Rockwell Automation 1775-S5_SR5,D17756.5.5 User Manual PLC-3 FAMILY I/0 - Manual (Page 115)

Diagnostics Methods
Chapter 7
7-3
The line number is the relative location of a command line from the
beginning of the message procedure containing the line. The first line of
each procedure is line number 1, and any following lines are numbered in
ascending sequence. Nested procedures begin with line 1 again, thus the
need for words 3, 4 and 5 in the error block.
Important: You do not enter the line numbers for a procedure; the scanner
automatically keeps track of the line numbers for you.
The error block retains its data even after the message procedures complete
execution. The block does not reset automatically.
Error Block Operation
Figure 7.1 illustrates how the error block works. In this figure, an
addressing error (invalid destination address) occurs in procedure SUB2,
which is nested 3 levels deep.
Word: of the error code gives the line number where error occured in procedure:
5 SUB2.
4 SUB1 that executed procedures SUB2.
3 MAIN that executed procedure SUB1.