Top Document: PDP-8 Summary of Models and Options (posted every other month) Previous Document: What is a PDP-5? Next Document: What is a LINC-8? See reader questions & answers on this topic! - Help others by sharing your knowledge Date of introduction: 1965 (Unveiled March 22, in New York). Date of withdrawal: 1968. Total production run: 1450. Also known as: Classic PDP-8 (to point out lack of a model suffix) Straight-8 (Again, points out the lack of a model suffix) PCP-88, an OEM label, used by Foxboro Corporation. AN/GYK-6, (Army-Navy Ground-based (Y)data-processing Komputer 6) Price: $18,000 Technology: Mostly standard DEC R-series logic modules; these were originally discrete component transistor logic, but around the time the PDP-8 was introduced, DEC introduced the Flip Chip, a hybrid diode/resistor "integrated circuit" on a ceramic substrate. These could directly replace some of the discrete components on some logic modules, and DEC quickly began to refer to all R-series modules as flip-chip modules; they even advertised the PDP-8 as an integrated circuit computer. A typical flip-chip module, the R111, had three 2-input nand gates and cost $14, with no price change from 1965 to 1970. Some special dual height R-series modules were designed specifically for the PDP-8. S and B-series logic modules were also used; these are similar to their R-series cousins, but with different speed/fanout tradeoffs in their design. Some logic modules have trimmers that must be tuned to the context, making replacement of such modules more complex than simply swapping boards. As with the system modules used in the PDP-5, the supply voltages were +10 and -15 volts and the logic levels were -3 (logic 1) and 0 (logic 0). Logic was packaged on boards that were 2.5 inches wide by 5 inches long. The card edge connector had 18 contacts on 1/8 inch centers. Some double height cards were used; these had two card edge connectors and were 5 1/8 inches high. Machine wrapped wire-wrap technology was used on the backplane using 24-gauge wire. The "negibus" or negative logic I/O bus used -3 and 0 volt logic levels in 92 ohm coaxial cable, with 9 coaxial cables bundled per connector card and 6 bundles making up the basic bus. 5 (later 4) more bundles were required to support data-break (DMA) transfers. The total bus length was limited to 50 feet, and bus termination was generally kluged in with 100 ohm resistors clipped or wrapped into the backplane, although a bus terminator card was sometimes used. Some time after the first year of production, flat ribbon cable made of multiple coaxial cables was used, and later still, shielded flat stripline cable was used (but this cut the allowed bus length by a factor of two). Core memory was used, originally made by FERROXCUBE, with a 1.5 microsecond cycle time, giving the machine an add time of 3 microseconds. 4K of core occupied an aluminum box 6 inches on a side and needed numerous auxiliary flip-chips and for support, as well as an array of boards from the core vendor. It is worth noting that the PDP-8 was about as fast as was practical with the logic technology used; only by using tricks like memory interleaving or pipelining could the machine have been made much faster. Reason for introduction: This machine was inspired by the success of the PDP-5 and by the realization that, with their new Flip-Chip technology, DEC could make a table-top computer that could be powered by a single standard wall outlet; of course, adding any peripherals quickly increased the power requirement! Reason for withdrawal: The PDP-8/I was less expensive, and after initial production difficulties, it equalled the performance of the PDP-8. Compatability: This machine defines the core of the PDP-8 instruction set, but with restrictions that were lifted on later machines. The Group 1 OPR instruction IAC cannot be combined with any of the rotate instructions. If RAR and RAL or RTR and RTL are combined, the results are unpredictable (simultaneous set and reset of bits of AC results in metastable behavior). The IOT 0 instruction was used for the internal type 189 ADC, and not for the later CAF (clear all flags) instruction. As a result, if the ADC option was not present, IOT 6004 (or microcoded variants) would hang the machine. The SWP instruction (exchange AC and MQ) never works, even if the extended arithmetic element is present. This works on later models when the EAE is present, although it was only documented with the introduction of the PDP-8/E. Finally, the EAE lacks the SCL (shift