Get email productivity tips!  
  Home    Up Contact Legal Stuff  

          Piercing the unknown

IBM pluggable units bring modularity to computer construction

    In 1948 IBM introduced a small electronic calculator, the IBM 604 Electronic Calculating Punch. Opinions may differ as to its smallness: one paper from 1951 describes it as “a miniature card-programmed electronic calculator”; from the image at right, you can form your own idea of how miniature it really was.
    Still, the 604 came only 2 years after the room-sized ENIAC, the first large electronic computer; and unlike that one-off monster machine it was meant for business use and was accordingly mass-produced. And this introduced a problem, for these early machines used thousands of discrete vacuum tubes, and many more resistors, capacitors and the like,
IBM 604 calculator
which had to be wired together by hand and which presented a maintenance nightmare afterwards. The 604 was the first machine to attempt an elegant solution to this problem, by introducing Pluggable Units.
Two IBM Pluggable Units from the 604 calculator
Click photo to enlarge
    You can see here the two pluggable units I managed to get my hands on. Each is a plug-in assembly that fits, in the footprint of a single miniature tube, both the tube itself and the additional components and wiring required to produce a specific logic element. The calculator had some 2,000 vacuum tubes, but a far smaller number of different pluggable unit types were needed. The two I have are a type IN-5 sporting a 5844 twin triode in a dual inverter circuit, and a type CD-1 containing a 6350 twin triode whose purpose I am unclear about.
    The advantages of this innovation are many. Construction for mass production was greatly simplified, since the circuits could be built in small modules and tested outside the final machine. Maintenance and fault tracing were also facilitated, since a suspect module could be replaced in seconds. And perhaps most interestingly, this construction method stacks tubes and their supporting passive components vertically, allowing much greater circuit density than with the older 2D construction paradigm of a flat chassis with all wiring hugging its surface. IBM had really done a good job here, and they were not slow to advertise this fact, although their ads -- of which I have the one seen here -- really took the liberty to over-dramatize, as you can see…

    The photos below show the construction of these units. Revolutionary for their day, they still depended on hand-soldered wiring and bulky discrete components. The metal-and-insulator framework, however, is standardized for most unit types.

Ad for the IBM 604 calculator
Click photo to enlarge
Detail of an IBM Pluggable Unit  Detail of an IBM Pluggable Unit
Click a photo to enlarge
Exhibit provenance::
    I bought the two units from two different US sellers on eBay, one of whom had the good fortune of buying an entire 604 decades ago for scrap metal -- and the good sense to hang onto the pluggable units until they became collectable. The magazine ad, from the Sept. 1951 issue of National Geographic, also comes from eBay.

More info:
    More on these units can be found on Paul Pierce’s site here. A wealth of information on the IBM 604 is available on Ed Thelen’s site here.
    You can see the wiring diagrams of these and other units here. Let me know if you determine the role of the CD-1 unit...

Back Index Next

Home | HOC | Fractals | Miscellany | About | Contact

Copyright © 2008 N. Zeldes. All rights reserved.