TTL

(electronics) A logic circuit containing two transistors, for driving large output capacitances at high speed. Abbreviated T²L; TTL.

(1) (Time To Live) A parameter in a network packet that sets a time 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.

In the DNS system, which converts host names to IP addresses, responses use a TTL field to keep the IP address in the user's cache for a set time. If a company is preparing to change its IP addresses, it can lower the time in the TTL field before the actual change is scheduled. If the address is then cached in the user's computer, it is valid for only a short time, and the company's name server will be queried more frequently. See DNS and DNS rebinding.

(2) (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 Widely used in all variety of electronic applications, especially prior to CMOS circuits becoming popular, TTL superseded the earlier RTL (resistor-transistor) and DTL (diode-transistor) logic designs, which used more power.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.

(3) (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.

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