Intel 4004 μ-Computer (SIM4-01)

Some time ago, there was an Intel 4004-based singleboard computer for sale on ebay. The name of this computer was printed on the board: “Intel 4004 μ-Computer”. I did not buy it (also because the price was too high), but I never had heard about it, plus it was based on the first commercially available microprocessor, the Intel 4004. Now, the Intel 4004 is not only well known for its historical importance, but also that it was not really meant to be the heart of a small computer as it was designed to be used in a calculator. So what is this “mu-Computer” contraption?

Intel designed the 4004 as the CPU chip in its 4-chip family from 1971, the MCS-4. The other MCS-4 chips were the 4001 (masked-programmed) 256 bytes ROM, the (80 x 4 bit) 4002 RAM, and the 4003 shift register. The 4003 was meant as the basis to provide MCS-4-based systems output ports e.g. in order to control displays or other peripheral devices. The MCS-4 chips used a 4-bit data bus (although the 4004 supported 8 bit commands). The clock frequency of the 4004 was 500 – 740 kHz.

In 1971 Intel was mainly a RAM and ROM manufacturer who sold only components to their customers. (Mask-programmed) ROMs were ROMs whose content needed to be set during the actual manufacturing of the chip silicon, so a change in ROM contents needed several weeks until a new ROM with the new content was available. This was of course a problem for the software developers because they could change their software quickly, but then had to wait weeks until the new version could be used. Therefore, also in 1971 Intel announced a new product, the first EPROM. This was a ROM type whose content can be changed in a matter of minutes as it was stored in a re-writable manner on the chip (EPROMs are the chips with the glass window in the middle and often a sticker on top of that window). So, developers could program their system, create a new version of their software on their system, and when the software was stable, a traditional (and much cheaper) ROM could be produced without the need of many iterations.

Therefore, Intel felt the need to support developers using their MCS-4 products with first, a development system based on the MCS-4 that accepted either ROM or EPROM chips and second, with an EPROM programming device that allowed to load new content to an EPROM (an EPROM burner in today’s technology). The first development system is the 4004 μ-Computer. “mu” like the greek μ sign for “micro”. How did that device look like (that later on basically every CPU manufacturer would produce and sell for promoting their own design)?

The 4 large sockets on the top left corner can hold either a ROM or an EPROM chip each. The small chip on the bottom left corner is the 4004 CPU (tiny because of the 4 bit bus), and the 4 sockets to the right of it can hold a 4002 RAM chip each, giving it a whopping maximum of 160 bytes of writable memory. The other chips on the board are not really interesting (at least from my point of view). I read that the board actually has no 4003 chip on it, probably because there is no real standard design in a MCS-4 computer for it as this depends on what devices you want to connect. There are no recognizable interfaces on the board, just an edge connector of no recognisable standard at the bottom. So how do you connect something to this card? Meet the Intel MCB4 (Micro Computer Connector Board) chassis:

The chassis (the box on the bottom helds an 4004 mu-Computer on the front slot, and the second device, the EPROM burner on the back slot. It offers some switches, some lights, some interfaces and one (E?)PROM socket. You connect a TTY to the chassis, and voila: an entire microcomputer, ready to use.

Now, if you looked at the first picture and wondered where it said “mu-Computer”, well it doesn’t, it says “SIM4-01”. This is the (slightly) later name for the board. See, the problem was that Intel did feel that it was a component manufacturer, not a producer of computers, and that it should not be perceived (especially by its computer manufacturing customers) as such. Therefore, after some management thinking, this was not the 4004 micro-computer, this was the MCS-4 simulation device (SIM4-01), and you will find it much more often under that latter name. If you are interested in the history of the 4004 mu-Computer and especially the aspect of Intel shying away from being perceived as a computer manufacturer, I can wholeheartedly recommend the excellent article of Zbigniew Stachniak on exactly this topic (see [Stachniak] in the references).

The 4004 mu-Computer was probably the first computer with a 4004 CPU, the first microprocessor. Therefore, one can probably also call him the first microcomputer (if its definition requires a microprocessor). It (and its descendants like the twin brother SIM4-01 and the younger, healthier brother SIM4-02) can even be the only historic, commercially available, programmable computer with a 4004 CPU.

Technical Data

Manufacturer: Intel
Model: 4004 mu-Computer (a.k.a. SIM4-01)
CPU: Intel 4004
ROM: 4 sockets for 256 bytes Intel 4001 ROM, or 1701 or 1702 EPROM each
RAM: 4 sockets for Intel 4002 RAMs (80 x 4 bit = 40 bytes each)
Interfaces: 72-pin edge connector, allowing for 6 I/O ports (2 in and 4 out), and TTY
Released: 1971
Initial price: ???


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