Date of introduction: 1974 (Announced in May for December delivery)
Date of withdrawal: 1984
Also known as: KIT-8/A (CPU plus 1K RAM), CLASSIC (CLASSroom Interactive Computer), DECDataSystem310 (an 8/A 500 sold as a word-processor)
Technology: This machine used the OMNIBUS with a new single-board CPU. The backplane was reoriented so that boards plugged into it from the front, with the board held horizontally. The new omnibus allowed a board format half again as wide as the original (formally, this was called hex height), but the extra 2 groups of contact fingers added to each wide board was largely unused. (the 6th contact group was not connected on most backplane slots; the 5th was unsupported on 8 of the 12 or 20 backplane slots, and was used primarily for additional power and ground distribution).
Reason for introduction: Using TTL MSI and LSI components, DEC was able to reduce the PDP-8 CPU to a single oversize board (formally, hex height, double width). Similarly, they were able to make an 4K
core memory board, and later, an 8K board in this format, and they were able to introduce a static RAM card using semiconductor memory. The minimum system was thus reduced to 3 boards. The relatively expensive lights and toggle switches on the front panel of the PDP-8/E were replaced with an octal membrane keypad and 4-digit 7-segment LED display.
The market for the PDP-8 was dominated by small systems, with fewer and fewer customers needing large-scale expandability. Thus, the 20 slot backplane of the early OMNIBUS machines was too
big; with the new single board CPU and memory, a 12 slot backplane was enough, allowing further cost reductions.
Reason for withdrawal: The market for the PDP-8 family was shrinking in the face of pressure from larger minicomputers and the new monolithic microcomputers. After 1975, many PDP-8 sales were to
captive customers who had sufficient software investments that they could not afford to move. Only the word-processing and small business markets remained strong for first-time PDP-8
sales, and in these, the specialized DEC VT-78 and DECmate machines were more cost effective than the open architecture OMNIBUS machines.
Compatability: The new PDP-8/A CPU was largely compatable with the PDP-8/E CPU, except that the combination of RTR and RTL (Group 1 OPR instructions) loaded the next address. The power-fail
auto-restart option included the standard skip on power low instruction, but also a new skip on battery empty instruction to test the battery used for back-up power on the new solid state
The standard parallel port on the M8316 was not software compatable with the earlier line-printer interfaces used with device code 66.
Standard configurations: The PDP-8/A was sold with a new short OMNIBUS backplane, mounted on its side above a power supply and a battery to back up the solid state memory. The minimum
configuration included a limited function control panel and the following components on the bus:
— M8315 — KK8A CPU board
– M???? — MS8A 1K to 4K solid state memory.
– M???? — MR8A ROM companion for the MS8A.
– M8316 — DKC8AA serial/parallel interface and clock.
The M8316 board contained a remarkable but useful hodgepodge of commonly used peripherals, including the console terminal interface, a parallel port, the power/fail auto-restart logic, and a 100 Hz real time clock.
The smallest PDP-8/A configuraton marketed was the KIT/8A, either just the KK8A and MS8A 1K boards for $572, or $1350 for a system that appears to have included the M8316 and a 4 slot backplane.
The 8/A 100, was a computer system with a 10 slot backplane and a poor power supply. The 8/A 400 was a better system with a 12 slot backplane, and the 8/A 420 had a 20 slot backplane.
The 8/A 600 and 620 were the 8/A 400 and 420 with the KK8E PDP-8/E CPU set allowing added speed and the use of the 8/E EAE.
Expandability: All PDP-8/E peripherals and options could be used with the PDP-8/A. For those configurations requiring more than 20 backplane slots, A pair of PDP-8/A backplanes could be connected
using BC08H cables, and there was a special cable, the BC80C, for connecting a hex wide 8A backplane to a PDP-8/E, -8/F or -8/M backplane.
By February 1975, the PDP-8/A was being sold in a workstation configuration, with the CPU and dual 8″ diskette drives in a desk with a video terminal (VT52) and optional letter quality printer
on top. For the educational market, this configuration was marketed as the CLASSIC. As an office system, such configurations were marketed as DECDataSystems.
The following additional PDP-8/A (hex) boards were offered:
— G649 \_ MM8AA 8K Core stack (too slow for 8/E CPU!).
– H219A / MM8AA 8K Core memory control.
– G650 \_ MM8AB 16K Core stack (ok for 8/E CPU!).
– H219B / MM8AB 16K Core memory control.
– M8349 — MR8F 1K ROM (quad, overlayable with core).
– M8317 — KM8A memory extender (with variations).
– M8319 — KL8A 4 channel RS232 or current loop serial I/O.
– M8433 — RL8A controller for 1 to 4 RL01/RL02 disk drives.
– M8410 \_ FPP8A floating point processor control
– M8411 / FPP8A floating point processor data path
The PDP-8/A model 800 was the same as the model 600, but with the FPP8A floating point processor included as part of the package.
— M8416 — KT8AA Memory management unit for up to 128K.
– — KC8AA Programmer’s Console (requires M8316)
– M8417 — MSC8DJ 128K DRAM MOS Memory.
Note that memory extension to 128K was a new PDP-8/A feature that was necessarily incompatable with the older PDP-8 memory expansion options, although the conventional PDP-8 memory expansion
instructions still operate correctly on the first 32K. Access to additional fields involved borrowing IOT instructions that were previously dedicated to other devices.
The MM8A core memory options require the use of a box with a G8018 power supply that provides +20V, while the semicondustor memory options require a G8016 power supply with built-in battery backup.
Also, the use of the MSC8 DRAM memory cards require a CPU that supports the memory stall signal; early PDP-8/E CPUs did not.
Survival: As with the PDP-8/E, these machines are moderately common on the surplus market and a modest number are still in use.
(Source: PDP-8 FAQ)