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How Far and how Fast Can You Go with RS-485?

The various serial-datacom protocols range from RS-232 to Gigabit Ethernet and beyond. Though each suits a particular application, you must in all cases consider cost and performance of the physical (PHY) layer. This article focuses on the RS-485 protocol, the applications best suited to that standard, and the ways you can optimize data rates as a function of cabling, system design, and component selection.

Definitions

What is RS-485? What is Profibus? How do they compare to other serial protocols and for what applications are they best suited? To answer those questions, the following overview compares the characteristics and capabilities of the RS-485 PHY with those of RS-232 and RS-422. (Throughout the article, “RS” refers to the respective ANSI EIA/TIA standards.)

RS-232 is a standard that originated as a communications guide for modems, printers, and other PC peripherals. It provided a single-ended channel with baud rates to 20kBits/s, later enhanced to 1MBits/s. Other RS-232 specifications include nominal ï5V transmit and ±3V receive (space/mark), 2V common-mode rejection, 2200pF maximum cable load capacitance, 300Ω maximum driver output resistance, 3kΩ minimum receiver (load) impedance, and 100 ft (typical) maximum cable length. RS-232 systems are point-to-point, not multi-droppable. Any RS232 system must accommodate these constraints.

RS-422 is a unidirectional full-duplex standard for electrically noisy industrial environments. It specifies a single driver with multiple receivers. The signal path is differential, and handles bit rates above 50MBits/s. The receiver common-mode range is ±7V, the driver output resistance is 100Ω maximum, and the receiver input impedance can be as low as 4kΩ.

RS-485 is a bidirectional half-duplex standard featuring multiple “bussed” drivers and receivers, in which each driver can relinquish the bus. It meets all RS-422 specifications, but is more robust. It has a higher receiver-input impedance and larger common-mode range (“7V to +12V).

Receiver input sensitivity is ±200mV, which means that to recognize a mark or space, a receiver must see signal levels above +200mV or below “200mV. Minimum receiver input impedance is 12kΩ, and the driver output voltage is ±1.5V minimum, ±5V maximum.

Drive capacity is 32 unit loads, i.e., 32 12kΩ receivers in parallel. For receivers of higher input impedance, the number of unit loads on one bus can be higher. Any number of receivers can be connected to the bus, provided the combined (parallel) load presented to the driver does not exceed 32 unit loads (375Ω).

The driver load impedance is 54Ω maximum, which in a typical 24AWG twisted-pair environment is 32 unit loads in parallel with two 120Ω terminators. RS-485 has become the best choice for POS, industrial and telecom applications. The wide common-mode range enables data transmission over longer cable lengths, and in noisy environments such as the floor of a factory. Also, the receivers' higher input impedance allows more devices to be dropped on the lines.

Profibus and Fieldbus . (See Reference 1 for more detail.) Used mainly in industrial plants, these buses are an extension of RS-485. The plant wiring systems measure sensors, control actuators, collect and display data, and conduct data communications between the process control system and the network of sensors and actuators. Note that older and existing plants have a complicated wiring infrastructure that is prohibitive to replace.

Profibus and Fieldbus are the overall system descriptions; RS-485 is the standard for the PHY layer of the network supporting them. Profibus and Fieldbus have slightly different specifications. Profibus requires a 2.0V minimum differential output voltage with RL = 54Ω, and Fieldbus requires a minimum differential output voltage of 1.5V, with RL = 54Ω. Profibus transmits at 12MBits/s, vs. 500KBits/s for Fieldbus. Skew and capacitance tolerance are tighter in Profibus applications.

Where do they best fit?

RS-232 : for communication with modems, printers, and other PC peripherals. The typical maximum cable length is 100 ft.

RS-422 : for industrial environments that require only one bus master (driver). Typical applications include process automation (chemicals, brewing, paper mills), factory automation (autos, metal fabrication), HVAC, security, motor control, and motion control.

RS-485 : industrial environments for which more than one bus master/driver is needed. Typical applications are similar to those of RS-422: process automation (chemicals, brewing, paper mills), factory automation (autos, metal fabrication), HVAC, security, motor control, and motion control.

What factors limit the data rate?

The following factors come into play when determining how far one can reliably transmit at a given data rate:

  • Cable length. At a given frequency, the signal is attenuated by the cable as a function of length.
  • Cable construction. Cat 5 24AWG twisted pair is a very common type of cable used for RS-485 systems. Adding shielding to the cable enhances noise immunity, and thereby increases the data rate for a given distance.
  • Cable characteristic impedance. Distributed capacitance and inductance slows edges, eating up noise margin and compromising the “eye pattern.” Distributed resistance attenuates the signal level directly.
  • Driver output impedance. Limits drive capability, if too high.
  • Receiver input impedance. If too low, limits the number of receivers the driver can handle.
  • Termination. A long cable can act like a transmission line. Terminating the cable with its characteristic impedance reduces reflections and increases the achievable data rate.
  • Noise margin. Bigger is better.

  • Slew rate of driver. Slower edges (lower slew rates) enable transmission over longer cable lengths.

Some empirical data

Given the background information above, we next consider an actual wired system such as that of Figure 1. The cable shown is one of the most common for RS-485 systems: EIA/TIA/ANSI 568 Category 5 twisted pair. The data rates obtained for cable lengths from 300 to 900 feet range from 1MBits/s to 35MBits/s.


Figure 1. The Test setup..



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System designers often choose a driver and receiver from two competing manufacturers, but most designers are primarily interested in how far and how fast the RS-485 driver can drive a signal. The performance of a Maxim driver (MAX3469) and an equivalent driver from another manufacturer are presented in Figures 2-3.


