Douglas Krantz - Technical Writer - Describing How It Works

How can I Fix an Intermittent RS485 Fail Message?

By Douglas Krantz | Maintenance

How can I Fix an Intermittent RS485 Fail Message?

How can I Fix an Intermittent RS485 Fail Message?

Greetings Douglas,

We are experiencing an issue with our Marine Fire Alarm System setup on a Ship fire detection system, and stumbled across your really useful website / guide.

We have 3 repeater panels on board (we believe that one is "master" and the other 2 "slave"). Currently we are receiving the Comms Error RS485 on both of the slave repeater units. This has now occurred twice in a week.

From the system diagram / tech documentation, we are led to believe that this may be an error with the CPU? However, having read your article The RS485 Network Keeps Having Communication Errors check the terminating resistors on the RS 485 network. Unfortunately, our technical documentation is limited / lacks detail. Are you able to offer any guidance on fault finding / investigation or any common component failures in your experience?

Thank You, BH

When troubleshooting, go with the easy and the cheap first; save time and money. Until you've exhausted trying everything that's easy, and made sure you've tried everything that's cheap; stay away from the time consuming and expensive stuff.

Also, the installation manual showing how to install the fire alarm system is available by contacting the technical support people at the fire alarm system's manufacturer. If they don't have it directly, they can help you obtain one.

Keep in mind that the installation manual shows you exactly how to install the system, and it may even have some troubleshooting look-up tables. However, because it is meant to show how to install the system, it doesn't have schematics showing details of all circuitry, or even the detailed electronic theory of operation. That information is proprietary, and no fire alarm manufacturer will give that information out to anyone.

When troubleshooting a fire alarm system, mostly you are making sure that everything is still installed exactly the way it was meant to be installed, and that any degradation to the system is dealt with. The installation manual helps when examining the fire alarm system to make sure it is still installed correctly.

RS485 Circuit

This is a government owned and operated, Offshore Patrol Vessel. Everything, including the fire alarm system was originally commissioned to make sure it worked exactly as advertised. The fire alarm system worked at one time, but now it isn't fully working. Something changed.

The weak spots in the vessel-wide fire alarm system will be the connections. This is where tarnish can build up and cause intermittent communication problems. Connections for the RS485 circuit can be found on the master fire alarm control panel, as well as all the slave panels.

You may be having a tarnish buildup-on-the-contacts issue; checking for tarnish in the contacts is an easy and cheap thing to do. Tarnish, remember, is a form of corrosion. In low voltage situations, like the RS485 circuits, tarnish on the connector contacts can interfere with the signals. See:

How is Tarnish Removed from Electronic Contacts?

Unplugging and plugging each connector half a dozen times is all that is needed to scrape off any tarnish buildup on the connector contacts. With screw terminals, usually just tightening the screws will fix the tarnish issue. Just make sure that the recommended torque for the screws isn't exceeded.

Being a vessel designed for ocean travel, the ambient air probably has at least some salt content; salt corrodes the contacts. While you're at the connectors, make sure that any air seals or gaskets aren't damaged or out of place.

Once you've checked all the connections, then for further troubleshooting, contact the technical support team for the fire alarm system. They will help you from there.

Douglas Krantz
Life Safety
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