Reducing fat in RS-485 designs that are high in saturated power is simple if you understand how to maintain good transmission quality at the same time. The following discussion covers the facts, myths and "dirty tricks" you should know to achieve this goal.
Table 1. The RS-485 and RS-422 standard
Industrial- and building-automation systems include a variety of remote data-acquisition devices, which send and receive data via a central unit that makes the data available to users and other processors. Data loggers and meter readers are typical of these applications. A near-ideal data-transfer link for this purpose is defined by the RS-485 standard, which interconnects the data-acquisition devices with a single twisted-pair cable.
Because many of the data-acquisition and data-collection devices in an RS-485 network are small, hand held, battery-powered gadgets, power conservation is necessary to control their heat buildup and extend battery life. Similarly, power consumption is a concern for handheld instruments and other applications that use the RS-485 interface for downloading data to a host processor. (For those not familiar with RS-485, please see "RS-485 history and description.")
Where does all the power go?
An obvious indicator of power loss is the transceiver's quiescent current (IQ), but modern parts reduce this factor dramatically. Table 2 compares the quiescent current of low-power CMOS transceivers with the bipolar, industry-standard, 75176.
Table 2. Quiescent current comparison for various RS-485 transceivers
Another power-consuming characteristic of RS-485 transceivers is apparent under the conditions of no load, driver-enabled and an alternating input signal. Because open RS-485 lines should be avoided at all times, the drivers "crowbar" their output structures during each output transition. This brief turn-on of both output transistors at once produces a spike in the supply current, but a sufficiently large input capacitor smooths these current spikes, producing an rms current that increases with data rate up to a maximum value. For MAX1483 transceivers, this maximum is about 15 mA.
Figure 1. Three external resistors form a termination and fail-safe-biasing network for this RS-485 transceiver
Connecting a standard RS-485 transceiver to a minimum load (one other transceiver, two termination resistors and two fail-safe resistors) lets you measure the dependency of supply current on data rate in a more realistic condition. Figure 2 illustrates ICC vs. data rate for a MAX1483 transceiver under the following conditions: standard 560-ohm/120-ohm/560-ohm resistors, VCC = 5V, DE = /RE\ = VCC and 1000 feet of cable.
Figure 2. Supply current for the MAX1483 transceiver varies with data rate as shown
As Figure 2 shows, the supply current increased to approximately 37 mA even for extremely low data rates, caused primarily by the addition of termination and fail-safe bias resistors. For low-power applications, this should demonstrate the importance of the type of termination used as well as how the fail-safe is achieved. Fail-safe is described in the next section and a detailed description of termination is found in the section, "Dirty tricks of termination."
For RS-485 receiver inputs between --200 mV and +200 mV, the output is undefined. That is, if the differential voltage on the RS-485 side of a half-duplex configuration is 0 V and no master transceiver is driving the line (or a connection has come loose), then a logic "high" output is as probable as logic "low." To ensure a defined output under these conditions, most of today's RS-485 transceivers require fail-safe bias resistors: a pullup resistor on one line (A) and a pulldown on the other line (B) as shown in Figure 1. Historically, the fail-safe bias resistors on most schematics were labeled 560-ohm, but to reduce power loss (when terminating one end only) this value can be increased to approximately 1.1k-ohm. Some designers terminate both ends with resistor values between 1.1k-ohm and 2.2k-ohm. The tradeoff is noise immunity vs. current draw.
Transceiver manufacturers first avoided external biasing resistors by providing internal pullup resistors at the receiver inputs, but that approach was effective only for detecting open circuits. The pullups used in these pseudo-fail-safe receivers were too weak to define the receiver output for a terminated bus. Other attempts (to avoid external resistors) violated the RS-485 specification by changing the receiver threshold to a level between 0 V and --0.5 V.
