Likely what is happening is you are overrunning the RS232
interrupt buffer, and that is causing a write into code
space.  Odd that the firmware is changing, though.

Does this thing have Flash RAM, or battery backed NVRAM (Dallas)?

Oftentimes, NVRAM using devices use the NVRAM for both firmware,
and data RAM space.  Things can get really ugly when that is done.

Also, to test my guess, put a delay between each character
sent to the FE-5680A.  Say, 2 or 3 ms.

Needless to say before you do this, you need to develop a way
of restoring the firmware.

-Chuck Harris

Bert Kehren via time-nuts wrote:
Sadly yes
The reason we have not released our GPSDO for the FE5690/50 and FE405 is
that we have experienced the same on all three devices. You are the 9th case
that I know of. We know very little as to what exactly causes it but we are
  incorporating circuitry to prevent it. What we know is that it is not a
particular code sequence on the RS232 port, but what happens the operating
code  is turning to mush. Some how the RS port is involved. Serious flaw on
all  FE devices since most likely the code was written by the same individual.
Bert Kehren


In a message dated 4/28/2016 9:02:28 P.M. Eastern Daylight Time,
[email protected] writes:

Just as I  started testing the GPS discipline board prototype, my FE-5680A
seems to have  developed a very odd problem.

If I give it power, it outputs a “kinda”  10 MHz sine wave while it sweeps
around looking for a physics lock. This is as  expected.

About the time that I would expect the lock light to turn on,  instead the
output just stops. The lock light never comes on.

This  unit’s been working fine for months now. It’s conceivable that I’ve
sent it  some sort of serial command it couldn’t digest, but using the
Windows  calibrator software seems to work - albeit there not being any output
from the  oscillator at  all.


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