> Does anyone know how to switch a 3A on at a certain time from OPL ?

The simplest approach is to use the Operating System service 
TimWaitAbsolute (call($0c89)).  This sleeps the current process (ie your 
Opl program) until the time specified in CX:DX.  When that time arrives, 
the machine is switched on again, if need be, to let your program 
continue.

Here's an example that uses TimWaitAbsolute.  It prints out "Waiting", 
and then two minutes later, switches the machine on (if need be) and 
prints "Done".

In this example, I use TimGetSystemTime (call($0289)) to get the current 
system time, into AX:BX.  The actual code below contains some messy 
conversions, to try to avoid overflow in the low word in adding on 120, 
but don't let this cause you to miss the essential simplicity of what the 
code is doing.

  PROC twomins:
    local ax%,bx%,cx%,dx%,si%,di%
    local t&,a%
    ax%=$200
    os($89,addr(ax%))
    t&=ax%*&10000 + bx%
    t&=t& + 120
    a%=addr(t&)
    print "Waiting"
    call($0c89,0,peekw(a%+2),peekw(a%))
    print "Finished waiting"
    get
  ENDP

If you want to be able to do something else in your program, as well as 
waiting for the absolute time, you need to use the TIM: device driver.

Regards, DavidW