Routine:        Pb4     
Function:       Program B4: As Pb3 except program B4 displays time elapsed since start of  experiment, not real-time
Called by:      Main
Calls:          Pb3
Entry:          None
Exit:           None
Preserved:      None


PB4     LD      IX,CLKSEC
        XOR     A
        LD      B,6
; Set all time bytes to zero and reset the 50 Hz counter, then jump to Pb3.
PB4A    LD      (IX),A
        INC     IX
        DJNZ    PB4A
        IN      A,(6)
        LD      (CLK50),A
        JP      PB3