RS-485 network

RS-485 receiver supply

Quiescent supply current

Driver supply current

TSB-89 guidelines

Two Implementations

The RS-485 standard is a widely used differential bus with applications in many networks. Since the 1970s, semiconductor manufacturers have developed transceivers to transmit and receive RS-485 signals over twisted-pair media. In addition to advances in signaling rate, robustness, and package size, the latest transceivers offer significant power savings over previous generations. In fact, smart transceiver selection and network design can significantly reduce the total power dissipation in the network.

During the operation of any RS-485 network, there should be at most one node transmitting with an active differential driver, one or more receiver nodes, the connecting media, and optional termination impedances. Power is dissipated in each of these components. A typical RS-485 network has several nodes, each with a differential driver and receiver (Fig. 1). The twisted-pair bus is terminated with the characteristic impedance at each end of the bus line.

Cable And Termination

The termination resistors commonly dominate power dissipation in the media and termination.^{1} With a 120-Ω resistor at each end of the bus line, the RS-485 standard requires a differential voltage of at least 1.5 V from the active driver. Thus, the active driver must cause a current through the termination resistors of at least:

I_{TERMINATION} = \\[1.5 V/(120 Ω || 120 Ω)\\] = 25 mA

and the power dissipated in the termination is at least:

P_{TERMINATION} = (1.5 V)·(25 mA) = 37.5 mW

Connected Receivers

The power dissipated in receivers can be calculated according to the unit load (UL) of each receiver. The input impedance of a one-UL (1UL) receiver can be approximated as 12 kΩ. Each 1UL receiver input pin will sink or source at most 1 mA. Therefore, the power dissipation for each receiver input is less than:

P_{RECEIVER INPUT} < (1 mA)2·(12 kΩ) = 12 mW

Typically, the voltage on the receiver inputs is much less than the extreme values (–7 V or 12 V). As a result, the receiver input current is much less than 1 mA, so the power dissipation in the receiver due to bus current usually can be neglected.^{2}

The receiver power dissipation due to internal circuits may be more significant than the power due to bus currents. Older RS-485 receiver designs have higher internal power dissipation than newer designs. Figure 2 shows the history of reduction in power-supply current for RS-485 receivers.

Early receivers dissipated up to 200 mW each \\[(40 mA) · (5 V)\\], for a total of 6.4 W for a network with 32 connected receivers. New reduced unit-load devices allow up to 256 connected receivers, with operating currents as low as 1 mA. These receivers dissipate less than 1.3 W in total. New receivers have low-power standby modes to further reduce power when the node is not actively listening to the network.

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Active Driver— Quiescent Current

During RS-485 network operation, either all drivers will be passive (the idle bus case) or one driver will be active.^{3} Power dissipation in the active driver is the sum of the power dissipation due to the network load, plus the power dissipation due to the internal circuits.

The internal dissipation depends on two components: a quiescent current and a switching current. Early transceivers had quiescent currents that were on the order of 40 mA or more. Newer generations of transceivers have significantly reduced quiescent currents (Fig. 3). The internal dissipation in the steady state is the product of the quiescent current and the power-supply voltage.

Most RS-485 transceivers use either a 5-V supply voltage or a 3.3-V supply voltage. Therefore, the internal power dissipation (not due to loading or switching) can be as high as 200 mW (5 V · 40 mA) for some older generation devices.

Recent designs have quiescent currents less than 1 mA, so internal quiescent dissipation can be less than 5 mW. Furthermore, new transceivers incorporate low-power standby modes to reduce power to microwatts when the node is not actively participating in the network.

Active Driver—Switching Current

The switching current depends on the signaling rate and accounts for brief periods of high-dynamic current during each transition. Figure 4 shows this effect for three example transceivers, each differing in the controlled driver slew rate.

In this example, the SN65HVD3082E is optimized for switching rates up to 200 kbits/s with relatively slow (1 μs) differential transition times. The SN65HVD3085E is for medium (up to 1-Mbit/s signaling rates), while the SN65HVD3088E has fast (7 ns) driver output transitions for signaling at rates up to 20 Mbits/s.

