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TTL(1) (Time To Live) A counter in a network packet that sets a limit to its validity. In order to prevent an IP packet from propagating endlessly through the network, the value in the TTL field is reduced by each router. When TTL reaches 0, the packet is discarded.
(2) (Time To Live) A timestamp in the DNS system, which converts hostnames to IP addresses. Responses use a TTL field to keep the IP address in the user's cache for a limited amount of time. After the time is up, the next request for that IP address must go back to the DNS system. See DNS and DNS rebinding.
(3) (Transistor-Transistor Logic) A digital circuit composed of bipolar junction transistors (BJTs). Widely used in all variety of electronic applications, especially prior to CMOS circuits becoming popular, TTL superseded the earlier RTL (resistor-transistor) and DTL (diode-transistor) logic designs, which used more power. In TTL, transistors are used to both isolate inputs and perform the logic switching. A "TTL" designation on a circuit input or output indicates a digital circuit rather than analog. See 7400 series, ECL, I2L and bipolar transistor.
(4) (Through The Lens) Refers to a single-lens reflex camera that lets the photographer view the scene through the same lens that captures the image. "TTL metering" means that the light is measured from behind the lens to determine the correct shutter and flash settings.