Troubleshoot RS-485 networks

In spite of the proliferation of more modern alternatives, RS-485 technology remains the mainstay of myriad communication networks. Here's an eight-step recipe for checking common failures and making troublesome RS-485 networks work. 1.RS-485 uses an unbalanced differential pair, which means that every device on the network must be connected to ground through a signal return to minimize noise...

03/01/2005


In spite of the proliferation of more modern alternatives, RS-485 technology remains the mainstay of myriad communication networks. Here's an eight-step recipe for checking common failures and making troublesome RS-485 networks work.

1. RS-485 uses an unbalanced differential pair, which means that every device on the network must be connected to ground through a signal return to minimize noise on the data lines. Data conductors should consist of a twisted pair of wires plus a shield if the environment is noisy.

2. Termination causes more problems than it solves in the vast majority of RS-485 networks. To check which nodes are terminated, power down each one and disconnect it from the network. Use an ohmmeter to measure between the A and B or + and - lines of the receiver. Terminated nodes will usually read less than 200 Ohms. Unterminated nodes will read greater than 4,000 Ohms.

3. It's not always clear which line is the A line and which is the B. Different manufacturers have adopted different labeling conventions, even though the B line should always be the one that is more positive in the idle condition. Thus, the A line is equivalent to - and the B line is equivalent to +. Check the idle network with a voltmeter. If the B line isn't more positive than A, there's a connection problem.

4. A tristate condition in an RS-485 network occurs when no devices are transmitting, and all are listening. This can cause all the drivers to go into a high impedance state, leaving floating wires feeding into all of the RS-485 receivers. A node designer typically cures this precarious state by installing pull-down and pull-up resistors on the A and B lines of the receiver to simulate an idle condition.

Networks & Communication
An oscilloscope trace can determine if the 1s and 0s of each byte have voltages well outside of the undefined state and if the minimum turnaround delay (equal to the time occupied by one byte of data) is being satisfied before the device reaches the tristate condition.

To check the biasing, measure the voltage from B to A with the network powered up and idle. At least 300 mV is required to keep well away from the undefined state shown in the graphic. If no termination resistors are installed, bias requirements are very forgiving.

5. A two-wire-plus-ground RS-485 network sends data up and down the wires. Since no two transmitters can successfully talk at once, there's a slice of time immediately after the last bit has been sent when the network appears to be idle, but in fact the node has not yet put its driver into the tristate condition. A collision with unpredictable results will occur if another device tries to talk during this interval. To check for collisions, use a digital oscilloscope to capture a few bytes of ones and zeros. Identify the time it takes for a node to reach the tristate condition at the end of a transmission, as shown in the figure. Make sure the RS-485 software is not trying to respond to a request faster than one byte time (a little more than 1 millisecond at 76.8 kilobits/second).

6. Every reliable mid-to-long distance networking technology has some form of isolation built in, except for RS-485. It's up to the system designer to ensure that the network does not include any ground loops. Isolating every node will increase the network's reliability by orders of magnitude.

7. Isolation is the first line of defense against power surges, but adding a multistage surge suppressor takes the edge off larger surges, keeping them in the range that the network's isolation can tolerate. It is best to install a surge suppressor at one location on the network with a high-quality ground. Tie it to the earth ground at the same point as the rest of the network equipment or the facility's electrical system.

8. Once the RS-485 network is up and running, write down every detail of its configuration. Include termination information, biasing, wire types, and information on spare parts. Buy some spares now, if they're affordable, and keep them in the cabinet.

For more information about RS-485 technology, see www.bb-elec.com .





The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by Control Engineering subscribers. Vote now (if qualified)!
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Maximize ROI with integrated control system approach; Microcontrollers vs. PLCs; Power quality; Accelerate and rewire IIoT; Traits for excellent engineers
HMI effectiveness; Distributed I/O; Engineers' Choice Award finalists; System Integrator advice; Inside Machines
Women in engineering; Engineering Leaders Under 40; PID benefits and drawbacks; Ladder logic; Cloud computing
Programmable logic controllers (PLCs) represent the logic (decision) part of the control loop of sense, decide, and actuate. As we know, PLCs aren’t the only option for making decisions in a control loop, but they are likely why you’re here.
This digital report explains how plant engineers and subject matter experts (SME) need support for time series data and its many challenges.
This article collection contains several articles on how advancements in vision system designs, computing power, algorithms, optics, and communications are making machine vision more cost effective than ever before.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Control room technology innovation; Practical approaches to corrosion protection; Pipeline regulator revises quality programs
Cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by Control Engineering subscribers. Vote now (if qualified)!
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Maximize ROI with integrated control system approach; Microcontrollers vs. PLCs; Power quality; Accelerate and rewire IIoT; Traits for excellent engineers
HMI effectiveness; Distributed I/O; Engineers' Choice Award finalists; System Integrator advice; Inside Machines
Women in engineering; Engineering Leaders Under 40; PID benefits and drawbacks; Ladder logic; Cloud computing
Programmable logic controllers (PLCs) represent the logic (decision) part of the control loop of sense, decide, and actuate. As we know, PLCs aren’t the only option for making decisions in a control loop, but they are likely why you’re here.
This digital report explains how plant engineers and subject matter experts (SME) need support for time series data and its many challenges.
This article collection contains several articles on how advancements in vision system designs, computing power, algorithms, optics, and communications are making machine vision more cost effective than ever before.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Control room technology innovation; Practical approaches to corrosion protection; Pipeline regulator revises quality programs
Cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by Control Engineering subscribers. Vote now (if qualified)!
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Maximize ROI with integrated control system approach; Microcontrollers vs. PLCs; Power quality; Accelerate and rewire IIoT; Traits for excellent engineers
HMI effectiveness; Distributed I/O; Engineers' Choice Award finalists; System Integrator advice; Inside Machines
Women in engineering; Engineering Leaders Under 40; PID benefits and drawbacks; Ladder logic; Cloud computing
Programmable logic controllers (PLCs) represent the logic (decision) part of the control loop of sense, decide, and actuate. As we know, PLCs aren’t the only option for making decisions in a control loop, but they are likely why you’re here.
This digital report explains how plant engineers and subject matter experts (SME) need support for time series data and its many challenges.
This article collection contains several articles on how advancements in vision system designs, computing power, algorithms, optics, and communications are making machine vision more cost effective than ever before.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Control room technology innovation; Practical approaches to corrosion protection; Pipeline regulator revises quality programs
Cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me