The Signetics 2650 was an 8-bit microprocessor introduced in mid-1975. According to Adam Osborne's book An Introduction to Microprocessors Vol 2: Some Real Products, it was "the most minicomputer-like" of the microprocessors available at the time.
Signetics sold development boards e.g. the PC1001 and its successor, the PC1500 "Adaptable Board Computer", ranging in price from A$165 to A$400. The chip by itself sold for around A$20. Several hardware construction projects and programming articles were published in magazines such as Electronics Australia and Elektor and related kits were sold by electronics stores. These factors led to its use by a number of hobbyists in many countries such as Australia, U.S.A., United Kingdom, the Netherlands and Germany.
The chip contained 7 8-bit general purpose registers, although only 4 were visible at any time. It was limited to a 15-bit address space (thereby addressing a maximum of 32KB of memory), since the upper bit of a 16-bit memory reference was reserved to indicate that the indirect memory addressing mode was to be used (a minicomputer-like feature).
The address space was further limited by the use of another 2 bits of the address to indicate the indexing mode for all logical and arithmetic (i.e. non branch) instructions. This divided the address space into 4 8Kb 'pages', only the current (i.e. the page the program was running in) could be addressed directly, the others only by indirect addressing.
While there were nine different addressing modes, the lack of any 16-bit registers and the 13 bit - 15-bit address space prevented widespread use. Despite this, an operating system ("2650 DOS") was available, along with 8k and 12k BASIC interpreters (sold by Central Data Corporation USA), and many games of the Hunt the Wumpus style. Most programs were written in assembly language.
Two types of video game console used the Signetics 2650 or 2650A. The first group of consoles are based on the video display controller 2636 Programmable Video Interface; the 1292 Advanced Programmable Video System and the closely related Interton VC4000 belong to this group. Those were released in Germany in 1976 and 1978 respectively. The second group of consoles were based on the Signetics 2637 as a video display controller; Emerson Arcadia 2001 which was released in 1982 and which used a Signetics 2650 running at 3.58 MHz as a CPU belong to this group together with many other ones software-compatible (Leonardo, Hanimex MPT-03 etc.).
The processor was also used in the Signetics Instructor 50, which was a small computer designed to teach the use and programming of the Signetics 2650 CPU.
The processor was most suited as a microcontroller, due to its extensive i/o support:
- Single bit i/o pins on the processor (sense/flag bits)
- Signals to directly address 2 8-bit i/o ports (control and data ports) using single byte instructions (port i/o). This circumvented the elaborate hardware other systems needed for memory-mapped i/o
- Signals to address another 256 i/o ports using an 8-bit address and 2 byte instructions, again, limiting the amount of hardware (address decoding) required.
Philips emphasized this use as a micro-controller with a demonstration program showing the 2650 controlling an intelligent elevator system. Also, at trade fairs they showed the 2650 controlling a miniature 'sort and stack' robot
Industrial Microcomputer System - IMS
For a short time starting 1979, Philips sold a modular 2650 computer called the 'IMS' - Industrial Microcomputer System, based on the Eurocard format in a 19" rack. It included CPU, PROM, RAM, input, output and teletype modules. This system was meant as a more intelligent programmable logic controller. For development, they later added DEBUG, DISPLAY, INTERRUPT and MODEST((E)Prom programmer) modules.
- 2650 original version with 1.25 MHz maximum clock frequency
- 2650A improved version (minor fabrication changes to improve stability) 1.25 MHz maximum clock frequency
- 2650A-1 as 2650A with 2Mhz maximum clock frequency
- 2650B-1 as 2650B with 2Mhz maximum clock frequency
The 2650B had the following changes and improvements over the 2650A:
- Two new signals- "Bus Enable" on pin 15 and "Cycle Last" on pin 25
- Program Status Word Upper bits 3 and 4 are setable and testable user flags (unused on the 2650A)
- Two new instructions to save and restore the lower status register in order to simplify interrupt processing
- Single byte register R0 instructions execute faster (1 cycle rather than 2)
In 1975 Signetics was sold to Philips and the 2650 was later incorporated into the Philips Semiconductors line. They made a version of the 2650 called the MAB2650A. Valvo, a subsidiary of Philips, sold the 2650 in Germany. Valvo also sold the VA200 single board (Eurocard) 2650 computer with 4kb PROM/EPROM, 1kb RAM and 4 I/O ports.
The 2650 came with a full complement of peripheral chips:
- 2621 Video Encoder (PAL)
- 2622 Video Encoder (NTSC)
- 2636 Programmable Video Interface
- 2637 Universal Video Interface
- 2651 Programmable Communication Interface
- 2652 Multi-Protocol Communications Circuit (incl. Synchronous Data Link Control (SDLC))
- 2653 Polynomial Generator / Checker
- 2655 Programmable Peripheral Interface
- 2656 SMI (System memory interface)
- 2657 Direct Memory Access
- 2661 Enhanced Programmable Communication Interface (EPCI)
- 2670 Display Character and Graphics Generator
- 2671 Programmable Keyboard and Communications Controller
- 2672 Programmable Video Timing Controller
- 2673 Video Attributes Controller
Philips Technical Note 083 describes how to interface the 2651 PCI to various other microprocessors, such as the 8080, 8085, Z80, 8048 and 6800
Descendants of the 2651/2661 serial communications chips are still sold as the Philips SC26 series.
The 2656 was specifically designed to augment, and interface with, the 2650 and make a 2-chip computer possible. It contained everything the 2650 lacked to make a complete computer:
- 2 kB 8-bit Mask-programmed ROM program memory
- 128 bytes 8-bit RAM memory
- Clock generator with crystal or RC network
- Power-on reset
- 8 general purpose I/O pins
The I/O pins could be used as an 8-bit I/O port or programmed to generate enable signals for extra RAM, ROM or I/O ports. This was achieved by Mask-programming a Programmable Logic Array in the 2656.
To develop and test the design before committing it to production, Philips sold the PC4000, a 2656 emulator board using PROM'S and FPLA's to emulate the ROM and PLA in the 2656.
- Microcomputer Digest Vol. 2 No. 1 July 1975
- Signetics Technical Note SP50; 2650 evaluation printed circuit board level system PC1001
- Signetics Technical Note SS50; PC1001 monitor program "PIPBUG"
- Build a 2650 Microcomputer system, Radio Electronics magazine: April, May, June 1977
- Holland Computer Club (HCC) 2650 user group
- Programmierbeispiele mit dem Mikroprozessor 2650, Johann Hatzenbichler, 1978 ISBN 3-921682-36-6
- Industrial Microcomputer System; System Specification, Philips Electronic Components and materials, 1980
- Philips 2650 Series microprocessor short-form manual 02-1979; 9398 209 50011
- VALVO VA 200 Mikrocomputer im Europa-Format: VALVO Applikationslaboratorium Marz 1978
- 2650PC-4000 memory interface emulator using PROM's and FPLA's