There are many great white papers on the subject [Reference 1]. This article provides missing details, or simplifications that can enhance the performance of your design.
Figure 1. Basic RS-485 topology
There are three primary tools available to the designer to manage EMI (shielding is covered separately at the end of the article).
1. Device speed
2. Transceiver operating voltage
3. Terminating resistor currents
It has been said before, but bears mentioning again. Do not use a baud rate faster than is needed for the application – and that includes the speed of the transceivers. Transceivers are available in different speed options that affect the rise/fall times of the signals. For instance, many RS-485 links run below 1 Mbps, so a device such as TI's SN75HVD12DR is a good choice. For 128 kbps links, the slower Intersil part would suffice.
The slower rise time of these parts (e.g., 100 nsec, [Ref. 2]) is more than fast enough for these applications, and minimizes EMI radiation. It may also reduce susceptibility from nearby noise sources since it has a slower response speed. Read the specs of the transceiver, as many standard devices will run at 10 Mbps or faster which is much higher than typically needed for these links.
Table 1. Example RS-485 transceivers and speeds
EMI is proportional to the voltage swing of any signal. Reduce the voltage swing and you can reduce the EMI radiated by the connections. Many newer devices are fully rated to operate at 3.3V while satisfying the minimum requirements of the RS-485 signalling standard. As a bonus, 3.3V is more common these days than 5V in many system designs. What do we give up by using a lower voltage? Speed capability and noise immunity are reduced at this voltage. But if the device is rated for the speed required, and shielding is used, a 3.3V RS-485 signal is usually adequate. Again, it is up to the designer to consider all relevant conditions, and check the datasheet. As a side note, resist the urge to add a capacitor across the input of a transceiver unless you compute the frequency response as being 5-10 times that of the signalling rate (1/2 the baud rate) to avoid signal degradation.
RS-485 has a wide operating voltage range, from an Rx threshold of 200 mV to a maximum differential signal of 10V. Usually 2V P-P is the minimum recommended drive level, and 3.3V devices will meet that criterion while interfacing just fine with 5V-powered receivers, providing reasonable signal-to-noise levels, especially for shorter runs. Keep in mind that if you need high speed (defined as >5 MHz), you may need 5V power, so check the datasheet.
Because EMI problems can be magnetic in nature, the currents flowing through the terminating resistors may be considered a factor. Magnetic interference can be more difficult to control as copper has a relative permeability of about 1, and may induce coupling from an aggressor circuit in spite of nearby shielding. Lower transient currents reduce the magnetic signature, and aid in minimizing coupling to other nearby circuits.
How do we do that? Isn’t the terminating resistor value fixed? No it isn’t, as long as your cable is not “electrically long” relative to the edge rate of your signals. There is no rule that says you can’t raise the value for other engineering reasons. If the primary issue or concern is susceptibility rather than radiation, then the lower the termination resistance, the better. But as in all engineering designs, there are trade-offs. Comparing a 5V/120Ω system with a 3.3V/499Ω system shows a factor-of-six reduction in current.