Support Thread: Possible Break Condition AHRS-II-P/INS/MRU

Posted by Will Dillingham on Aug 19, 2020 11:16:14 AM
Find me on:

Inertial Labs logo - new2-1

Purpose: The purpose of this article is to help diagnose an accidental break condition. This same situation is capable of occurring for both INS models and MRU models.

Last Updated: July 2019



The customer connects the AHRS-II-P with the Auto-pilot. Then power is switched on for the AHRS-II-P unit – neither the LED glows nor the unit outputs any data and this remains the same for a long time if left untouched.

However, when they cut off the power supply to AHRS-II-P and give back power once again, it starts working immediately i.e. both the LED glows and the unit outputs data.

Thanks & Regards,


Inertial Labs Service Member:

Based on the description of the issue, it is possible that there is the break condition on the TX line at power on. When they power-cycle the device, the computer has already configured the port and it is in the Stop condition.

To check if our assumptions are correct, please ask your customer to disconnect the AHRS-II-P from the computer and check if the LED will light up every time you power on the device.

Please let us know about the result.
Thank you!



As requested, the customer connected only the power with no connection to PC and the LED glows immediately without any delays.

Another query they have is, as per the pin configuration, pin numbers 11 to 14 are RS-422 TX/RX lines. They want to know, in them, which are positive and which are negative.

Thanks & Regards,


Inertial Labs Service Member:

The fact that the AHRS is powering up with no issues when you disconnected it from the computer means that your computer sets “break” condition on the COM port to which AHRS is connected - this makes AHRS enter firmware update mode (see details on “Firmware update“ operating mode in section 6.1 “Operational modes of the Inertial Labs AHRS-II“ on page 27 of the AHRS-II ICD).

The way the device enters the Firmware update mode is - it captures the receiving line of the serial port at the power-up. If the break condition is recognized - the device enters the firmware update mode and stays there until power cycled again. The break condition (in case of RS422 converter you use) is when the voltage on pin1 (PC-TX+) of DB9 connector is below the voltage on the pin 2 (PC-TX-), i.e. infinite logical zero. See more details about a break condition here:

As for pin connections, referenced in Section 5 of the ICD, pins A and Y are positive, and B and Z are negative.

Thank you!

Inertial Labs has adjusted customer names, contact details, and has removed/modified information from this conversation thread to protect the privacy of the parties that were involved and effectively communicate solution.

Topics: MRU, AHRS, INS, Break, Off, Restarting, AHRS-II-P, Condition, On

      Knowledge Base Documents