IBM Pluggable Units
Pluggable units TR-56, D-11, IN-6, CF-1

IBM Pluggable Units

This is a project to document early IBM pluggable units and an early training kit that uses them. Two of these training kits have surfaced, one of them is in Mark Metzler's collection, listed as "An unkown IBM vacuum tube treasure". I now have one too, but Mark's is in better shape.

These are some of the first modular components (or field replacable units) used in computers. Each consists of (usually) one vacuum tube in a small chassis consisting of the tube socket, two phenolic spacers and a base with a 9-pin tube plug. The spacers have 10 notches to hold component leads (wires) in place. Resistors, capacitors and other components are wired between the tube socket and the base. Intermediate connections between components are made at the spacers by bending the leads over to adjacent notches and soldering to other leads.

Each unit has an insulated curved handle with an identifying code stamped on the end. The code consists of two letters and a number. The two letters indicate what kind of circuit it is and the number tells which variation. Common letter codes are IN for inverter, CF for cathode follower, TR for trigger (flip-flop) and D for diode.

Mark's training box came with this complement of modules:


The basic circuits are similar to those used in the NORC built by IBM in the early 50's. The logic levels are +10v and -25v. This illustration is from Millman and Taub, "Pulse and Digital Circuits", 1956 in the classic McGraw-Hill Electrical and Electronic Engineering Series.

Pluggable units schematics

I once obtained a bag of IBM pluggable units at an electronic surplus store. Four of them are shown above. I traced out the circuits and made individual schematics. Mark Metzler has begun tracing his units, and I recently obtained three more and traced them too.

IBM 650

Here are the drawings for the pluggable units in the IBM 650. Note that these somewhat later units are different, they have 10 pins at the base instead of 9 and they use different supply voltages. The supply voltages in the IBM 650 are indicated by numbers in squares in the drawings. The voltages are as follows, referenced to the number in the square.

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