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How to Troubleshoot RS485 Communication Issues

Also see How to build a RS485 network →

Basic Wiring Problems and Termination Errors

For RS485 networks where no communication can be made at all, it is recommended to start by testing for basic electrical termination errors:

• Perform a continuity test using a continuity meter
• Confirm tha the polarity of the terminations on each devices is correct. This is the most common source of problems, compounded by inconsistent labelling of terminals by different manufacturers. (for example, "A" and "B" may mean the exact opposite for different manufacturers!)
  • You won't break the devices by having the RS485 D+ and D- terminations the wrong way around, so it's safe to try swapping them to troubleshoot this particular issue.
• For more complex RS485 networks with multiple devices, a "divide and conquer" approach can be useful, for example if you have 5 devices on a network with a data logger at one end, remove the last 4 devices and see if you can read the first one, then add further devices back into the network until you locate the problem.

Misconfiguration Issues

RS485 devices on the same serial network need to be configured to ensure that all devices (including the "master" / data logger) are using the same serial communication settings:

• Baud rate is the number of bits of data transmitted on the serial bus per second. We generally recommend 9,600 or 19,200 as effective baud rates providing a good compromise between communication speed and reliability.
• RS485 support the use of a parity bit which also needs to be set to the same value on all devices. It's actually fine to use Parity "None" if you are communicating using Modbus, as the Modbus protocol includes it's own (more robust) CRC checksum to check for data transmission errors. There is no problem using parity "Even" or "Odd" however, if that is the preferred or only option available for a device.
• Where the parity is set to "None", a third parameter, the number of stop bits also needs to be set to the same value on all devices (options being 1 or 2 stop bits). Where the parity is even or odd, this is not a consideration as these parity values only support 1 stop bit.
• Some devices may not allow you to change any of these parameters. In this case, consult the manufacturer's documentation to find out the default settings and configure your data logger to match.
• The data logger or Modbus Gateway may allow configuration of a timeout value when reading devices. Most devices will respond quickly to modbus requests, however this value can be increased on some gateways or data loggers if required (including on the tGW-735 gateway, described in more detail below).

Poor Signal Quality on the RS485 Serial Bus

A well installed RS485 network, using the correct kind of shielded cable, with 120Ω terminal resistors installed at both ends can extend for up to 1,200 m with multiple devices along it's length. However, various issues can effect the quality of the signal on an RS485 bus:

• Noise generated by external electrical equipment, for example AC cables running in the same tray or conduit.
• Incompatible devices from different manufacturers may not sit well together on the same RS485 network. Finding out which makes of device work well together is a matter of trial and error.
• Mistakenly having more than one device with the same unit ID (address) can create unexpected problems with communication.
• Placing more than one modbus master on the same RS485 network is not permitted and causes signal interference, generally preventing either "master" device from communicating effectively.

Diagnosing poor serial signal quality

We recommend the use of a suitable oscilloscope to allow you to visualise the signal on the cable. For use in the field, compact USB oscilloscopes that work with software on your laptop offer a good solution. Using an oscilloscope is beyond the scope of this documentation. Please see the user manual for any oscilloscope you may choose to use.

A clean signal should be a solid, square waveform with little "noise":

Troubleshooting a network connected to an ICP-DAS tGW-735

The ICP-DAS tGW-735 is a device widely used by Juggle Energy and PAD Technology to interface between a LAN and an RS485 serial network.

This is a small but powerful device that is able to both queue and cache requests from more than one "master" (data logger) on to a single RS485 network.

There are three RS485 ports on this device. Note that they are numbered from right to left on the physical device, which is important when you are configuring the individual RS485 ports on the built-in admin web interface.

Checking Configuration

In general, the default TCP port number for modbus requests is 502.

The tGW-735 allows independent access to the three physical ports by assigning different TCP ports to them:

This can be modified if required, however in general it's fine to leave the gateway set up like this. The Juggle JBox, legacy MOXA "GoconFigure" data logger and the ABB PLCs that we use are all capable of accessing modbus on these three different port numbers.

However, when troubleshooting an RS485 network via a tGW-735 it is important to check if the default port numbers have been changed.

Confirming this, along with other relevant settings can be completed using the built in admin web interface on the gateway.

If you need any further help please reach out to our support.