Ken Shirriff has reverse-engineered and powered up a TR-3 trigger module from the IBM 604 Electronic Calculating Punch, introduced in 1948. The 604 was a plugboard-programmed calculator the size of a double refrigerator: it read numbers from punch cards, performed up to 60 calculations, and punched results back into the card at 100 cards per minute. IBM marketed it as the equivalent of 150 engineers, rented it for $550 a month, and built over 5,600 units. The machine contained about 1,300 vacuum tubes, packaged in pluggable modules so faulty ones could be swapped quickly.

The TR-3 is what we would now call a flip-flop: a circuit with two stable states that holds one bit until forced to switch. Triggers generated timing signals and stored pulses in the 604, and its decimal arithmetic counters were built from them. The module uses a 2033 dual triode — two triodes in one glass envelope sharing heater and cathode connections, with a heater drawing 6.3 volts at 300 milliamps, a voltage inherited from 6-volt car batteries. The 604 ran at 50 kilohertz, three orders of magnitude faster than the 50-hertz electromechanical accounting machines of the era.

Shirriff explains the circuit as two tube inverters wired in a cross-coupled loop: each inverter's plate output feeds the other's grid through 200K and 1K resistors, so whichever side conducts keeps the other off. Because plate outputs swing between 50 and 150 volts while grids need voltages near zero, a -100 volt bias supply and resistor dividers act as level shifters. Inputs arrive through 40 pF capacitors, and the module is triggered by negative pulses that turn a conducting triode off, flipping the state. The two outputs sit at different voltage levels, which complicated Shirriff's plan to attach neon indicator bulbs to both.

In practice the circuit proved temperamental: depending on supply voltages, it would oscillate or latch permanently, since it must be stable enough to hold a state yet unstable enough to flip on a pulse. Notably, the later IBM 650 abandoned this design in favor of diode-logic gates driving an inverter loop, making state changes a matter of Boolean logic rather than analog voltage interplay.

The article closes with lineage: the trigger circuit was invented in 1918 by British physicists William Eccles and Frank Jordan as a kind of relay. ENIAC (1945) built decimal counters from ten triggers in a ring; IBM engineers patented a four-trigger binary-coded decimal counter in 1946 and used it in the 603 and 604. Modern chips use the same idea by the millions — just microscopic, transistor-based, and renamed flip-flops.