Internal communication signals in a stored program controlled system comprising a plurality of units configured to process signals are provided by an optical beam line which is proximal to all of the plurality of units. The beam line is configured to contain optically encoded signals, which comprises signals transmitted between and/or among the plurality of units. Each unit includes a probe for injecting optically encoded signals in the beam line and/or and for receiving optically encoded signals from the beam line. Processing units include at least one aperture for passage of the beam line. Processing units may also include a movable portion that is moved during installation of the processing unit and then replaced after the main body is in place.