micky wrote:

How did my alarm code change when I never touched it?

(not a computer but it does have a digitally stored program, and solid
state memory for the settings, less than 1000 bytes, I think.)

About 9 years ago I started to install a burglar alarm, but I didn't
finish before I had to leave on my trip.

After I got home, I didn't care anymore.

I'm leaving on another trip soon and I started in on the alarm agailn.

Even thogh the trouble light is lit, It didn't need much to hook it up,
but I couldn't arm it. Pressing the default arm code just let to a
longer error-beep.

Eventually did a reset, what they call a Factory Default Main Panel.

Now the default arm code works.

I'm sure I never changed the code. My previous alarm worked for 15
years or more and I used the default code all that time, never
considered chaning it**, and I was still trying to get the thing to
work. Another reason not to change it.

So can things like this get messed up just be sitting around?. In my
basement with cinder block walls. The only source of radiation is the
30 y.o. oil furnace. That coudln't do it. A small tv in the room
too. There's a microwave upstairs.

**There are occasional burglaries around here, but none of these people
know what the default alarm code is for anything. It's not like I live
on the same street as the Vanderbilts or Morrgans.

BTW, it's funny. I wanted to see if the siren worked but the code
didnt' work to arm the alarm. The third time I tried, I noticed the
Away button. This contol panel doesn't require a code when leaving
because they figure if you got in, you're trustworthy. So I armed it.
Then I opened the door until the siren went off. Then I rmemebered I
had no code to disarm it!!. I tried the code several times, again, but
it didnt' work. I unplugged the thing, but when I plugged it in
again, it armed itself. Of course. Otherwise, power failures when the
battery is dead would disarm alarms. So I was trapped in the house.
Eventually I disconnected the control panel from the siren driver, so it
didn't make any noise the next time.

Haven't a clue what is the hardware which you seem to deliberate omit in
your posts. Could be the code gets stored in EEPROM but I doubt it
since it would take sufficient voltage to reprogram the bits in the
chip. Could be there is a capacitor on the PCB used to hold the code.
That capacitor discharged long ago so any value stored in volatile
memory is gone. Could be a battery is used instead of a capacitor.
Just like your PC, a dead battery means the CMOS table's values (a copy
of the BIOS and where you do your tweaks, like changing a login
password) is corrupted or all values are unusable. That means the BIOS
copy of the settings get loaded into the CMOS table.

If it uses EEPROM, something is wrong with the hardware that had it lose
your stored code. If it uses a capacitor, well, those things only stay
charged for a limited time. If it uses a battery, replace it as it is
probably too old and doesn't have enough capacity (might have enough
voltage under a no-load test but cannot supply any current, even a tiny
bit, without a voltage drop).