A troubleshooting guide for RS-485

Biasing the network

To maintain the proper idle-voltage state, you most apply bias resistors to force the data liens to the idle condition. These bias resistors are pullup resistors on the data B line (typically to 5V) and pulldown resistors (to ground) on the data A line. In a four-wire configuration, place the bias resistors on the receiver lines.

The value of the bias resistors depends on termination and number of system nodes. Generate enough DC bias current to maintain at least 200 mV between data A and B lines.

You can place bias resistors anywhere in the network or split them among multiple nodes. However, the parallel combination of all bias resistors in a system must be equal to or less than the calculated biasing requirements.

Symptoms of under-biasing (resistor value too high): decreased noise immunity to complete data failure. Over-biasing (resistor value too low) has less effect: its primary result is increased load on the drivers. Post-powered RS-232-to-RS-485 converters can be sensitive to over-biasing.

Get the right cabling

Don’t overlook the signal ground conductor when ordering cable. An extra twisted pair must be specified to have enough conductors to run a signal ground. A two-wire system then requires two twisted pair; a four-wire system requires three twisted pair. Because the added cost of shielded cable is usually minimal, use it.

Transmission line losses consist of AC losses (skin effect), DC conductor loss, leakage, and AC losses in the dielectric. In high-quality cable, conductor and dielectric losses are approximately equal. Polyethylene cables offer much lower attenuation than PVC cables.

Surge protection: Isolation

Unwanted energy, which may come from high-voltage cables running near the data cables, creates the potential for a fault condition due to insulation failures or inadvertent contact by an installer.

This surge could contact any number of conductors in the data cable, presenting a differential surge. Although voltages and currents associated with this surge are much lower than those modeled by ANSI or IEC, they can be steady state on the data network.

To protect each device, reference each to only one ground, eliminating the path through the device for surge currents searching for a return.

With an isolated port, the isolated circuitry floats to the level of the transient without disrupting data communications. As long as the circuitry’s floating level does not exceed the breakdown rating of the isolators (typically 1,000 V – 2,500V), the port is protected.

Surge protection: Shunting

Selected voltage rating is typically 6-8 V. Because these devices typically add capacitive load to the data liens, derate the total line length; several hundred feet is usually adequate.

Install protective devices as close as possible to the port to be protected.

Also, you must provide an extremely low impedance connection to the local earth ground of the unit being protected. This ground is crucial to proper operation. Use heavy gauge wire and keep it as short as possible. If cable length exceeds one meter, you must use copper strap or braided cable.

Be aware that, besides the high-frequency nature of transients, there can be an enormous amount of current present. For example, several thousand amps typically result from applications of the combination wave test in the ANSI and IEC specification.

Use 100Ω resistors in series with the signal ground path to limit ground currents.

Special considerations for fault conditions

Data systems potentially exposed to short circuits to power conductors require an added fuse-type device in addition to shunting type suppression. Choose a low fuse value—125 mA s typical—so the fuse will open before the shunt device is damaged.

Some troubleshooting considerations:

• Get a schematic of each serial port to assist in troubleshooting and repairs.
• Ensure a 200 mV – 6V range for attenuation on balanced-differential transmission lines.
• Use a signal ground line to keep common-mode voltage within the -7 V to +12 V range.
• Use a driver that disconnects from the transmission line when a particular node is not transmitting.
• Specify isolated equipment wherever possible for reliability.
• Add fuse-type devices to shunting protection to guard against short circuits.

About the Author

Mike Fahrion, director of product management at B&B Electronics is an expert in data communications with 20 years of design and application experience. He oversees development of the company’s rugged M2M connectivity solutions for wireless and wired networks based on serial, Ethernet, wireless and USB communication technologies. Fahrion has particular expertise in reliable connectivity solutions for devices deployed at the “edge” of networks in remote, harsh or uncontrolled environments. Fahrion is a speaker and widely-published author, including his politically-incorrect newsletter, uConnections, with over 50,000 monthly subscribers. Fahrion holds a BSEE from Iowa State University.