Examining circuit boards from the Space Shuttle's I/O Processor
Examining circuit boards from the Space Shuttle's I/O Processor
The Space Shuttle's five1 general-purpose computers played a critical role in each flight: controlling the<br>engines, monitoring thousands of sensors, displaying data to the astronauts, and navigating<br>the Shuttle.<br>Each computer consisted of two 60-pound aluminum-alloy boxes: the box on the right is the CPU, a<br>32-bit processor that executed 420,000 instructions per second.<br>These computers were designed before microprocessors became popular, so the processor was<br>built from multiple boards crammed with simple chips and they used magnetic core memory rather than DRAM chips.
The Space Shuttle IOP and CPU (AP-101B). Photo courtesy of RR Auction.
The box on the left is the I/O Processor (IOP): the link between the CPU and the rest of the Shuttle.<br>It implemented the input/output capabilities for the computer, primarily<br>24 high-speed networks that connected the computer to the Shuttle's systems and sensors.<br>But the IOP wasn't just a peripheral; it was a separate programmable computer,<br>more complicated than the main CPU.<br>The IOP had an unusual architecture: it was one of the first multi-threaded computers, implementing 25 virtual processors (with<br>two completely different instruction sets) that ran on one physical processor.
I obtained two circuit cards from the I/O Processor,2 each a 9"×3" rectangle<br>packed with tiny chips and other components.<br>In IBM lingo, each card is called a "page" (remember this term).<br>The top page is a network interface, providing four network connections,<br>each handling 1 million bits per second. (The IOP contained six of these cards for<br>its 24 network connections.)<br>The bottom page held the microcode for the IOP's processors, the low-level code that defined<br>each instruction.<br>The rows of white-and-gold chips stored the microcode's bits in tiny metal fuses,<br>programmed by blowing a fuse for each 1 bit.<br>In this article, I'll explain how the I/O Processor worked, and the roles of these two pages.
Two pages from the Space Shuttle I/O Processor: the "MIA" interface page and the PROM page.
The MIA interface page
The Space Shuttle had 28 data bus networks that linked the computers to the rest of the Shuttle,<br>with each computer attached to 24 of the networks.3<br>The large number of networks provided both high performance and reliability, with at<br>least two networks between a computer and any Shuttle system.<br>Eight networks were assigned to flight-critical systems, with each CRT display and engine controller connected to four networks for redundancy.
The page below is one of the six network interface pages in the I/O Processor.<br>Space Shuttle engineers loved acronyms, so this page has the<br>cryptic name MIA for "Multiplexer Interface Adapter".<br>(Many of the networks<br>were connected to boxes called Multiplexer/Demultiplexers, which<br>provided the link between the network and the diverse analog and digital components of the Space Shuttle.5)<br>The MIA interface page is tightly packed with integrated circuits and other components.<br>The page holds two printed-circuit boards, one on each side of the page.<br>The boards on both sides are almost identical,4 as you can see by comparing the<br>photo above and the photo below. (Main difference: the connector switches sides.)
The network interface page, called the MIA (Multiplex Interface Adapter).<br>The page has extensive rework; thin brown "bodge" wires snake around the page to<br>repair errors or implement updates.
Each board implements two network interfaces, so the page supports four networks.<br>Each network transmits data across a pair of wires, twisted together and shielded, rather than<br>a coaxial cable.<br>Although the network transmits digital data, the signals transmitted across the network<br>are physical voltages that will weaken with distance and will have distortion and noise.<br>Thus, the interface page must convert these analog signals back to 0's and 1's.
The right half of the board holds the analog circuitry.<br>It is dominated by a large golden module labeled "IBM", with 46 pins.<br>This is a hybrid module, consisting of tiny components such as transistor dies, resistors,<br>capacitors, and potentially IC dies, connected by bond wires thinner than a hair.<br>It's not quite an integrated circuit, but a collection of individual components mounted on a ceramic wafer.<br>Hybrid modules were popular for aerospace applications, since a board of analog components<br>could be shrunk down to a single (expensive) module.<br>This module contains the analog circuitry for two I/O ports: the drivers<br>to transmit network signals along with the amplifiers and comparators to receive signals.
Various discrete components are mounted next to the hybrid module: resistors, glass capacitors6, inductors, and small square transformers.<br>The transformers provide the coupling between the interface board and the network.<br>As with Ethernet, transformers provide isolation between the computer...