Published July 1999

THE ART AND SCIENCE OF RS-485

by Bob Perrin

Start • Arm Yourself • RS-485 101 • Getting Grounded • Shielding • Topology • Termination • Idle-state Biasing • Transients • Review Time • Sources

TERMINATION

Assuming the network cable is long enough for transmission-line effects to arise, what termination technique should be used to mitigate reflections?

There are quite a few termination methods available. National Semiconductor has published a 10-page application note that describes seven distinct techniques [7]. The four techniques that I will review are shown in Figure 6.

Figure 6—Several termination methods are widely used on RS-485 networks.

Unterminated networks are low power, low cost, and simple to build. The disadvantage, of course, is that data rates must be quite slow or cable length must be short for the network to operate reliably.

A parallel termination offers excellent data rates but is limited to networks that only have one driver. The driver must be located on one end of the network and the termination resistor must be located on the far end.

The resistor should have the same value as the characteristic impedance (Z_o) of the transmission line. Cable manufacturers publish Z_o for their network cables. The larger the Z_o, the less power Rp (which is equal to Z_o) must dissipate as heat.

The most common RS-485 twisted pairs have a Z_o of 100–120 ohms. Category 5 (CAT-5) cable offers a 100-ohm Z_o, typically has four pairs, and is widely available. The Belden RS-485 cables (9841-9844) have a Z_o of 120 ohms. Alpha Wire cables (Alpha 6222-6230) have a Z_o of 100 ohms.

The third termination technique is a bidirectional termination, which offers excellent signal integrity. With this technique, the line drivers can be anywhere on the network. The disadvantage is power consumption. This technique is probably the most reliable RS-485 termination technique.

The fourth and most dubious technique is called AC termination. The idea is to use the capacitor as a DC blocking element to reduce power consumption. In practice, I have never seen this technique do anything except butcher signal integrity. The National Semiconductor application note describes a design methodology for this type of termination [7]. I’m willing to believe this technique is useful in some applications, but I’m also pretty sure a fair degree of tweaking is required to get this system to function reliably.

The last subject related to termination is what to do with unused conductors in a data cable. Unused conductors will self-resonate and couple noise into the data wires. If the unused cables are left open, they will resonate at all sorts of strange frequencies. If they are grounded at one end, they will resonate at L/2. If they are grounded at both ends, they resonate at L/4.

The best method for minimizing energy on an unused conductor is to dissipate the energy as heat. In short, terminate both ends of the unused conductor to ground with resistors (a bidirectional termination). The resistors should be equal to the characteristic impedance of the line.

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