user6547: The unsolved: when I connect power and signal lines to my Arduino - while power is on - I potentially kill it. I have fried 2 so far.
Situation:

I have build an DMX-controlled lighting fixture, which consists of the fixture and a Controller Box.
The fixture houses 2 Arduino's (5V), 2 steppers, 2 drives (36V), 2 fans (24V) and some temperature sensors.
The controller box houses 1 Arduino, amongst other electronics.

The controller box receives DMX and does calculations an send controls via I2C to the fixture.
There is a 8 pole connector between fixture and controller box. 
1 - GND
2 - 5v
3 - 24v
4 - 36v
5 - CLK (i2c)
6 - SDA (i2c)
7 - signal line (used as charge pump)
8 - reset

All wires are shielded(but the shield is only connected AFTER the initial pins make contact)
When the power on the controller box is ON and i connect the light Fixture with the 8-pole connector my arduino goes to heaven.
My own idea's so far:

Voltage spike, because the connection wires is grounded after it connects the pins.
Some strange flyback? via the Arduinos in the light fixture? I connected everything straight to the 5v pins, not using the regulator.
no idea

Any help on a way to search for the most likely cause of the problem would be greatly appreciated. 

user10147: If you ever drive an I/O more than a diode drop above or below its power supply rails, the internal ESD protection structure will begin to conduct.  The internal structure is only there to keep the part alive during normal handling, i.e. in a factory but not for the situation described above.  If the current in this structure is allowed to go high enough, it looks like an SCR and "Latches" up.  When latch up occurs the device, like an SCR, will keep conducting as much current as the power supply rails are capable of until the current is removed and usually results in device destruction.  The way around this is to limit the current that may flow into a pin, and prevent current from forward biasing the ESD structure:  http://www.tayloredge.com/reference/Electronics/EMIESD/InputProtection.pdf

6547: <p>The unsolved: when I connect power and signal lines to my Arduino - while power is on - I potentially kill it. I have fried 2 so far.</p>

<p>Situation:</p>

<ul>
<li>I have build an DMX-controlled lighting fixture, which consists of the fixture and a Controller Box.</li>
<li>The fixture houses 2 Arduino's (5V), 2 steppers, 2 drives (36V), 2 fans (24V) and some temperature sensors.</li>
<li>The controller box houses 1 Arduino, amongst other electronics.</li>
</ul>

<p>The controller box receives DMX and does calculations an send controls via I2C to the fixture.</p>

<p>There is a 8 pole connector between fixture and controller box. </p>

<pre><code>1 - GND
2 - 5v
3 - 24v
4 - 36v
5 - CLK (i2c)
6 - SDA (i2c)
7 - signal line (used as charge pump)
8 - reset
</code></pre>

<p>All wires are shielded(but the shield is only connected AFTER the initial pins make contact)</p>

<p>When the power on the controller box is ON and i connect the light Fixture with the 8-pole connector my arduino goes to heaven.</p>

<p>My own idea's so far:</p>

<ol>
<li>Voltage spike, because the connection wires is grounded after it connects the pins.</li>
<li>Some strange flyback? via the Arduinos in the light fixture? I connected everything straight to the 5v pins, not using the regulator.</li>
<li>no idea</li>
</ol>

<p>Any help on a way to search for the most likely cause of the problem would be greatly appreciated. </p>
 10147: <p>If you ever drive an I/O more than a diode drop above or below its power supply rails, the internal ESD protection structure will begin to conduct.  The internal structure is only there to keep the part alive during normal handling, i.e. in a factory but not for the situation described above.  If the current in this structure is allowed to go high enough, it looks like an SCR and "Latches" up.  When latch up occurs the device, like an SCR, will keep conducting as much current as the power supply rails are capable of until the current is removed and usually results in device destruction.  The way around this is to limit the current that may flow into a pin, and prevent current from forward biasing the ESD structure:  <a href="http://www.tayloredge.com/reference/Electronics/EMIESD/InputProtection.pdf" rel="nofollow">http://www.tayloredge.com/reference/Electronics/EMIESD/InputProtection.pdf</a></p>
