Reverse engineering a vintage diode-transistor logic chip

Microscope analysis identifies an initially unknown Signetics ST680A as a 1965 integrated circuit containing four two-input NAND gates built with diode-transistor logic (DTL). Its unpassivated silicon die reveals NPN transistors, diodes, and 16 serpentine silicon resistors, but removing stubborn epoxy and then metal made the circuit connections difficult to reconstruct. The output transistors are larger to handle more current, and each can sink 2.5 mA. The chip also reflects an early packaging milestone: Signetics’ 600 series helped establish the inexpensive plastic dual-in-line package, whose regular pin spacing simplified assembly and circuit-board design.

In each DTL gate, input diodes steer current: when both inputs are high, two transistors turn on and pull the output low; if either input is low, the first transistor switches off, the output transistor remains off, and a resistor pulls the output high. This yields NAND logic. The design isolates inputs and uses an extra diode to improve noise tolerance, but its resistor-based high output is weak and wastes power when the output is low. Two apparently unused diodes on the die reveal a manufacturing economy: Signetics reused the same underlying silicon for other chips by changing only the metal wiring. The shared layout could implement a three-gate, three-input NAND chip or a dual five-input NAND chip, reducing the cost of producing distinct circuits. The ST680A’s power and ground pins also use a diagonal arrangement that differs from the later familiar 7400-series convention.