Information on hardware systems based on the 2650 microprocessor: systems I am working on, systems that I know about, some remarks on the 2650B variant, and information on how to read Signetics chip codes.

Systems I am working on

I am currently working on three systems.

my P1 computer, which is a close copy of the Central Data board.

a rare Signetics TWIN system, on loan from the Eindhoven Computer Association.

As a nice constrast to these “work” machines I recently acquired a “play” machine: an Interton VC 4000. A lot of information on this machine can be found on internet, and there is an active community of people around this and similar game consoles based on the 2650.

Philips IMS

I also have a CPU and memory card which I think belong to the Philips Industrial Microcomputer System. The 2650 microprocessor normally sits in the empty socket on the photo below. There is very little information to be found on internet. MODEST (Microcomputer Development System) was avaialble to develop and debug software for the IMS. MODEST had access to I/O devices such as a terminal or an audio cassette interface that were not available or necessary during industrial deployment.

Also see: MODEST a Novel Development System for an Industrial Microcomputer System.

The mysterious 2650B

More recent documentation from Signetics mentions their 2650B variant of the 2650 microprocessor. The 2650B brings two new instructions to store (STPL) and load (LDPL) the lower Program Status Word to and from memory directly, which makes returning from interrupt handlers a lot easier. It also speeds up the execution of instructions that use register addressing (e.g. lodz,r1). The 2650B is not pin-compatible with the 2650/2650A, as it replaces the two input pins ADREN and DBUSEN (see below) with one input pin BEN and one output pin CYLAST.

Meaning of the pin signals:

ADREN = Address Bus Enable.

DBUSEN = Data Bus Enable. Used with ADREN to release the busses during DMA.

BEN = Bus Enable. Combines ADREN and DBUSEN into a single signal.

CYLAST = Cycle Last. Indicates that the last cycle of the current instructions is being executed.

I have never seen an 2650B, not on photos on internet anywhere and definitely not in real life. I suspect that the 2650B has always been rare, and I never encountered a hardware design specifically for the 2650B.

Chip codes

Chips are marked with numbers that not always have an obvious meaning. To complicate matters, sometimes some of the markings are missing! This makes identifying ICs challenging. The following guide will help.

Date code. The date code indicates the date of manufacture. It consists of four digits: the first two indicate the year, the last two digits are the weeknumber. So “7835” indicates that the IC was manufactured in week 35 of 1978 (around the end of August).

Product identification. The marketed name of the IC (2650 or 2650A). The product identification is followed by the letter I for ceramic packaging (mostly white, some dark purple) or by the letter N for plastic packaging (black). The product number is sometimes followed by a letter of unknown purpose. If the product identification is omitted, use the mask code to identify the IC.

Mask code. In addition to the product identification, Signetics also uses a mask code. The mask code is followed by a letter indicating packaging (often I or N), and may be followed by a letter, the same as in the product identification. The mask codes are not listed in the documentation or manuals, but I have deduced some codes; see the table below. For example “CP1004I” is for a ceramic packaging of the 2656, and “CT429NC” is a plastic packaging of the 2650A. For the 2650A the separate “-1” marking indicates the high-speed version. Mask codes may differentiate between versions of the same product. For example, the Slave card in the Signetics TWIN uses a special version of the 2650(A) microprocessor indicated by the mask code CT354 or CT904; the 2608 ROM chips would have a different mask code depending on its contents.

ExampleMask codeProduct Identification
2650I Signetics chip IDCT3192650 Microprocessor
CT429 Signetics chip code with 2650A chip ID CT4292650A Microprocessor (note the -1 marking)
CI140 Signetics chip code CI1402650A, alternative packaging
CT9042650A special mask, for TWIN slave card
CT4302621 Universal Sync Generator (PAL)
CT4552636 Programmable Video Interface
CP10022656 System Memory Interface preprogrammed with Pipbug 2 and Pipla.
CP1004 Signetics chip codeCP10042656 System Memory Interface
CN00352608 ROM with Pipbug
Signetics mask codes.

Hardware emulators:

WinArcadia: Windows utility to emulate the Central Data board and the Signetics Instructor, but also many other systems.

2650 emulator written in Tcl/Tk.

AllDatasheet.com: download info on integrated circuits, even old ones.

Other vintage 2650 computers:

Phunsy (Philipse Universal System)

ETI-685 kit published in the Australian ETI Magazine.

78up5 kit published in Electronics Australia, and its modern reimplementation.

Modern redesigns:

Signetics 2650 Microcomputer at Tholin’s Place.

Signetics 2650 Single Board Computer

RetroShield 2650 for Arduino Mega