# D-17B

The D-17B (D17B) computer was used in the Minuteman I NS-1OQ missile guidance system. The complete guidance system contained a D-17B computer, the associated stable platform, and power supplies.
The D-17B weighed approximately 62 pounds (28 kg), contained 1,521 transistors, 6,282 diodes, 1,116 capacitors, and 5094 resistors. These components were mounted on double copper-clad, engraved, gold-plated, glass fiber laminate circuit boards. There were 75 of these circuit boards and each one was coated with a flexible polyurethane compound for moisture and vibration protection. The high degree of reliability and ruggedness of the computer were driven by the strict requirements of the weapons system.

## Design constraints

High reliability was required of the D-17B. It controlled a key weapon that would have just one chance to execute its mission. Reliability of the D-17B was achieved through the use of solid-state electronics and a relatively simple design. Simpler DRL (diode–resistor) logic was used extensively, while less-reliable DTL (diode–transistor) logic (which provides gain and inversion) was used only where needed. In the late 1950s and early 1960s, when the D-17B was designed, transistors lacked today's reliability. Reliability was also enhanced by the rotating-disk memory with non-destructive readout (NDRO). In actual real-time situations, Minuteman missiles achieved a mean time between failures (MTBF) of over 5.5 years.
The Soviets had much larger rockets and could use vacuum tubes in their guidance systems. The Minuteman I, II and III weighed 29,500 kg, 31,746 kg and 35,000 kg respectively, versus the Soviet R-7 missile (1959) at 280,000 kg. The US planners had to choose either to develop solid state guidance systems (which weigh less) or consider the additional cost and time delay of developing larger rockets.

## Specifications

Minuteman I D-17B computer specifications
Year: 1962
The D17B is a synchronous serial general-purpose digital computer.
Manufacturer: Autonetics Division of North American Aviation
Applications: Guidance and control of the Minuteman I ICBM.
Programming and numerical system:
    Number system: Binary, fixed point, 2's complement
    Logic levels: 0 V for logical 0 (false), -10 V for logical 1 (true)
    Data word length (bits): 11 or 24 (double precision)
    Instruction word length (bits): 24
    Binary digits/word: 27
    Instructions/word: 1
    Instruction type: One and half address
Number of instructions: 39 types from a 4-bit op code by using five bits of the operand address field for instructions which do not access memory.
Execution times in microseconds:
    Add: 78+1⁄8 μs
    Multiply: 546+7⁄8 μs or 1,015+5⁄8 μs (double precision)
    Divide: (software)
(Note: Parallel processing such as two simultaneous single precision operations is permitted without additional execution time.)
Clock channel: 345.6 kHz
Addressing:
    Direct addressing of entire memory
    Two-address (unflagged) and three-address (flagged) instruction
Memory:
    Word length (bits): 24 plus 5 timing
    Type: Ferrous-oxide-coated NDRO disk
    Cycle time: 78+1⁄8 μs (minimal)
    Capacity (words): 5,454 or 2,727 (double precision)
Input/output:
    Input lines: 48 digital
    Output lines: 28 digital
       12 analog
       3 pulse
    Program: 800 5-bit char/s
Instruction word format:
Registers:
    Phase and voltage output registers
Construction (arithmetic unit only): transistor-diode logic is used.
    Timing: Synchronous
    Operation: Sequential
Storage :
| Medium | No. of Words | Access (μs) | |
| Rotating disk | 2,688 | 5,000 avg. | (general purpose channels) |
| Rotating disk | 41 | | (rapid access loops) |
| | | 40 | (1 word loop) |
| | | 160 | (4 word loop) |
| | | 320 | (8 word loop) |
| | | 640 | (16 word loop) |
Input
    48 digital lines (input)
    26 specialized incremental inputs
Output
28 digital lines (output) 12 analog lines (output) 13 pulse lines (output) 25,600 word/s maximum I/O transfer rate
Physical characteristics
    Dimensions: 20 in high, 29 in diameter, 5 in deep
    Power: 28 VDC at 25 A
    Circuits: DRL and DTL
    Weight: 62 pounds (28 kg)
Construction:
    Double copper clad, gold plated, glass fiber laminate, flexible polyurethane-coated circuit boards
Software:
    Minimal delay coding using machine language
    Modular special-purpose subroutines
Reliability: 5.5 years MTBF
Checking features: Parity on fill and on character outputs
Power, space, weight, and site preparation
    Power, computer: 0.25 kW
    Air conditioner: Closed system
    Volume, computer: 1.55 cu ft (44 L)
    Weight, computer: 70 lb (32 kg)
    Designed specifically to fit in cylindrical guidance package.
The word length for this computer Is 27 bits, of which 24 are used In computation. The remaining 3 bits are spare and synchronizing bits. The memory storage capability consists of a 6000 rpm magnetic disk with a storage capacity of 2985 words of which 2728 are addressable. The contents of memory include 20 cold-storage channels of 128 sectors (words) each, a hot-storage channel of 128 sectors, four rapid access loops (U, F, E, H) of 1, 4, 8, and 16 words respectively, four 1-word arithmetic loops (A, L, H, I), and a two 4-word input buffer input loops (V, R).
The outputs that can be realized from the D-17B computer are binary, discrete, single character, phase register status, telemetry, and voltage outputs. Binary outputs are computer generated levels of +1 or −1 available on the binary output lines.

## Instruction set

Special features of the D-17B computer include flag store, split-word arithmetic, and minimized access timing. Flag store provides the capability of storing the present contents of the accumulator while executing the next Instruction. Split-word arithmetic is used in performing arithmetic operations on both halves of a split word at the same time. A split word on the D-17B consists of 11 bits. Minimized access timing is the placing of instructions and data in memory so that they are available with minimum delay from the disk memory.

## Guidance software

Autonetics was the associate contractor for the Minuteman (MM) guidance system, which included the flight and prelaunch software. This software was programmed in assembly language into a D17 disk computer. TRW provided the guidance equations that Autonetics programmed and was also responsible for the verification of the flight software. When MM I became operational, the flight computer was the only digital computer in the system. The targeting was done at Strategic Air Command (SAC) Headquarters by the Operational Targeting Program developed by TRW to execute on an IBM 709 mainframe computer.
Sylvania Electronics Systems was selected to develop the first ground-based command and control system using a programmable computer. They developed the software, the message processing and control unit for Wing 6. To support the deployment of the Wing 6 system, TRW, Inc. developed the execution plan program (EPP) from a mainframe computer at SAC and performed an independent checkout of the command and control software. The EPP assisted in assigning targets and launch time for the missiles.
The MM II missile was deployed with a D-37C disk computer. Autonetics also programmed functional simulators and the code inserter verifier that was used at Wing headquarters to generate and test the flight program codes to go into the airborne computer.