What Causes a Signal Bounce in a Bus?

By Mike Wallace

Signal bounce occurs when a signal is transmitted down a line, reaches a destination, and gets reflected back to its source. Signal bounce is highly undesirable in a network of computers since the bounced signal can interfere with other signals. One network topology is the bus, where all computers are connected to a single main line, or bus. There are several different causes to signal bounce on a bus network.

Missing Terminating Resistor

At either end of the bus, there must be a terminating resistor that absorbs the signal so that it cannot bounce. It is, instead, dissipated as heat through the resistor. If both ends of the bus lack a terminating resistor, signal bounce will occur. The terminating resistor must match the impedance load of the transmission line to prevent signal bounce. For computer networks, this is 50 Ohms. The term "Ohms" refers to a physical measurement of resistance to electrical current.

Improper Terminating Resistor

The terminating resistor must absorb all of the energy of the transmitted signal to avoid signal bounce. This occurs when the impedance of the terminating resistor matches that of the transmission line, or bus. Luckily, networking equipment is standardized so finding an improper terminating resistor is unlikely. However, a malfunctioning terminating resistor may behave outside of its nominal specifications, and therefore behave like an improper terminating resistor. If a terminating resistor does not measure an impedance of 50 Ohms at the frequency that the network operates at, it may cause signal bounce.

Improper Cabling

Network cabling is rated at a maximum frequency at which it can transmit data. Exceeding this frequency can alter the electrical characteristics of the network, and, thus, necessitate the need for non-standard terminating resistors to absorb signals. It is often easier to replace the cabling with the appropriate kind than attempt to track down a non-standard electrical component of any sort.

Fault in the Network

A fault in the network, such as a malfunctioning network port, can cause signal bounce. The reason for this is that network ports themselves behave like terminating resistors. If they malfunction, then they may reflect the entire signal back down the bus.