Developer and vintage computing enthusiast Chris Burrows has been working on a retro-themed microcomputer built around a Signetics 2650 eight-bit processor — and a Raspberry Pi RP2350B-powered Olimex Pico2-XL microcontroller board to stand in for every other required component.
“The [Raspberry Pi] RP2350 generates the 1MHz clock and the [Signetics] 2650 (an eight-bit MCU [Microcontroller Unit] from the 1970s) executes the code,” Burrows explains. “The Signetics Monitor ROM, Pipbug (or other ROM code) and the user’s application are loaded into RAM on the RP2350 which then services the OPREQ signals from the 2650.”
The Signetics 2650 rides again, driving a game of Othello on a new minimalist microcomputer design. (📷: Chris Burrows)
The Signetics 2650 was released in 1975 as a single-chip processor designed to compete with multi-chip minicomputers of the era — with a design led by IBM’s John Kessler and based on the company’s IBM 1130 minicomputer of ten years’ earlier. Unlike the IBM 1130, though, the S2650 is an eight-bit chip, with a for-the-time high clock speed of 1.25MHz thanks to a low-power NMOS process node. Despite being more powerful than Intel’s rival 4004 and 8008, the chip was not a huge success — in part thanks to that very desire to compete with minicomputers, which saw it implementing features that were simply unnecessary for microcomputers while missing others.
Despite this, the chip still has its fans — including Burrows, who has built a functional system around an original S2650 dubbed the S2650 IcePi. Aside from the processor itself, there’s only one other major component involved: the Olimex Pixo2-XL and its RP2450B microcontroller, which acts as everything from clock source to ROM.
“Being able to access up to 48 GPIO [General-Purpose Input/Output] pins came in very useful,” Burrows says of the project. “There are two USB/RS232 adapter boards on this prototype. One communicates directly with the 2650 SENSE and FLAG serial IO running at 4800 Baud. The second USB/RS232 adapter connects to RX/TX on the RP2350 to communicate via the Astrobe terminal. The opportunities for tracing and debugging in real-time are endless as all of the data and address traffic to/from the 2650 is intercepted by S2650 IcePi. This came in very useful during development.”
The project is documented on the Astrobe forum, along with partial source code.