Maxim's MAX3080 and MAX3471 family of transceivers solved both of these problems by specifying a precise receiver-threshold range of --50 mV to --200 mV, thereby eliminating the need for fail-safe bias resistors while complying fully with the RS-485 standard. These parts ensure that 0 V at the receiver input produces a logic "high" output. Further, this design guarantees a known receiver-output state for the open- and shorted-line conditions.
How to save power
As shown in Table 2, transceivers differ greatly in their draw of quiescent current. So the first step in conserving power is to choose low-power parts (like, for example, the MAX3471 (2.8 uA with driver disabled, up to 64 kbits/s). Because a transceiver's power consumption increases substantially during transmission, another goal is to minimize duty time for the drivers by transmitting short data telegrams with long waiting periods between them. The structure of a typical serial-transmission telegram is shown in Table 3.
Table 3. A serial transmission telegram
An RS-485 system including one-unit-load receivers (32 addressable devices) can have the following bits: 5 address bits, 8 data bits, start bits (all frames), stop bits (all frames), parity bits (optional) and CRC bits (optional). The minimum telegram length for such a configuration is 20 bits. For secure transmissions you must send additional information such as the data length, sending address and direction, which leads to telegram lengths up to 255 bytes (2,040 bits).
This variation in telegram length trades bus time and power consumption for data security, with the telegram structure defined by standards such as X.25. For example, the transmission of 20 bits at 200 kbits/second takes 100 us. Using the MAX1483 to send data every second at 200 kbit/s requires an average current of
(100 us*53mA + (1s -- 100 us)* 20(A)/1s = 25.3 uA.
Table 4. Telegram length vs. current consumption using a MAX1483 driver
When a transceiver is in idle mode, its driver must be disabled to achieve minimum power consumption. The effect of telegram length on the power consumption of a single MAX1483 driver, operating with defined breaks between transmissions, is shown in Table 4. The use of a shutdown mode can further limit power consumption in a system that provides a fixed-time polling technology or longer deterministic breaks between transmissions.
Figure 3. IC transceivers differ greatly in their supply current vs. data rate
In addition to these software considerations, the hardware offers plenty of room for improvement in power consumption. Figure 3 compares the supply current drawn by various transceivers while transmitting a square wave over a 1,000-foot cable with drivers and receivers enabled. The 75ALS176 and MAX1483 are terminated with standard 560-ohm/120-ohm/560-ohm networks at each end of the bus and the "true fail-safe" parts (MAX3088 and MAX3471) have only the 120-ohm termination resistors at each end of the bus. At 20 kbits/s, the supply current ranges from 12.2 mA (MAX3471 with VCC = 3.3V) to 70mA (75ALS176). Thus, there is a big power saving immediately when you choose low-power parts with a true fail-safe feature that also eliminates the need for biasing resistors (to ground and to Vcc). Make sure the RS-485 receiver you choose produces valid logic-output levels for both shorted and open conditions on the differential receive lines.
Dirty tricks of termination
As mentioned above, termination resistors eliminate the reflections caused by impedance mismatches, but their drawback is the additional power dissipation. Their effect is shown in Table 5, which lists the supply current for various transceivers (drivers enabled) for the conditions of no resistors, termination resistors only and termination/fail-safe-bias resistors combined.
Table 5. Using termination resistors and failsafe bias resistors increase supply current
No termination: The first approach in minimizing power consumption is to eliminate termination resistors altogether. This option is available only for short cables and low data rates, which allow reflections to settle before data is sampled in the receiver. As a rule of thumb, no termination is needed when the rise time of a signal is at least four times longer than the one-way propagation delay through the cable. In the following steps, this rule is employed to calculate the maximum usable length for an unterminated cable:
Step 1: For the cable in question, find the one-way velocity of propagation - usually provided by cable manufacturers as a percentage of the speed of light in free space (c = 3x10^8 m/s). A typical value for standard insulated PVC cable (consisting of a #24 AWG twisted pair) is 8in/ns.
