Open-collector output — what it means for the bus
The SN74LS03DR is a quad 2-input NAND gate from the 74LS low-power Schottky family. Its defining feature is the open-collector output: the output transistor's collector is left unconnected internally, so the high-level output voltage is determined by an external pull-up resistor tied to a supply rail — typically 5 V or a higher voltage the downstream logic can tolerate. This means the part can drive a wired-AND bus where multiple open-collector outputs share a common pull-up: any gate pulling low forces the bus low, which is the standard architecture for interrupt lines, I²C-like handshaking, or shared reset signals. The propagation delay is 32 ns typical at 5 V with a 15 pF load — a 74LS-class speed that keeps up with most 5 V microprocessor buses running below 10 MHz. The input thresholds are 0.8 V (low) and 2.0 V (high), compatible with standard 5 V TTL and CMOS logic families.