Control Systems Before Computers
Controls before computers consisted of relays, switches and wires. When memory was needed, latching relays were generally used. Sorters must keep track of quite a bit of information for every board in the system. Magnetic drum systems were used to record the board information. The magnetic drum was a non-magnetic metal cylinder with magnetic tracks mounted on it. A set of electrical coils would either erase or record a magnetic pulse onto the track. The drum was rotated by a motor synchronized to the sorter chains. At normal sorter speeds, the drum would rotate about once every 2 minutes. The magnetic pulses would move around the drum as it rotated and were read by reed relays. When the right combination of pulses closed the correct relays, a diverter was activated and the board was diverted into a bin.
Here are some photos of a magnetic drum control system: