The RS-485 Design Guide

This article serves as an introductory guide to designers new to RS-485 by discussing the main aspects of the standard. Studying the additional application notes referenced at the end of this article can help a designer to accomplish a robust data transmission design in the shortest time possible.

Standard Purpose
RS-485 only defines the electrical characteristics of drivers and receivers used in balanced multipoint transmission lines. As an electrical-only standard, RS-485 is commonly referenced by higher level standards as their physical layer.

Network Topology
Bus nodes are networked in a daisy-chain, or bus topology (Figure 1). That is, each node connects to the main cable trunk via short stubs. The interface bus is usually designed for half-duplex transmission, meaning that only one signal pair is used, across which the driving and receiving of data must occur at different times.

This implementation requires the protocol controlled operation of all nodes via direction control signals, such as driver/receiver enabled signals, to ensure that only one driver is active on the bus at any time. Having more than one driver accessing the bus simultaneously leads to bus contention, which must be avoided at all times.

Signal Levels
RS-485 drivers must provide a differential output of a minimum of 1.5V across a 54 load, while RS-485 receivers must detect a differential input with a minimum of 200mV (Figure 2). These two values provide sufficient margin for a reliable data transmission, even under severe signal degradation across the cable and connectors. This robustness is the primary reason why RS-485 is good for long distance networking in noisy environments.

Cable Type
RS-485 Applications benefit from differential signaling over twisted pair cable. This is because noise from external sources couples equally into both signal lines as common-mode noise, which is rejected by the differential receiver input.

Industrial RS-485 cables are of the sheathed, unshielded, twisted pair type (UTP) with a characteristic impedance of 120 and 22 AWG. Figure 3 shows the cross section of a single pair, UTP cable for half-duplex networks.

Beyond network cabling, it is mandatory that the printed circuit board layouts and the connector pin assignments of RS-485 equipment keep both signal lines close and equidistant to another to maintain the network's electrical characteristics

Bus Termination and Stub Length
Data transmission lines should always be terminated and stubs should be as short as possible to avoid signal reflections on the line. Proper termination requires matching the terminating resistors, RT, to the characteristic impedance, Z0, of the transmission cable. Because RS-485 recommends cables with Z0 = 120, the cable trunk is commonly terminated with 120 resistors, one at each cable end (Figure 1 right).

Applications in noisy environments often add common-mode noise filtering by replacing the 120 resistors with two R-C low-pass filters (Figure 4). It is important to match the resistor values (preferably with precision resistors) to ensure equal roll-off frequencies of both filters. Larger resistor tolerances cause the filter corner frequencies to differ and common-mode noise to be converted into differential noise, thus compromising the receiver's noise immunity.

The electrical length of a stub (the distance between a transceiver and cable trunk) should be shorter than 1/10 of the driver's output rise time, and is given through: (1)

where: LBus = maximum length of an unterminated cable (ft) tr = driver (10/90) rise time (ns) v = signal velocity of the cable as factor of c c = speed of light (9.8 x10^8 ft/s or 9.8E8 ft/s)

Table 1 lists the maximum stub lengths of the cable in Figure 4 for various driver rise times.

Failsafe
Failsafe operation is a receiver's ability to assume a determined output state in the absence of an input signal. Three possible causes can lead to the loss of signal (LOS): 1) open-circuit: caused by a wire break, or by disconnecting a transceiver from the bus 2) short-circuit: caused by an insulation fault connecting the wires of a differential pair to another, 3) idle-bus: occurring when none of the bus drivers is active

Because the conditions above can cause conventional receivers to assume random output states when the input signal is zero, modern transceiver designs include biasing circuits for open-circuit, short-circuit, and idle-bus failsafe, that drive the receiver output to a determined state, when the input signal is zero.

While these failsafe transceivers claim to reduce component count, their worst case noise margin of 10mV necessitates the design of external failsafe circuitry

An external failsafe circuit consists of a resistive voltage divider that generates sufficient differential bus voltage, to drive the receiver output into a determined state. To ensure sufficient noise margin, VAB must include the maximum differential noise in addition to the 200 mV receiver input threshold. The values for the failsafe bias resistors, RB, are then calculated for worst case conditions, that is maximum noise at minimum supply:

(2)

with: VAB = 200 mV + VNoise

For a minimum bus voltage of 4.75 V, (5V " 5%), VAB = 0.25 V, and Z0 = 120, RB yields 528. Inserting two 523 resistors in series to RT, (Figure 5, left), establishes a single failsafe circuit at one bus end.

Figure 5. External idle-bus failsafe biasing.

Editor's note
Problems with editing tools caused some equations and other elements to be removed from text, then further problems made it impossible to fix them in timely fashion. It is these problems, not errors in the author's original text, that prompted most of the criticism below. These errors have now been fixed.

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