NORD Drivesystems BU0550 - Manual (Page 17)

2 Creation of PLC programs
BU 0550 GB-0813 17
2.4 Debugging
As programs only rarely function the very first time, the NORD PLC provides several possibilities for finding
faults. These possibilities can be roughly divided into two categories, which are described in detail below.
2.4.1 Observation points (Watchpoints)
The simplest debugging variant is the Watchpoint function. This provides a rapid overview of the behaviour
of several variables. For this, an observation point is set at an arbitrary point in the program. When the PLC
executes this program line, up to 5 values are saved and displayed in the observation list ("Holding point"
window). The 5 values to be observed can be selected via the context menu (right mouse button) in the
input window or the variable window.
2.4.2 Holding points (Breakpoints)
Via holding points it is possible to deliberately stop the PLC program at a specific line of the program. If the
PLC runs into a holding point, the AE, Accumulator 1 and Accumulator 2 are read out, together with all the
variables which have been selected under the menu item "Debug variables" (right mouse menu).
Up to 5 Breakpoints can be set in a PLC program.
This function is started via the symbol. The program now runs until a holding point is triggered. Further
actuation of the symbol bar allows the program to continue running until it reaches the next holding point.
If the program is to continue running, the symbol is actuated.
2.4.3 Single Step
With this debugging method it is possible to execute the PLC program line for line. With each individual
step, all the selected variables are read out of the FI PLC and displayed in the "Observation list" window.
The values to be observed can be selected in the input window or the variable window by means of the right
mouse button menu.
The prerequisite for debugging in single steps is that at least one holding point (see Section 2.4.2) has been
set before debugging is started. The debugging mode is switched on by actuating the symbol. It is only
possible to debug the program in single steps via the symbol after the program has run into the first
holding point.
Some command lines contain several individual commands. Because of this it is possible that two or more
individual steps must be executed before the step indicator moves forward in the input window. The actual
position is shown by a small arrow in the left PLC Editor window.
When the symbol is actuated, the program continues running until the next holding point.
If the program is to continue running, the symbol is actuated.