PDP-8 Clones

Introduction

The Soviet Block countries didn't have much in the way of home developed systems. Designing and building a computer is only half the challenge, the biggest effort is the man hours required to create a reliable operating system and the associated programming languages. A faster way to leapfrog these challenges is to acquire this technology by copying or stealing someone else's. DEC's PDP-8, PDP-11 and later VAX systems were the choice of Soviet copycats. By building a clone they immediately had access to a wealth of operating systems, programming languages and applications.

PDP-8 and PDP-11 systems were shipped with circuit manuals intended for maintenance, but could be easily used to re-build a PDP from the schematics. These systems were intended for engineers or technical staff who could maintain these systems and hence the reason for the detailed bus specifications found in the DEC handbooks and the schematic manuals provided to users. By building clone computers, the effort to develop operating systems and programming languages was a matter of acquiring DEC software copies and loading them into the cloned PDP-8s. The intellectual property developed at DEC by way of many man hours of development was simply and easily copied and acquired. The Russian PDP-8s were remarkably similar to the DEC PDP-8s in their packaging, size and looks.

The Hungarian TPA series, was built by KFKI (the Central Research Institute for Physics). The original machine in this series, the TPA1001, was built from the description in DEC's Small Computer Handbook. Only after the series was in production, when a machine was exhibited in Ljubljana, Yugoslavia, was full DEC compatibility demonstrated, when a DEC user booted the TPA machine from a DEC paper tape. By the end of the TPA production run, around 900 PDP-8 clones had been built. Given the Soviet era central planning for the computer industries in Eastern Europe, it is quite possible that the Electrotechnika and Electronica models may have been TPA machines packaged for use in the USSR and other Soviet Block countries. There was a decree that computer development in Hungary was to cease, with all computers to be purchased from the USSR. In response, the people at KFKI ceased developing computers and began developing "Stored Program Analyzers" or, the acronym for which is TPA in Hungarian, hence why these computers all were named TPA. Similar to why DEC named their machines "Programmed Data Processors" (PDP) instead of minicomputers.

Figure 1 - Russian Saratov-2 PDP-8 Clone, Source: Hackaday.com (Ralph Mirebs photo credit)

According to Thomas Lopez at the Universidad de las Ciencias Informáticas in Havana, the Cuban CID-201, CID-201A and CID-201B minicomputers were built from the description in DEC's Introduction to Programming. The first prototype, implemented with DTL IC logic, was operational in April 1970. Given that DEC never built a DTL implementation of the PDP-8, the Cuban hardware qualifies as an original re-implementation of the architecture. The Cuban efforts were written up in Datamation Magazine in December 1973. In the mid to late 1970s, there was some interchange between the Cuban and Hungarian groups. By that time, both Cubans and Hungarians were at work on PDP-11 clones.

Although much is written about Soviet clones, there were also Western firms building PDP-8 clones. The CESI (Computer Extension Systems, Inc.) computer had 128K words of local RAM on each processor card and allowed up to 4 processor cards per OMNIBUS, along with 128K words of global shared memory. The CPU was based on three AMD 2901 bit-slice ICs to build the 12-bit ALU and data paths and were controlled by an 80-bit microword (a fairly wide control store microword to program for).

Figure 2 - Russian Saratov-2 PDP-8 Clone, Source: Hackaday.com (Ralph Mirebs photo credit)

Figure 3 - Russian Saratov-2 PDP-8 Clone, Source: Hackaday.com (Ralph Mirebs photo credit)

Figure 4 - Russian Saratov-2 PDP-8 Clone, 16KW Memory (Four blocks of 4KW cores) Source: Hackaday.com (Ralph Mirebs photo credit)

The following table lists compatible or semi-compatible PDP-8 clones. Sources:

• Adapted from: https://en.wikipedia.org/wiki/PDP-8

Related Links:

• Douglas W. Jones, University of Iowa, PDP-8 Collection
• Professor Mark Csele - Vintage Digital PDP-8 Computers
• Computermuseum der Stuttgarter Informatik
• Hackaday - Soviet Computing