A vintage logic chip hid spare diodes so one silicon design could become three different products
A close look inside an early logic IC finds spare components that let one silicon layout serve several jobs.
Two unused components inside an old chip can look like a mistake. In Ken Shirriff’s teardown, they are a clue to how the manufacturer stretched one design across several products. The silicon had options before the customer ever ordered an option.
Shirriff examined a Signetics ST680A, a diode-transistor logic chip from a family introduced in the 1960s. The particular package’s date code points to 1971. That distinction keeps the design’s age separate from the specimen’s age: an old circuit can have a younger manufacturing date, much like a book can have a later printing.
The investigation began with a chip supplied by CuriousMarc. Opening the package exposed the die, but cleaning it removed the metal connections. Shirriff reconstructed the circuit using remaining evidence of those connections. This is a particularly unforgiving version of taking something apart to find out how it works (the wiring diagram dissolved).
The circuit contains four NAND gates. Diodes combine the inputs, while transistors provide inversion and drive the outputs. The visible structures let Shirriff follow those functions across the die rather than treating the chip as an unexplained rectangle with a part number. A digital truth table has a physical floor plan underneath it.
Two diodes appeared unused in that four-gate arrangement. The explanation emerged when the layout was compared with other members of the family. With a different pattern of metal connections, the same silicon could produce three gates with three inputs each, or two gates with five inputs each. The latter configuration has a use for those extra diodes.
That makes the metal layer a manufacturing decision with consequences for the product catalog. Reusing the underlying layout reduces how much has to change to offer another gate arrangement. It also explains why inspecting one finished product can reveal components whose purpose belongs to a different member of its family.
One alternate arrangement needs a connection to cross beneath a metal path. Shirriff identifies a silicon crossunder that makes this possible. Even with only a handful of gates, arranging the connections is part of the engineering. The simplest schematic can become surprisingly demanding once every line must occupy an actual place.
There is a nice reversal at the end of the detective work. TubeTime found period databooks after the circuit had been reconstructed. The documentation would have answered the original question sooner, but it would not have shown readers the route from a spare diode to a family of chips. Sometimes finding the manual late produces the better tour.
Those spare parts were already employed.