The Bedstead


      The Bedstead is a way of mounting a long cipher text, punched onto paper tape in the Baudot teleprinter code, and allowing

      it to   be read optically at very high speed, 5,000 characters per second.


      The Baudot teleprinter code. Each character is made up of 5 bits either dot representing a hole in the tape or blank, no hole.

      In modern speak, 1 or 0.


      The row of smaller holes near the centre are the sprocket holes used on mechanical teleprinters to step the tape forwards.

      There are thus two elements on one side of the sprocket holes and three on the other side. Elements 1 and 2 on one side,

      3, 4 and 5 on the other side.


The paper tape is loaded onto the tape wheels and drawn vertically in front of a light source. This means that a data character is horizontal and this character is projected by a lense onto photo cells.


The paper tape is looped over the wheels to absorb its length. The centre wheel can be slid up or down to take up the length.


The paper tape is joined into a loop using four strips of parcel tape stuck each side of the tape and leaving the sprocket holes uncovered.


The loop has to have about 5 feet of blank tape each side of the join.




















      The start and end of the cipher text on the tape is indicated by special holes punched onto the paper tape. The start

      hole is aligned between tracks 3 and 4 and is punched two positions in front of the first cipher character.


      The end indicator hole is aligned between tracks 4 and 5 and is punched two positions after the last cipher character

      on the tape.










                 End, Stop hole                           Start hole






Tape image



                                          The projected tape image                 The tape being read


       End cell


Sprocket cell


  5 Data cells


      Start cell




                                                          The photo cells 
































                                                                                              The J Rack master switch panel



                                                       The near/far reader switch