D 17B

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, but less-reliable DTL (diode-transistor) logic was used only where needed. In the late 1950s and early 1960s, when the D-17B was designed, transistors lacked today's reliability. DTL provided, however, either gain or inversion. 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 failure (MTBF) of over 5.5 years.

The Soviets had much larger rockets and could use vacuum tubes (thermionic valves) in their guidance systems. (The weights of the minuteman I and II are classified, but the minuteman III was 35,000 kg versus the Soviet R-7 missile (1959) of 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 complementLogic levels: 0 or False, 0V; 1 or True, -10VData word length (bits): 11 or 24 (double precision)Instruction word length (bits): 24Binary digits/word: 27Instructions/word: 1Instruction 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:

Add (microseconds): 78 1/8Multiply (µs): 546 7/8 or 1,015 5/8 (double precision)Divide: (software)

(Note: Parallel processing such as two simultaneous single precision operations is permitted without additional execution time.)

Clock channel: 345.6 Hz

Addressing:

Direct addressing of entire memoryTwo-address (unflagged) and three-address (flagged) instructions

Memory:

Word length (bits): 24 plus 5 timingType: Ferrous-oxide-coated NDRO diskCycle time (µs): 78 1/8 (minimal)Capacity (words): 5,454 or 2,727 (double precision)

Input/output:

Input lines: 48 digitalOutput lines: 28 digitalProgram: 800 5-bit char/s

Instruction word format:

+--------+--------+------+--------+---------+--------+--------+ | TP | T24 21 | 20 | 19 13 | 12 8 | 7 1 | 0 | +--------+--------+------+--------+---------+--------+--------+ | Timing | OP | Flag | Next | Channel | Sector | Timing | | | | | Inst. | | | | | | | | Sector | | | | +--------+--------+------+--------+---------+--------+--------+

Registers:

Phase and voltage output registers

Arithmetic unit (excluding storage access):

Add: 78 µsMultiply: 1,016 µs

Construction (arithmetic unit only): transistor-diode logic is used.

Timing: SynchronousOperation: Sequential

Input

48 digital lines (input)26 specialized incremental inputs -Medium- -Speed- Paper/Mylar Tape 600 chars/sec Keyboard Manual Typewriter Manual

OUTPUT

-Medium- -Speed- Printer Character 78.5–2,433 ms (Program Control) Phase - Voltage (Program Control)

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 deepPower: 28 VDC at 25 ACircuits: DRL and DTLWeight: 62 pounds (28 kg)

Construction:

Double copper clad, gold plated, glass fiber laminate, flexible polyurethane-coated circuit boards

Software:

Minimal delay coding using machine languageModular 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 kWAir conditioner: Closed systemVolume, 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

D-17B Instruction RepertoireNumeric Code Code Description------------ ---- -----------00 20, s SAL Split accumulator left shift00 22, s ALS Accumulator left shift00 24, 2 SLL Split left word left shift00 26, r SLR Split left word right shift00 30, s SAR Split accumulator right shift00 32, s ARS Accumulator right shift00 34, s SRL Split right word left shift00 36, s SRR Split right word right shift00 60, s COA Character output A04 c, S SCL Split Compare and .ivt10 c, S TMI Transfer on minus20 c, s SMP Split multiply24 c, s MPY Multiply30 c, s SMM Split multiply modified34 c, s MPM Multiply modified40 02, s BOC Binary output C40 10, s BCA Binary output A40 12, s BOB Binary output B40 20, s RSD Reset detector40 22, s HPR Halt and Proceed40 26, s DOA Discrete output A40 30, s VOA Voltage output A40 32, s VOB Voltage output B40 34, s VOC Voltage output C40 40, s ANA And to accumulator40 44, s MIM Minus magnitude40 46, s COM Complement40 50, s DIB Discrete input B40 52, s DIA Discrete input A40 60, s HFC Halt fine countdown40 70, s LPR Load phase register44 c, s CIA Clear and Add50 c, s TRA Transfer54 c, s STO Store accumulator60 c, s SAD Split add64 c, s ADD Add70 c, s SSU Split subtract74 c, s SUB Subtract

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.