count load) instruction that is present on later models. On machines with 8K or more, an attempt to change the data field to a non-existant field caused a bizarre double-indirect and skip instruction execution that must be accounted for in memory diagnostics. Standard configuration: The PDP-8 was sold as a CPU with 4K of memory, a 110 baud current loop teletype interface and an ASR 33 Teletype. In addition, the standard in-cabinet logic includes support for the full negibus interface, including data-break (DMA) transfers. Both a rack-mount model with rosewood trim and an elegant plexiglass enclosed table-top configuration were standard. Under the skin, the basic machine occupies a volume 33 inches high by 19 inches wide by 22 inches deep. The two halves of the backplane are mounted vertically, like the covers of a book, with the spine in back and circuit modules inserted from the two sides. Sliding the CPU out of the relay rack or removing the plexiglass covers allows the backplane to swung open to access the wires-wrap. Expandability: In-cabinet options include the type 182 extended arithmetic element (EAE) ($3,500), the type 183 memory extension control subsystem ($3000), and the type 189 low performance analog to digital converter ($1450). Prewired backplane slots were reserved for all of these, as well as the optional type 129 data channel multiplexor ($2700). Expansion beyond 4K of memory requires rack mounting space (at $690 per CAB-8 rack). Each type 184 memory module adds a 4K field of memory ($10,000), seven modules may be added. The rack-mount CPU occupied a large part of one rack, allowing room for a single memory expansion module below the CPU; generally, a second rack was needed for added peripherals or memory. At the end of the production run, some PDP-8 systems were sold with PDP-8/I memory, allowing room for an additional 4K without need for an expansion chassis. These nonstandard machines were very difficult to maintain! Peripherals: At the time of introduction, the following negibus peripherals were offered. -- Type 750 high speed paper tape reader and control ($3500). -- Type 75A high speed paper tape punch and control ($4000). -- Type 138 analog to digital converter ($4500). -- Type 139 analog multiplexor ($3300). -- Type 30N precision CRT display ($13,400). -- Type 34B oscilloscope display ($3600). -- Type 370 high speed light pen ($1625) -- Type 350 incremental (CalComp) plotter and control ($8,900 up). -- Type 451 card reader and control ($14,900). -- Type 451B fast card reader and control ($25,600). -- Type 450 card punch control for IBM Type 523 punch. -- Type 64 (later 645) Mohawk line printer and control ($28,900). -- Type 250 (RM08) serial magnetic drum (256K words for $43,600). -- Type 552 DECtape control (for type 555 DECtape drives, $9500). -- Type 555 dual DECtape transport, $7400). -- Type 57A magnetic tape control with IBM type 729 drive ($15,200). -- Type 580 magnetic tape system with one transport ($19,700). By 1966, the following peripherals had been added to the line: -- Type AA01A three-channel digital to analog converter. -- Type CR01C card reader control. -- Type TC01 DECtape control for up to 8 TU55 transports. -- Type 251 drum (8-256 tracks, 8 sectors/track, 128 words/sector). -- Type 645 line printer control. -- Type 680 data communications system (allows 64 teletypes). By 1967, the following peripherals had been added to the line: -- Type AF01 analog to digital converter and multiplexor. -- Type AX08 parallel digital input port. -- Type 338 Programmed Buffered Display (vector graphics). By 1968, the following new peripheral had been added: -- Type DF32 fixed head disk system (32K to 256K words). -- Type BE01 OEM version of the TC01 (no blinking lights). -- Type BE03 dual TU55 drive for the TC01 or BE01. Finally, as DEC abandoned the negibus, they introduced the DW08B negibus to posibus converter so newer posibus peripherals could be used on older negibus machines, and the DW08A posibus to negibus converter to allow use of old peripherals on new machines. Survival: Many classic PDP-8 systems survive to this day in working condition. User Contributions:Top Document: PDP-8 Summary of Models and Options (posted every other month) Previous Document: What is a PDP-5? Next Document: What is a LINC-8? Single Page [ Usenet FAQs | Web FAQs | Documents | RFC Index ] Send corrections/additions to the FAQ Maintainer: jones@cs.uiowa.edu (Douglas W. Jones)
Last Update March 27 2014 @ 02:11 PM
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