Step 2: For the RS-485 transceiver, find its minimum rise time (tr min) from the data sheet specifications. A MAX3471, for example, specifies 750ns.
Step 3: Divide the minimum rise time by 4. For the MAX3471, tr min/4 = 750ns/4 = 187.5ns.
Step 4: Calculate the maximum cable distance for which no termination is required: 187.5ns(8in/ns)(1ft/12in) = 125ft.
Figure 4. Resistive terminations represent a major loss of power
Thus, the MAX3471 can maintain decent signal quality while transmitting and receiving at 64 kbits/s over a 125-foot cable without terminations. Figure 4 shows the dramatic reduction obtained in MAX3471 supply current when 100 feet of cable with no termination resistor is substituted for 1,000 feet of cable and a 120-ohm termination resistor.
Figure 5. An RC termination cuts power loss, but requires careful selection of the C value
RC termination: At first glance, the ability of an RC termination to block dc current is very promising. You find, however, that this technique imposes specific conditions. The termination consists of an R and C in series across the differential receiver inputs (A and B) as shown in Figure 5. Although R always equals the cable's characteristic impedance (Z0), the choice of C requires some judgment. Large C values provide good terminations by allowing any signal to see an R that matches Z0, but large values also increase the driver's peak output current. Unfortunately, longer cables require larger C values. Entire articles have been written on optimizing the C value with respect to this tradeoff. You can find detailed equations for this purpose in the References.
Average signal voltage is another important factor that is often overlooked. Unless the average signal voltage is dc-balanced, a dc stair-stepping effect causes significant jitter due to the pattern-dependent skew known as intersymbol interference. In short, RC terminations are effective in reducing the supply current but they tend to ruin the signal quality. Because RC terminations impose so many constraints on their use, a better alternative in many cases is no termination at all.
Schottky-diode termination: Schottky diodes offer an alternative termination when power dissipation is a concern. Unlike other termination types, they do not attempt to match the line impedance. Instead, they simply clamp the over- and undershoots caused by reflections. As a result, voltage excursions are limited to the positive rail plus a Schottky-diode forward drop in one direction and to ground minus a Schottky drop in the other direction.
Figure 6. Though expensive, Schottky-diode terminations offer many advantages
Schottky-diode terminations waste little power because they conduct only in the presence of overshoots and undershoots. On the other hand, standard resistor terminations (with or without fail-safe bias resistors) draw power continuously. Figure 6 illustrates the use of Schottky diodes for the purpose of eliminating reflections. Schottky diodes don't implement fail-safe operation, but the choice of threshold voltage allowed by MAX308X and MAX3471 transceivers lets you implement fail-safe operation with this type of termination.
The best available approximation to an ideal diode (zero forward voltage Vf, zero turn-on time tON and zero reverse-recovery time trr) is the Schottky diode, which holds great interest for its value in replacing power-hungry termination resistors. As a disadvantage, Schottky-diode terminations in an RS-485/RS-422 system cannot clamp all reflections. Once a reflection decays below the Schottky's forward voltage, its energy is unaffected by the termination diodes and persists until dissipated by the cable. Whether or not this lingering disturbance is a problem depends on signal magnitudes at the receiver inputs.
A major disadvantage of Schottky terminations is their cost. One termination requires two diodes and because the RS-485/RS-422 bus is differential, this number is again multiplied by two (Figure 6). Multiple Schottky terminators on a bus is not uncommon.
Figure 7. Supply current in an RS-485 system varies considerably with data rate and termination type
Schottky-diode terminations have many advantages for RS-485/RS-422 systems and power-saving is foremost among them (Figure 7). No calculations are required, because the specified maximums for cable length and data rate will be met before any limitation is imposed by the Schottky terminators. As a further advantage, multiple Schottky terminators at various stubs and receiver inputs improves signal quality without loading the communications bus.
