6 Bus Termination and Stub Length

Data transmission lines should always be terminated and stubs should be as short as possible to avoid signal reflections on the line. Proper termination requires the matching of the terminating resistors, R_T, to the characteristic impedance, Z0, of the transmission cable. Because the RS-485 standard recommends cables with Z₀ = 120 W, the cable trunk is commonly terminated with 120-W resistors, one at each cable end (see Figure 6-1, left).

Applications in noisy environments often have the 120-Ω resistors replaced by two 60-Ω, low-pass filters to provide additional common-mode noise filtering, (see Figure 6-1, right). It is important to match the resistor values, (preferably with 1% precision resistors), to ensure equal rolloff frequencies of both filters. Larger resistor tolerances, (i.e., 20%), cause the filter corner frequencies to differ and common-mode noise to be converted into differential noise, thus compromising the receiver’s noise immunity.

The electrical length of a stub, (the distance between a transceiver and cable trunk), should be shorter than 1/10 of the driver’s output rise time, and is given through:

Where:

• L_Stub = maximum stub length (ft)
• t_r = driver (10/90) rise time (ns)
• v = signal velocity of the cable as factor of c
• c = speed of light (9.8 x 10⁸ ft/s).

Table 6-1 lists the maximum stub lengths of the cable in Figure 5-1, (78% velocity), for various driver rise times.