The upper crate should be switched on before the lower crate
      (in general, the crate, which by it's root board feeds the
      clock to the other crates, should be switched-on first, in 
      order to avoid the other crates running without clock)

  happens frequently:
  - Often a very short power off (< 1sec) is enough to cure problem
    (i.e. switch on the PSU for >= 10 sec, if red LED "OL" does not
    turn green, switch off for just less than about 1 sec)
  - Otherwise try (dirty!) clock trick: remove clock input of root board,
    switch on machine, re-attach clock cable, press hard reset button of
    root board.
  - If completely desperate: detach the PCs and try power-on. If power-on
    still no possible detach 1-4 processing boards. After successful
    power-on wait for O(0.5h) before attaching PCs and PBs again.

  after power up:

    Sasha's Trick:  Leave PSU on for a few minutes and than
    	try power-off-on again
    	(Explanation: Thermal effect of PSU)

    Walter's Trick:	Connect inhibit/OK signals of various PSU
    	in such a way, that 3.3 V comes up before 5V
    	(Explanation: On-chip exceptions)

    	There are actually two ways to do that: [Maurizio]

    	a) Clean:

    	- Resistor 10 k between Emitter of 3.3 V unit 
    	  Ground.

    	and

    	- Connect Emitter of 3.3 V unit with Inhibit-Input
    	  of 5.5 V unit

    	b) Dirty:

    	Connect Inhibit-Output of 3.3V unit with Inhibit-
    	Input of 5V unit

    Delay Circuit [Neuricam]:

	Generates inhibit for 5 V for O(few ms) after 
	3.3 V starts rising. Monitored by an oscilloscope,
	the net effect is that the 3.3 V lines reach a voltage 
	level of about 0.5...1 V BEFORE the 5 V lines
	(although the 3.3 V lines reach their final nominal 
	value O(20 ms) AFTER the 5 V lines)

    Brute force (not recommended): Increase current limitation