Indicators

Front panel indicator lights, aka blinkenlights

As well as System/360 being an engineering icon, a fascinating feature to today's eyes is the line’s extensive use of indicator lights on the front panels. [Although the casual term “front panel” is almost always used today, the IBM documentation refers to it as the “system control panel”.] These indicator lights are used to display the contents of the many registers, triggers, and latches that make up the internal logic of the CPU and associated circuitry.

The extent of front panel indicator usage varied across the model line, and the Model 65 can be considered a 'medium' example in this regard. Some higher-performance models had far more individual indicators, but not necessarily proportionally more functionality – looks could be deceptive. For example, a Model 91 (pictured below) has a front panel crammed with indicators and is about three times the size of a Model 65 panel. However, each Model 91 indicator is dedicated to displaying one single bit of information. The Model 65 uses six-position roller switches which allow a single indicator to selectively display any of six different internal bits of information.