The D-17B weighed approximately 62 pounds (28 kg), contained 1,521 transistors, 6,282 diodes, 1,116 capacitors, and 504 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.
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 failures (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 remain 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.
Minuteman I D-17B computer specifications
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 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.
(Note: Parallel processing such as two simultaneous single precision operations is permitted without additional execution time.)
Clock channel: 345.6 Hz
Instruction word format:
+--------+--------+------+--------+---------+--------+--------+ | TP | T24 21 | 20 | 19 13 | 12 8 | 7 1 | 0 | +--------+--------+------+--------+---------+--------+--------+ | Timing | OP | Flag | Next | Channel | Sector | Timing | | | | | Inst. | | | | | | | | Sector | | | | +--------+--------+------+--------+---------+--------+--------+
Arithmetic unit (excluding storage access):
Construction (arithmetic unit only): transistor-diode logic is used.
|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)|
-Medium- -Speed- Paper/Mylar Tape 600 chars/sec Keyboard Manual Typewriter Manual
-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
Reliability: 5.5 years MTBF
Checking features: Parity on fill and on character outputs
Power, space, weight, and site preparation
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.
D-17B Instruction Repertoire Numeric Code Code Description ------------ ---- ----------- 00 20, s SAL Split accumulator left shift 00 22, s ALS Accumulator left shift 00 24, 2 SLL Split left word left shift 00 26, r SLR Split left word right shift 00 30, s SAR Split accumulator right shift 00 32, s ARS Accumulator right shift 00 34, s SRL Split right word left shift 00 36, s SRR Split right word right shift 00 60, s COA Character output A 04 c, S SCL Split Compare and .ivt 10 c, S TMI Transfer on minus 20 c, s SMP Split multiply 24 c, s MPY Multiply 30 c, s SMM Split multiply modified 34 c, s MPM Multiply modified 40 02, s BOC Binary output C 40 10, s BCA Binary output A 40 12, s BOB Binary output B 40 20, s RSD Reset detector 40 22, s HPR Halt and Proceed 40 26, s DOA Discrete output A 40 30, s VOA Voltage output A 40 32, s VOB Voltage output B 40 34, s VOC Voltage output C 40 40, s ANA And to accumulator 40 44, s MIM Minus magnitude 40 46, s COM Complement 40 50, s DIB Discrete input B 40 52, s DIA Discrete input A 40 60, s HFC Halt fine countdown 40 70, s LPR Load phase register 44 c, s CIA Clear and Add 50 c, s TRA Transfer 54 c, s STO Store accumulator 60 c, s SAD Split add 64 c, s ADD Add 70 c, s SSU Split subtract 74 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.
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.