Figure 2. Graph of jitter for a given bit rate and cable length. Jitter is measured at ±100mV differential.



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Figure 3. Graph of jitter for a given bit rate and cable length. Jitter is measured at 0V differential.
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Signal integrity is tested by observing the driver's differential output. Set the oscilloscope to look for trigger points between the 80mV and -400mV thresholds (chosen because receivers have an input range of 20mV to -200mV, plus a noise margin). Then, when pulses (bits) begin to “run together,” use eye patterns to determine the overall contributions of distortion, noise, and attenuation to the parameter called intersymbol interference (ISI).

ISI forces you to reduce the bit rate to a level that allows an adequate distinction between pulses. Tests of the Figure 1 circuit show a consistent and clear correlation between trigger points and eye patterns. The eye patterns exhibit 50 percent jitter, measured using methods documented in National Semiconductor's Application Note 977. Measuring jitter at 0V differential and 100mV differential yields the data shown in Figures 4-5.


Figure 4. Eye pattern for an RS-485 driver device comparable to the MAX3469 from Maxim.



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Figure 5. Eye pattern for Maxim's MAX3469..



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For a given point-to-point connection, the bit rate associated with a particular cable length can be illustrated at ±100mV differential (Figure 4) or 0V differential (Figure 5). Thresholds of +100mV and -100mV ensure that the receiver switches properly, because we know they can switch correctly with differential signals greater than 200mV. (The data of Figure 5 applies only to an ideal receiver, which would be able to switch at a 0V differential input.)

Eye diagrams and failure modes

At 39MBits/s and 340 feet of CAT5 cable, the driver output of Figure 2 exhibits an eye pattern in which signals cross in the middle of the eyeï¾a condition indicating possible bit errors. The Maxim part at the same data rate, however, (Figure 3) shows no such condition. It offers better performance due to symmetrical output edges and lower input capacitance.

For the tests described above, the two drivers are comparable. At higher data rates over longer cable lengths, however, the Maxim driver is more robust. Figure 5 provides an estimate of how fast and how far the Maxim part can drive data in a point-to-point network. Empirically, the appearance of bit errors corresponds approximately to the 50 perent jitter limit.

Research data from various sources

Generally accepted industry-wide maximums for distance and data rate seem to be 4000 feet and 10MBits/s, but (of course) not at the same time. Combining the latest devices with careful system design, however, can provide higher throughput over longer cable lengths.

Pre-emphasis is a technique that improves data rate vs. distance, and is applicable to RS-485 communications (Figure 6). RS-485 transceivers without driver pre-emphasis or receiver equalization generally acquire 10 percent jitter across 1700 feet of cable when operating at a fixed data rate of 1MBits/s. Adding driver pre-emphasis at that rate doubles the distance to 3400ft without increasing the jitter. As an alternative, pre-emphasis can increase the data rate for a given distance. Drivers operating at 400kBits/s without pre-emphasis generally acquire 10 percent jitter over 4000ft. Adding pre-emphasis lets you transmit up to 800kBits/s for that distance.


Figure 6. Data rate vs. cable length.



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Another way to calculate maximum cable length for reliable transmissions is to use the attenuation vs. frequency table supplied by the manufacturer for Cat 5 cable. A rule of thumb for allowable attenuation is “6dBV over the run of cable. That value can be combined with the manufacturer's attenuation data, to calculate maximum cable length for a given frequency.

Tips and Tricks

Available RS-485 transceivers have several features that can enhance system performance:

Pre-emphasis (mentioned earlier) reduces inter-symbol interference. See Maxim app note 643.

Reduced unit-load receivers: low-load devices are available down to 1/8 unit load, enabling up to 256 devices on one bus. Such devices also enable lower bus loading, which in turn allows a longer cable or higher data rate.

High-speed devices: currently available drivers are capable of data rates up to 52MBits/s, achieved with special attention to low propagation delay and low skew.

ESD protection: does not enhance data rate, but can make the difference between a working system and one whose data rate is zero (broken). Available devices offer built-in ESD protection to ï15kV.

Proper wiring: (See Reference 2 for a detailed discussion.) RS-485 specifies differential transmission, which requires two signal wires in addition to a ground wire (commonly a 24 AWG twisted pair) to transmit the signal. The two signal wires carry signals opposite in polarity, and greatly reduce the problems of radiated EMI and EMI pickup. The common characteristic impedance of this wire is 120Ω, which is also the resistance used to terminate each end of the cableï¾in the interest of reducing reflections and other transmission-line effects. Figures 7-8 illustrate properly wired systems.


Figure 7. Single transmit, single receive network.



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Figure 8. Multiple-transceiver network.



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Thus, RS-485 networks can achieve reliable data transmissions in electrically noisy environments. By considering the tradeoff between data rate and cable length, you can design a system that achieves data rates in excess of 50MBits/s over cable lengths of hundreds of meters, and without repeaters.

The author wishes to thank Miguel Mendoza for his lab work in support of this article.

References

1. Maxim AN1833, “Using RS485/RS422 Transceivers in Fieldbus Networks,” 12/27/02.
2. Maxim AN763, “Guidelines for Proper wiring of an RS-485 Network,” 7/12/01.
3. Maxim AN736, ” RS485 Differential Data Transmission System Basics,” 3/16/01.
4. Maxim AN643, “Preemphasis Improves RS485 Communications” 1/22/01.
5. Application Note 977, National Semiconductor.
6. TI Databook, “Data Transmission Circuits, Vol. 1,” 1995/1996, pp. 4-9 to 4-24, and 4-37 to 4-48.

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