construction articles published in Electronics Australia. The project came together across three issues: the low-cost Video Display Unit appeared in February 1978, the keyboard interface in April 1978, and the complete 2650 Mini Computer System in May 1978. Together they formed a capable, self-contained machine that was, for the time, remarkably affordable to build from scratch.

Scanned copies of all three issues are available on the Internet Archive:

Feb 1978
Low-Cost Video Display Unit Electronics Australia, February 1978
Apr 1978
Keyboard Interface for the Video Data Terminal Electronics Australia, April 1978
May 1978
2650 Mini Computer System Electronics Australia, May 1978
Parts availability. Finding components for the original design today is a challenge — the 2650 chip itself, the 2608 PIPBUG ROM, and the supporting Signetics peripheral ICs have long since disappeared from general circulation. Rather than let the machine sit silent, a Python emulator was written to bring the 2650 back to life in software.

The Python emulator

The emulator recreates the 2650 CPU, making it possible to write and run 2650 programs using the built in assembler.

2650 CPU

Full instruction set emulation.

Video Display Unit

A 16-line × 64-character VDU, representing 1,024 bytes of VRAM.

LEDs & Soft Switches

Eight LEDs and soft switches driven by I/O port engagement.

7-Segment Display

Eight digits of 7-segment display emulating the 74LS138 decoder and DM9368 driver chips.

VDU — 16 lines × 64 characters

The terminal displays 16 lines of 64 characters, with a blinking cursor, driven entirely by the 2650's I/O port writes.

PIPBUG implementation is under development. The 2608 ROM monitor (PIPBUG) is currently being implemented. Once complete it will provide the familiar command-line interface for loading, examining, and executing 2650 programs — just as on the original hardware.

LEDs and soft switches

The emulator renders the eight output LEDs in real time as I/O port values change. Soft switches are toggled via the keyboard, feeding back into the 2650's sense inputs.

I/O SIGNAL LEVELS DATA: 7 6 5 4 3 2 1 0 [ 0x17 ] ADDR: [ 0x00 ]

Seven-segment display

Eight 7-segment digits emulate the 74LS138 for digit selection and the DM9368 BCD-to-7-segment latch/driver.

Coming soon. This is an ongoing project — additional documentation, source code, assembly program examples, and screenshots of the emulator in action will be added as development continues.