Data transmission lines must be terminated when the signal round-trip time (towards the bus end and back to the signal source) is longer than the rise/fall time of the active driver. If unterminated, signal reflections returning to the source will distort the initial driver signal and cause data errors. Proper termination requires that the energy of the signal propagating along the bus is fully converted into heat through the implementation of termination resistors at both cable ends. The values of the terminating resistors, RT, should match the characteristic impedance, Z0, of the transmission cable. Because the RS-485 standard recommends the use of twisted pair cables with a Z0 = 120 Ω, the value of the termination resistors should also be 120 Ω. This termination method is known as parallel termination. Note that the RS-485 standard allows for a wider range of characteristic cable impedances (120 Ω ±30 Ω) to enable the use of various cables with either lower or higher characteristic impedance, such as CAT-5 cable (Z0 = 100 Ω) and Profibus cable (Z0 = 150 Ω).