Comprehensive Guide to RS-485 Design

1 Introduction

In 1983, the Electronic Industries Association (EIA) approved a new balanced transmission standard known as RS-485. Investigations reveal that RS-485 is highly acclaimed and has been widely applied in industrial, medical, and consumer products, becoming the dominant specification for industrial interfaces.

This application report provides design guidelines for engineers who are unfamiliar with the RS-485 standard, helping them achieve robust and reliable data transmission designs in the shortest time possible.

2 Standards and Features

RS-485 is merely an electrical standard. Compared to complete interface standards that define functional, mechanical, and electrical specifications, RS-485 only defines the electrical characteristics of drivers and receivers using balanced multipoint transmission lines.

However, many higher-level standards reference RS-485, for example, China’s electric meter communication protocol standard DL/T645 explicitly specifies RS-485 as the physical layer standard.

The main features of RS-485 are:

•Balanced Interface

•Multipoint using a single 5V power supply

•–7V to +12V bus common mode range

•Up to 32 unit loads

•10Mbps maximum data rate (distance of 40 feet)

•4000 feet maximum cable length (at a rate of 100kbps)

3 Network Topology

The RS-485 standard recommends using a daisy chain to connect its nodes, also known as shared line or bus topology (see Figure 3-1). In this topology, the drivers, receivers, and transceivers used are connected to the backbone via short stubs.

Full-duplex implementation requires two pairs of signals (four wires), and a full-duplex transceiver, which has separate bus access lines for the transmitter and receiver lines. Full-duplex mode allows nodes to send data on one pair while receiving data on another pair.

In half-duplex mode, only one pair of signals is used, requiring data to be driven and received at different times. Both implementations require control via a direction control signal (e.g., driver/receiver enable signal) to ensure that only one driver is active on the bus at any given time. Multiple drivers accessing the bus simultaneously can lead to bus contention, which must be avoided through software control.

4 Signal Levels

Drivers compliant with the RS-485 standard can provide a differential output of no less than 1.5V on a 54Ω load, while receivers compliant with the standard can detect differential inputs as low as 200mV. Even in cases of severe signal attenuation in cables and connectors, these two values still provide ample margin for highly reliable data transmission. This robustness is the main reason why RS-485 is very suitable for long-distance networking in noisy environments.

5 Cable Types

Transmitting differential signals over twisted pairs is advantageous for RS-485 applications because external interference sources couple equally to both signal wires in a common mode, and this noise is filtered out by the differential receiver.

Industrial RS-485 cables are categorized as shielded, unshielded, twisted pair, and unshielded twisted pair, conforming to 22-24AWG wire gauge with a characteristic impedance of 120Ω.

Besides network wiring, the RS-485 standard mandates that the printed circuit board layout and connectors of the devices be consistent with the electrical characteristics of the network, which can be achieved by keeping the two signal lines on the printed circuit board as close and equal in length as possible.

6 Bus Termination and Stub Length

To avoid signal reflections, data transmission lines should always be terminated, and stubs should be as short as possible. Proper termination requires matching the terminal resistance RT with the characteristic impedance Z0 of the transmission cable. The RS-485 standard recommends using Z0 = 120Ω cable, so the cable trunk is typically terminated with a 120Ω resistor at each end (see the left half of Figure 6-1).

In noisy environments, applications typically replace the 120Ω resistors with two 60Ω resistors, forming a low-pass filter to provide additional common-mode noise filtering capability (see the right half of Figure 6-1). It is essential to match the resistor values (preferably using 1% tolerance resistors) to ensure that both filters have the same frequency roll-off. A larger resistor tolerance (i.e., 20%) can result in different cutoff frequencies for the filters, and common-mode noise can convert to differential noise, thus reducing the receiver’s immunity to interference.

The electrical length of the stub (the distance between the transceiver and the cable trunk) should be less than 1/10 of the driver output rise time and is derived from the following formula:

Table 6-1 lists the maximum stub lengths corresponding to the rise times of various drivers at a rate of 78% (see Figure 5-1).

7 Failure Protection

Failure protection enables the receiver to output a defined state in the absence of an input signal.

There are three possible causes for signal loss (LOS):

1.Open Circuit:The cable is broken, or the transceiver is disconnected from the bus.

2.Short Circuit:The wires of the differential pair contact each other due to insulation failure.

3.Bus Idle:This occurs when all bus drivers are inactive.

Under the above conditions, when the input signal is zero, traditional receivers output a random state. Modern transceivers incorporate a bias circuit that can protect against open circuits, short circuits, and bus idle conditions, allowing the receiver to force an output of a defined state even in the event of signal loss.

The downside of these failure protection designs is that the worst-case noise tolerance is only 10mV, so in interference environments, external failure protection circuits must be added to increase noise tolerance.

