5 Avoiding antennas through line termination

The data link in Figure 3 is terminated with 100-Ω resistors, as suggested by the RS-422 standard, matching the characteristic impedance of the bus cable. A myth exists that bus cables of a few meters in length or data links operating at low data rates don’t need termination. Don’t believe it. Operating the bus without termination can turn the transmission line into a nasty receiver/transmitter antenna. The lack of termination resistors, which usually absorb the incident wave power sent by the driver, causes standing waves to occur; and the entire incident wave is reflected into the bus. The reflected waves mix with other incident waves, thus yielding standing waves for signal frequencies whose quarter wavelengths, or multiples thereof, equal the length of the data link.

Depending on their location, the wave nodes (minima) and antinodes (maxima) can have varying effects on the bus transceivers (see Figure 5). A driver close to an anti- node sees a high impedance and therefore transfers insufficient energy to the bus. A driver close to a node sees a very low impedance or a short. The resulting output current can exceed the driver’s maximum drive capability and even trigger its current limit at around 250 mA. Receivers located at antinodes can be damaged by excessively large input signals that exceed the receiver’s common-mode input range. Receivers close to nodes experience insufficient signal strength and are highly susceptible to noise and EMI. Any of the foregoing events will result in data errors from either the transmission or the reception of wrong data.