At very low rates, all transceivers have supply currents of less than 1 mA, which is the quiescent current level. As the signaling rate increases, the switching current also increases, reaching a much higher level at the rated maximum speed.

The internal power dissipation due to switching currents can be calculated as the product of the average signaling current and the power-supply voltage. For fast signaling rates, power dissipation due to signaling will be more significant than the quiescent power.

Active Driver— Load Current

Power dissipated in the active driver also depends on the current it delivers to the network. The network current is the sum of the termination current and current to the receivers. The maximum total network current is:

I_{NETWORK} = I_{TERMINATION} + I_{RECEIVERS} < 25 mA + 32 mA = 57 mA

The most common type of RS-485 transceiver uses a 5-V supply. The efficiency of the fully loaded driver circuits is relatively low, since only 1.5 V of differential signal is required to be delivered to the network load, an efficiency of:

Efficiency_{(5 V-DRIVER)} = 1.5 V/5 V = 30%

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Therefore, up to about 70% of the signal power is dissipated inside the driver circuit. Typically in a network with no significant ground offset, only the termination current is significant, totaling about 25 mA. So for a typical driver, the power dissipation is:

P_{DRIVER LOAD TYP.} = 25 mA · (5 V – 1.5 V) = 87.5 mW

Thus, in the maximum steady-state case, an older RS-485 transceiver may dissipate up to a total of almost 300 mW (200 mW due to quiescent current plus 90-mW driver power due to load current) when driving a fully loaded network. When switching currents are included, total driver power will be even higher.

Reducing Network Power

There are several ways to reduce the total power dissipation. First, select a transceiver with low-quiescent current. Many newer devices have much lower quiescent power than older devices. For example, the SN65HVD3082E dissipates less than 5 mW in the unloaded quiescent state. Also, newer receivers have higher input impedance, providing fractional unit loading. This allows more nodes on the same bus segment and reduces the receiver load current required from the driver.

Another option for reducing the total power dissipation is to use transceivers with 3.3-V power supplies. Transceivers with 3.3-V supplies are available from several manufacturers, and they will meet the same RS-485 standards as transceivers with 5-V supplies. However, 3.3-V transceiver designs are more efficient in terms of signal-to-power ratio. Since at least 1.5 V of differential signal is still required, this yields a minimum efficiency of:

Efficiency_{(3.3V-DRIVER)} = 1.5 V/3.3 V = 45%

As before, in a network with no significant ground offset, only the termination current is significant, totaling about 25 mA. So for a typical driver:

P_{DRIVER LOAD TYP.} = 25 mA · (3.3 V – 1.5 V) = 45 mW

In this case, driver power dissipation due to loading is reduced by almost 50%, compared to the 5-V supply case.

In addition to lower dissipation due to load current, a transceiver with a 3.3-V supply also has lower dissipation due to switching and quiescent currents. The power savings are directly related to the lower voltage, assuming similar current levels.

Higher Termination Impedance

The termination resistors at each end of the RS-485 trunk line reduce reflections from the ends of the transmission line, improving signal fidelity. In systems where the signaling rate is low or bus length is short, it’s possible to remove the termination without significant degradation of the network reliability. However, this can significantly improve the power budget.

The curves for 5% and 20% jitter in Figure 5 are based on TSB-89 guidelines for terminated RS-485 networks.^{4} The tradeoff between bus length and signaling rate is constrained by the electrical properties of the cable media and the effect of high-frequency attenuation, which causes inter-symbol interference.

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The termination guideline curve is based on reflections due to impedance mismatch at the bus endpoints. A rule of thumb is that if the transmission-line delay from one end of the bus to the other is less than 10% of the bit time (unit interval), termination may be omitted without significant degradation of signal quality.