When data rates are high and the cable is long, flea power in an RS-485 system is difficult to achieve because line terminations are required. In that case, transceivers with "true fail-safe" receiver outputs can save power even with terminations by eliminating the need for fail-safe bias resistors. Software communication structures also reduce power consumption by placing the transceiver in shutdown or by disabling the driver when not in use.
For lower data rates and shorter cables, the power differences are huge: Sending data at 60 kbits/s over a 100-foot cable using a standard SN75ALS176 transceiver with 120-ohm terminations draws 70 mA from the system power supply. On the other hand , using a MAX3471 under the same conditions draws only 2.5 mA from the supply.
RS-485 History and Description
The RS-485 standard was jointly developed by two trade associations, the Electronic Industries Association (EIA) and the Telecommunications Industry Association (TIA). The EIA once labeled all its standards with the prefix "RS" (Recommended Standard). Many engineers continue to use this designation, but the EIA-TIA has officially replaced "RS" with "EIA/TIA" to help identify the origin of its standards. Today, various extensions of the RS-485 standard accommodate a large variety of applications.
The RS-485 and RS-422 standards have much in common and are often confused for that reason. Table 1 compares the two. RS-485, which specifies bidirectional half-duplex data transmission, is the only EIA/TIA standard that allows multiple receivers and drivers in "bus" configurations. EIA/TIA-422, on the other hand , specifies a single, unidirectional driver with multiple receivers. RS-485 parts are backward-compatible and interchangeable with their RS-422 counterparts, but RS-422 drivers should not be used in an RS-485 system because they cannot relinquish control of the bus.
The differential signal paths of RS-485 and RS-422 systems provide reliable data transmission in the presence of noise and their differential receiver inputs can also reject large common-mode voltages. However, to guard against the much higher voltage levels associated with electrostatic discharge (ESD) a different class of protection is needed.
The charged capacitance of a human body enables a person to destroy integrated circuits just by touching them. A single such contact can easily occur during the installation of an interface cable. To guard against such damage, Maxim's interface ICs incorporate "ESD structures." These structures protect the transmitter outputs and receiver inputs of RS-485 transceivers against ESD levels up to +-15kV.
To guarantee the specified ESD protection, Maxim tests the positive and negative power-supply terminals repeatedly, in 200-V increments, to verify a sequence of levels up to +-15 kV. Devices with this rating (per the Human Body Model or the IEC 1000-4-2 specification) are designated by an additional "E" suffix on the part number.
Data rates and driver loading
The load presented to an RS-485/RS-422 driver is quantified in terms of the unit load, which is defined as the input impedance for one standard RS-485 receiver (12k-ohm). Thus, a standard RS-485 driver is capable of driving 32 unit loads (32 12k-ohm loads in parallel). The input impedance for some RS-485 receivers, however, is higher: 48k-ohm (1/4 unit load) or even 96k-ohm (1/8 unit load). As many as 128 or 256 such receivers, respectively, can connect to a single RS-485 bus. You can connect any combination of receiver types, provided their parallel impedance does not exceed 32 unit loads (i.e., is not less than 375-ohm).
Consequences of high data rates
Faster transmissions require higher slew rates at the driver output and higher slew rates produce higher levels of electromagnetic interference (EMI). Some RS-485 transceivers minimize EMI by limiting their slew rates. Lower slew rates also help to control the reflections caused by fast transitions, high data rates or long cables. Essential for minimizing reflections are termination resistors that match the cable's characteristic impedance. For common RS-485 cables (a twisted pair of 24AWG wires) this means a 120-0hm resistor at both ends.
1. California Micro Devices Staff, "ST-103: The Dynamics of AC Termination," California Micro Devices Corp., 1998.
2. California Micro Devices staff, "ST-104: The Dynamics of Schottky Diode Termination," California Micro Devices Corp., 1998.
3. Johnson, H. and Graham, M., "High Speed Digital Design, A Hand book of Black Magic," 1993.
4. Axelson, Jan, "Designing RS-485 Circuits," Circuit Cellar Issue 107, June 1999.