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WS-AP3825i booting to boot mode when a long untermintated UTP cable is plugged into the console port

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Information

 
TitleWS-AP3825i booting to boot mode when a long untermintated UTP cable is plugged into the console port
Symptoms
WS-AP3825i  booting to boot mode when a long untermintated UTP cable is plugged into the console port.
Environment
Identifi Wireless

WS-AP3825i

 
Cause
The console port is using a simple design of RS 232 standard (https://en.wikipedia.org/wiki/RS-232) , using RX, TX and GND only.

The purpose of the console port is for troubleshooting purposes only, so during reboot of an AP it is assumed that a laptop is connected or the connection is properly terminated in some way, such as being connected into a terminal server.

Leaving long UTP cables un-terminated to the DB9 side will transform the cable into an antenna that will capture anything and provide feedback to the AP - including the signals transmitted by AP during the boot sequence (TX will induce into the RX)

The longer the serial cable is the more energy will be collected - this is why there is a greater chance that an unterminated short cable would work properly while a longer one would most likely not work.

The AP will enter in Boot mode if receives ANY character.


 
Resolution
To resolve the UTP cable connected to the console port must be terminated.

The cable could be plugged into a KVM to terminate it, although because the GND is on the cable this might create a hazardous ground loop (when the GND on both ends do not have same potential ).

The following diagram illustrates a solution:



rs232 terminatoin
Additional notes

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