Reverse engineering circuitry in a Spacelab computer from 1980
Reverse engineering circuitry in a Spacelab computer from 1980
Spacelab was a reusable laboratory that could be carried in the cargo bay of the Space Shuttle, providing lab space for astronauts<br>and experiments.<br>Spacelab was controlled by a French-built minicomputer, called the Mitra 125 MS.<br>Unlike modern computers, this computer didn't contain a microprocessor chip.<br>Instead, its 16-bit processor was constructed from several boards of chips.<br>In this article, I reverse-engineer one of the processor boards, shown below, part of the computer's Arithmetic/Logic Unit (ALU).
The Mitra 125 MS computer, built by CIMSA, with one of the ALU/register cards shown.
Spacelab consisted of a pressurized cylindrical laboratory that held<br>experiments, computers, and work areas for researchers.<br>A tunnel connected the laboratory to the Shuttle, allowing researchers to move between the Shuttle and Spacelab.<br>Spacelab also supported up to five<br>unpressurized "pallets" that were exposed to space, holding experiments such as telescopes and sensors.<br>The illustration below shows the tunnel, the Spacelab laboratory, and a pallet installed in the Shuttle's cargo bay.1
Illustration of the Spacelab-3 mission. From NASA.
Because Spacelab was a European project, it used a European computer, the Mitra 125 MS.<br>The Mitra line started in 1971 when a French company called CII introduced the Mitra 15 minicomputer,<br>a 16-bit computer that used magnetic core memory.<br>Mitra is a French acronym2 that translates as "Mini-machine for Real-Time and Automatic Computing."<br>As the name suggests, Mitra was both small and designed for real-time computing, making it suitable for controlling experiments.<br>The Mitra 15 was a popular computer, with almost 8000 units sold.
In 1975, CII produced a successor called the Mitra 125.<br>The Mitra 125 improved on the Mitra 15 by adding memory management, I/O processors, higher performance, and additional instructions.<br>Spacelab used the Mitra 125 MS minicomputer,3 a militarized variant of the Mitra 125 that was produced by a company<br>called CIMSA.<br>A Spacelab mission had three of these computers: the Subsystem Computer controlled<br>and managed Spacelab itself, while the Experiment Computer handled the experiments. A Backup Computer could take over if either<br>computer failed.1<br>These computers were part of Spacelab's Command and Data Management Subsystem, which controlled experiments and collected data.4
The three computers were normally mounted in the Spacelab laboratory underneath the Work Bench Rack (details). The computers were controlled through<br>a keyboard and a color CRT display, called the Data Display System (DDS).<br>The computer installation and a DDS are visible in the photo below.
This photo shows astronauts inside Spacelab (but not in space). The Spacelab computers were mounted under the Work Bench (right arrow). The Data Display System (left arrow) provided the interface to the computers. Photo is STS-51B Crew Portrait, 1984.
For some Spacelab missions, the laboratory was omitted entirely, providing more room for experiment pallets.<br>In this case, the computers were mounted in a small pressurized cylinder called the igloo.<br>The researchers remained in the Shuttle, controlling experiments through two Data Display Systems that were<br>mounted in the Shuttle's rear flight deck (photo).
The 74181 ALU chip
The Spacelab computer didn't use a microprocessor chip. Instead, like most minicomputers at the time, it was built from simple<br>integrated circuits that were combined to implement the computer's circuitry.<br>Unlike modern CMOS integrated circuits, these chips contained bipolar transistors, which were fast, but large and<br>power-hungry, a technology known as TTL (transistor-transistor logic).<br>Electronics hobbyists of a certain age will recall the popular 7400 series of TTL chips.<br>The Spacelab computer was built from the military grade of these chips, the 5400 series.
The most complex chip in the computer was probably the '181 Arithmetic/Logic Unit (ALU) chip, containing about 170 transistors.<br>The arithmetic/logic unit is the heart of a computer, performing arithmetic operations as well as Boolean logic operations.<br>In 1970, Texas Instruments put a complete 4-bit arithmetic/logic unit on a single chip, called the 74181.<br>Since the chip was fast, compact, and inexpensive,<br>it was widely used, providing the ALU in computers<br>from the popular PDP-11 and Xerox Alto to the powerful VAX-11/780 "superminicomputer".
The 74181 provides a full set of binary logical operations, including AND, OR, XOR, and complement.<br>For arithmetic, it includes addition, subtraction, incrementing, and decrementing.5<br>Inconveniently, the 74181 doesn't support shifting right.<br>Moreover, multiplication and division were much too complicated to be included in the 74181.<br>Instead, a processor implemented multiplication and division through repeated addition or subtraction, combined with...