= H =
1. The signal emitted by a Level 2 Ethernet
transceiver at the end of every packet to show that the
collision-detection circuit is still connected. 2. A periodic
synchronization signal used by software or hardware, such as a bus
clock or a periodic interrupt. 3. The `natural' oscillation
frequency of a computer's clock crystal, before frequency division
down to the machine's clock rate. 4. A signal emitted at regular
intervals by software to demonstrate that it is still alive.
Sometimes hardware is designed to reboot the machine if it stops
hearing a heartbeat. See also breath-of-life packet.
--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.