CHAPTER 5 SYSTEMS: COMMAND AND DATA HANDLING—THE BRAINS OF THE INTERNATIONAL SPACE STATION 98 Table 2. Comparison between the Standard MDM with the Enhanced Unit. In recent years, the C&C, GNC, PL, and EXT MDMs were upgraded as indicated in parentheses. See also Chapter 6. Component Function Standard MDM Enhanced MDM 80386 Processor Chip (Pentium 266 MMX) Microprocessor (CPU) of the MDM 12 MHz 16 MHz (144 MHz) Electrically Erasable Programmable Read-Only Memory Nonvolatile storage area for the MDM and application software. This includes the MDM boot-up software. 1 MB 1 MB Dynamic Random Access Memory Volatile storage area where applications execute 2 MB 8 MB (64 MB) Analog to Digital Chip Converter Converts analog data received from I/O cards to digital data Present Present, but only to measure the internal temperature of the MDM Math Coprocessor Chip that assists the CPU in performing certain types of operations increasing the computer’s speed Not present Present Figure 2. Photographs of an MDM. The top picture shows the MDM with all the various computer cards. The SSMMU is the wide device at the right of the card set. A picture of the computer cards is shown in the bottom image. The ISS MDMs come in two styles: standard and enhanced. Table 2 lists the basic properties of the ISS MDMs. The main difference is that the enhanced ones have a bit more memory (8 MB versus 2 MB), a faster processor (16 MHz vs 12 MHz), and can hold an additional memory card whereas the others cannot. The standard MDMs come in several sizes depending on how many I/O cards they can hold i.e., 4, 10, or 16. MDMs within a class are interchangeable. Whole boxes are not generally retained as spares on orbit, but a few generic MDMs or spare cards are present. If a specific MDM experiences a fatal failure, a new box or card is installed and the appropriate software is installed. Not all enhanced boxes contain a hard drive. Tier 1 and Tier 2 MDMs are of the enhanced type. Since the standard MDMs do not need to read data off of a disk or store data, they do not require hard drives and, at that point, resemble a tablet more than a desktop PC. All the system software is resident in nonvolatile electronic memory on a circuit card. Figure 2 shows the layout of a basic MDM. Two of the enhanced MDMs—C&C and Payload