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Posted - 22 Sep 2010 :  12:54:05  Show Profile  Email Poster
PN99: Protecting the ZEUS/ZyWAN RS422/485 interface from destruction and common mode noise and surges
Sept 22, 2010

This note is provided to highlight customer reported damage to the ZEUS RS422/485 interface in the field. On investigation, the cause has been attributed to grounding nonisolated RS422/485 ports together where the ground potential difference has exceeded the current rating of the ferrite beads used to EMI filter the ZEUS RS422/485 ground.

As little as 100mV is enough to destroy the ferrite beads.

PN99 PDF (89 kB)

Along with PN99, we have also gathered together, in one ZIP file, a number of Application Notes concerning subjects such as grounding, using isolated repeaters, surge management, and so on. Also included are datasheets on sample isolators and protection components.

Collection of Application Notes and Product Datasheets (ZIP, 2 MB)


1519 Posts

Posted - 08 Jun 2011 :  17:34:58  Show Profile  Email Poster
Since issuing PN99, we have identified another means by which traces or components on the ZyWAN motherboard can be critically damaged if power polarity reverses.


In short, ground paths created by external connections to the ZyWAN defeat its input power protection circuits, and can lay the groundwork for damage to the motherboard.


Place a fuse in the Ground input to the ZyWAN.


- Test the wiring harness voltage polarity before plugging in to the ZyWAN.
- Test power polarity by plugging the wiring harness into the ZyWAN before mechanically/electrically connecting the ZyWAN to the vehicle chassis. If it doesn't boot, confirm power polarity.


Built-in Protection

The ZyWAN motherboard isolates the electrical and chassis ground inputs. The shells and shields of ZyWAN connectors are connected to the chassis of the ZyWAN. This is where problems can begin.

Protection Can Be Defeated by External Connections

Other equipment connected to the ZyWAN may connect the shields of its equipment electrical ground of the system. This connection can defeat the reverse-voltage protection built into the ZyWAN.

For example, tri-band antennas typically have a common ground between antennas. If any one of the devices connected to that antenna connects the antenna shield to electrical ground, a connected ZyWAN will be at risk of damage.

Once Vulnerable, ZyWAN Can be Damaged by Common Wiring Faults

Once a ZyWAN is at risk, any number of external power wiring faults can cause excessive ground current to run through the ZyWAN. These include the external power supply being reverse-connected, or another connected piece of equipment losing its ground.

Resulting Damage to Motherboard Traces, Components

Eurotech has observed damage to the input ground trace of the ZyWAN (ZEUS) motherboard, as well as to ferrite beads on the RS422/485 port.

One Scenario for Damage

1. A ZyWAN is installed in a system that connects its chassis to electrical ground, as described above, with a RS-422/485 cable connected.
2. External, un-fused power is incorrectly applied with reverse-polarity. This burns out the input ground trace on the motherboard.
3. The installer corrects the wiring and reconnects the system. With the normal ground gone, all ground current now returns through the serial port ground. This ultimately burns the small filter inductors open. System is now entirely inoperable.

In some cases, we have seen that the ZyWAN continues to operate with a burned ground trace, but only when a 422/485 cable is connected.

Edited by akidder 26-Oct-2015: The fault of burning out the ground trace due to reverse polarity connections will only occur if the power source is unfused, or the fuse has a value high enough not to protect the board's power traces. I've updated point #2 in the failure scenario above.
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