The external failure protection circuit consists of a resistor divider that can generate sufficient bus differential voltage to drive the receiver to produce a defined output state. To ensure sufficient noise tolerance, in addition to the 200mV receiver input threshold, VAB must also include the measured maximum differential noise, VAB= 200mV + V_noise.

The minimum bus voltage is 4.75V, (5V – 5%), VAB = 0.25V, and Z0 = 120Ω, then RB is 528Ω. Inserting two 523Ω series resistors into RT will establish the failure protection circuit shown in Figure 7-1.

8 Bus Load

The output of the driver depends on the current it must provide to the load; therefore, adding transceivers and failure protection circuits to the bus increases the total load current required. To estimate the possible maximum bus load number, RS-485 specifies a unit load (UL) assumption, indicating approximately 12kΩ of load impedance. Compliant drivers must be capable of driving up to 32 of these unit loads. Nowadays, transceivers typically reduce the unit load to, for example, 1/8 UL, allowing up to 256 transceivers to be connected to the bus.

The failure protection bias can contribute up to 20 unit loads to the total bus load, thus reducing the maximum number of transceivers N to:

Therefore, when using 1/8-UL transceivers, a maximum of 96 devices can be connected to the bus.

9 Data Rate and Bus Length

At a given data rate, the maximum bus length is limited by transmission line losses and signal jitter. When the jitter of the baud period is 10% or more, data reliability drops sharply. Figure 9-1 shows the relationship curve between cable length and data rate for traditional RS-485 cables at 10% signal jitter.

10 Minimum Node Spacing

The RS-485 bus is a distributed parameter circuit, and its electrical characteristics are primarily determined by the inductance and capacitance distributed along the physical medium (including interconnected cables and printed circuit board traces).

Adding capacitance to the bus in the form of devices and their interconnections reduces the bus impedance and causes the impedance of the medium and load portions of the bus to be mismatched. When the input signal reaches these locations, part of it reflects back to the signal source, causing distortion of the driver output signal.

To ensure that the voltage level of the first signal transmitted from the driver output remains valid when it reaches the receiver input, the minimum load impedance anywhere on the bus must be Z’> 0.4 x Z0, which can be achieved by maintaining a minimum distance d between bus nodes:

Where CL is the lumped load capacitance, and C is the medium capacitance per unit length (cable or PCB trace).

Equation 4 shows the functional relationship between minimum device spacing and distributed medium and lumped load capacitance; Figure 10-1 graphically presents this relationship.

The load capacitance comes from the bus pins of the line circuit, connector contacts, printed circuit board traces, protection devices, and any other physical connections to the trunk. Therefore, the electrical distance from the bus to the transceiver (stub region) should be as short as possible.

The capacitance values of various capacitors are as follows:

5V transceiver capacitance is typically 7pF, while 3V transceiver capacitance is about twice that of 16pF. The capacitance of the PCB traces varies depending on their structure, adding approximately 0.5~0.8pF capacitance per centimeter. The capacitance of connectors and suppressor devices can vary widely. The distributed capacitance of the medium ranges from 40pF/m (low capacitance unshielded twisted pair cable) to 70pF/m (backplane).

11 Grounding and Isolation

When designing remote data links, designers must assume significant ground potential differences (GPD) exist. These voltages Vn will superimpose on the transmission line in the form of common-mode interference. Even if the total superimposed signal is within the common-mode range at the receiver input, relying on local grounding as a reliable current return path is dangerous (see Figure 11-1a).

Since remote nodes may draw power from different parts of the electrical device, modifying such devices (i.e., during maintenance) can cause ground potential differences to exceed the common-mode range of the receiver input. Therefore, a data link that works normally today may stop functioning at some point in the future.

It is also recommended not to connect the remote ground directly via a grounding wire (see Figure 11-1b), as large loop ground currents can introduce common-mode noise onto the signal line.

To connect the remote ground directly, the RS485 standard recommends isolating the device ground from the local system ground by inserting resistors (see Figure 11-1c). Although this method reduces loop currents, the presence of large loop grounds still makes the data link sensitive to noise generated somewhere along the loop. Thus, a robust data link has not yet been established.

A method for establishing an RS-485 data link that can tolerate thousands of volts of ground potential difference and is robust for long-distance transmission is signal and power supply isolation (see Figure 11-2).

In this case, power isolators (e.g., isolated DC/DC converters) and signal isolators (e.g., digital capacitive isolators) can prevent current from flowing between remote system grounds and avoid generating loop currents.

While Figure 11-2 only shows the detailed connections of two transceiver nodes, Figure 11-3 provides examples of multiple isolated transceivers. All transceivers except one are connected to the bus through isolation. The non-isolated transceiver on the left provides a single ground reference for the entire bus.

Source: TI “RS-485 Design Guide”

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