- The IBM 604 Electronic Calculating Punch (1948) was a programmable calculator the size of a double refrigerator, programmed via a plugboard, processing 100 cards per minute and renting for $550/month, with over 5600 units produced.
- It used about 1300 vacuum tubes and introduced pluggable tube modules for easy replacement; circuits were built from modules like the TR-3 trigger, a flip-flop circuit with two stable states for storing information.
- The TR-3 trigger module was reverse-engineered, using a dual triode vacuum tube with cross-coupled inverters, a negative bias voltage, and capacitor-coupled inputs, though it was temperamental and later replaced by diode logic in the IBM 650.
- The trigger circuit (Eccles-Jordan trigger, 1918) was crucial for digital computers, enabling state machines and step-by-step operations; it led to decimal counters and binary-coded decimal (BCD) in the IBM 604.