If the signal propagation delay through the twisted-pair cable (typically 5 ns/m) is a small fraction of the duration of one bit, the reflections will die out in a fraction of the bit time and can be ignored. “RS-422 and RS-485 Standards Overview and System Configurations” discusses termination options and waveforms showing the effect on signal dynamics.^{5}

For example, with a cable length of 100 m for a signaling rate of 38.4 kbits/s, we are below the “termination” guideline. Each bit has a duration of 26 μs, and the end-to-end transmission line delay (t_{TLD}) is about 0.5 μs. Hence, allowing even several complete t_{TLD} delays for the reflections to stabilize does not affect a significant fraction of the bit time.

The signal along the transmission line will be at a reliable level for most of each unit interval. Conversely, for the same 100-m cable length, a signaling rate of 1 Mbit/s is above the termination guideline. Therefore, termination is strongly recommended. The unit interval is only 1 μs, so reflections could significantly affect the fidelity of each data bit.

In applications where the termination may be omitted, power dissipation in the driver decreases considerably. A fully populated network of 32 unit loads with no termination resistors has characteristics primarily dependent on the connected receivers.

The total receiver load can be modeled as 375-Ω resistors (12 kΩ/32) connected to a common-mode point representing the ground offset across the network. If the ground offset is negligible, the load is 750 Ω connected across the differential bus lines.

Here, the load current is bounded by the supply voltage divided by 750 Ω, which yields less than 7 mA for a 5-V supply and less than 5 mA for a 3.3-V supply. This compares to a load current of 25 mA found above for a terminated network with no common-mode loading.

For worst-case conditions of maximum allowed ground offset, receiver impedances may still have a total current of up to 32 mA. However, because the differential output voltage from the active driver is higher with reduced loading, less power is dissipated in the active driver.

Reduced Power Implementation

To illustrate the various methods of power reduction, we compare two scenarios: one has high-power dissipation, the other with power reduction steps applied. In both cases, assume a signaling rate of 115.2 kbits/s and a cable length of 30 m (~100 feet).

Next, assume that 32 nodes are connected, and there are no long periods of idle time with no driver active. For simplicity, assume the ground offset is negligible over the average time considered. There may be brief periods of high-ground offset, but on average the common-mode voltage is negligible.

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For the first scenario, assume an older transceiver, 5-V supply, and termination resistors at each cable end. For the second, assume a new transceiver with lower quiescent current, 3.3-V supply, and no termination resistors. The table compares each network power dissipation component and total power.

Using low-power transceivers and removing the termination resistors reduces the total network power by about 85%. The average power saved per connected node is about 150 mW. The most significant reduction comes from choosing newer transceivers with low current when in receiving-only mode. However, all other network power dissipation components have considerable reduction.

Potential Impact Of Power Savings

According to several industry trade sources, the worldwide market for new RS-485 transceivers has been in the range of hundreds of millions of nodes annually for several years. Assuming that only a fraction of these nodes is in service, there could be several hundred million RS-485 nodes in operation at any one time.

If the techniques above could be applied with an average savings per node of about 100 mW, the worldwide reduction in energy expenditure could be in the tens of megawatts. Since the average household power consumption is about a kilowatt, this savings is equivalent to the power for tens of thousands of homes.

References

1. Copper wire resistance is usually negligible compared to termination resistance. In applications where the network length is unusually long, losses in the wire may be considered. This may affect the total impedance seen by the active driver.

2. With significant ground offset (RS-485 allows a range of –7 V to 12 V) between a receiver and active driver, the receiver input current may reach 1 mA for a 1UL device. However, large ground offsets are due typically to electrical noise and are short in duration. In the worst case during a maximum ground offset event, total power dissipation of all connected receivers due to input bus current is limited by:

P_{RECEIVER TOTAL} < 64 · 12 mW = 768 mW

3. More than one driver may be active for brief periods, usually due to a protocol error or problem with network timing. Known as bus contention, this condition is typically considered a fault condition.

4. TIA TSB-89 “Application Guidelines for TIA/EIA-485-A” 2006.

5. “RS-422 and RS-485 Standards Overview and System Configurations,” by Manny Soltero, Jing Zhang and Chris Cockrill; updated by Kevin Zhang, Clark Kinnaird, and Thomas Kugelstadt, SLLA070D