Top Document: PDP-8 Summary of Models and Options (posted every other month) Previous Document: What is a PDP-8/S? Next Document: What is a PDP-8/L? See reader questions & answers on this topic! - Help others by sharing your knowledge Date of introduction: 1968 (Announced before December '67) Date of withdrawal: 1971. Total production run: 3698. Technology: DEC M-series logic modules, called M-series flip-chips as the term flip-chip was applied to the module format instead of to DEC's hybrid integrated circuits. M-series modules used TTL chips, with a +5 volt supply, packaged on the same board format used with the original flip-chips, but with double-sided card-edge connectors (36 contacts instead of 18). Modules were limited to typically 4 SSI ICs each. The M113, a typical M-series module, had 10 2-input nand gates and cost $23 in 1967 (the price fell to $18 in 1970). Wire-wrapped backplanes used 30-gauge wire. The PDP-8/I, as originally sold, supported the then-standard PDP-8 negibus. 4K words of core were packaged in a 1 inch thick module made of 5 rigidly connected 5 by 5 inch two-sided printed circuit boards. Connectors and support electronics occupied an additional 32 backplane slots. Nominally, the core memory (which, curiously, used a negative logic interface!) was supposed to run at a 1.5 microsecond cycle time, but many early PDP-8/I systems were delivered running at a slower rate because of memory quality problems. DEC went through many vendors in the search for good memory! The memory interface was asynchronous, allowing the CPU to delay for slow memory. DEC continued to make the classic PDP-8 until the problems with memory speed were solved. Reason for introduction: This machine was developed in response to the introduction of DIP component packaging of TTL integrated circuits. This allowed a machine of about the same performance as the original PDP-8 to fit in about half the volume and sell for a lower price. Reason for withdrawal: The PDP-8/E made slight performance improvements while undercutting the price of the PDP-8/I. Compatability: The core of the PDP-8 instruction set is present, and unlike the original PDP-8, IAC can be combined with rotate in a single microcoded Group 1 OPR instruction. Combined RAR and RAL or RTR and RTL produce the logical and of the expected results from each of the combined shifts. If the extended arithmetic element is present, the SWP (exchange AC and MQ) instruction works, but this was not documented. On large memory configurations, memory fetches from a nonexistant memory field take about 30 microseconds (waiting for a bus timeout) and then they return either 0000 or 7777 depending on the memory configuration and the field that was addressed. A front panel bug prevented continue after load-address without first clearing the machine. Standard configuration: CPU with 4K of memory, plus 110 baud current loop teletype interface. Pedestal, table-top and rack-mount versions were made. The pedestal mounted version was futuristic looking; the table-top version split the pedistal, with the CPU on the table and the power supply (the base of the pedistal) on the floor beside the table. The standard rack-mounted version had the power supply bolted to the right side of the rack while the CPU, mounted on slides, slid out of the left side of the rack. Expandability: 4K of memory could be added internally, and additional memory could be added externally using a rack-mounted MM8I memory expansion module for each 4K or 8K addition over 8K. The backplane of the PDP-8/I was prewired to hold a Calcomp plotter interface, with the adjacent backplane slot reserved for the cable connection to the plotter. There may be other built-in options. Initially, the CPU was sold with bus drivers for the PDP-8 negibus, allowing this machine to support all older DEC peripherals, but later machines were sold with posibus interfaces, and many older machines were converted in the field. A posibus to negibus converter, the DW08A, allowed use of all older PDP-8 peripherals, with small modifications. The change from negibus to posibus during the period of PDP-8/I production leads to confusion because surviving CPUs and peripherals may have any of three I/O bus configurations: Negibus, early posibus, or final posibus. The early posibus used the same connectors and cables as the negibus, with only 9 conductors per connector, while the final posibus used both sides of the connector paddles for 18 bus lines per connector. Y-shaped cables for converting from one physical bus layout to the other were available. To add to this confusion, some negibus PDP-8/I systems were rewired to use 18 conductor posibus cables with negative logic! Eventually, an add-on box was sold that allowed PDP-8/E (OMNIBUS) memory to be added to a PDP-8/I. Additionally, Fabritek sold a 24K memory box for the 8/I and PDP-12. Survival: Many PDP-8/I systems are in operating condition, some still performing in their original applications! User Contributions:Top Document: PDP-8 Summary of Models and Options (posted every other month) Previous Document: What is a PDP-8/S? Next Document: What is a PDP-8/L? 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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