116 Programmable Logic Controllers: Hardware and Programming
Input (I):
File used to hold data for
the input instructions.
XIC (examine if closed):
A PLC instruction for
a normally open input
device.
XIO (examine if open):
A PLC instruction for a
normally closed input
device.
Output (O):
File used to hold the
data for the output
instructions.
Status (S2):
File that holds the status
bit flags of an operating
PLC ladder logic
diagram.
Timers (T4):
File that holds the
contents of the timer
instructions.
Counters (C5):
File used for the counter
instructions.
how they are accessed through the main program file will be studied
in later chapters. If you select a main program file or a subroutine file
in the project tree area, its ladder logic diagram will be displayed in
the ladder view area.
In the
Data Files
area, there are icons for nine data files. The fol-
lowing describes each file:
• Input (I)—The input file holds the input bit status of the input
modules. There are 12 input ports in module zero of the Allen-
Bradley fixed SLC 500 PLC. These inputs can be examine if open
(XIO) or examine if closed (XIC).
• Output (O)—The output file holds the output bit status of the
output modules. In the Allen-Bradley fixed SLC 500 PLCs, there
are eight output ports in module zero.
• Status (S2)—The status file holds the flag or status bits of
the ladder logic diagram. If a central processing unit fault is
generated in the PLC, you should click this icon. Then, find
the fault indicator status bit and read the reason the message
was generated. Finally, correct the program to solve the fault
condition. Figure 6-21 displays a typical
Status
dialog box in
which the reason for the PLC fault is listed.
• Timer (T4)—The timer file holds information related to the
number of timers used in the ladder logic program. Depending
on the size of the PLC RAM, there could be timers with
addresses from T4:0 to T4:255. This means that, theoretically, a
PLC ladder diagram can have access to 256 timers. In practice,
the number of timers is limited due to lack of enough RAM
memory in the PLC.
• Counter (C5)—The counter file holds information related to
counters that can have any address from C5:0 to C5:255. The
Ladder view area Project tree area
Figure 6-20. The project tree area contains the program and data file. The ladder view
area displays the ladder diagram from the main program file (LAD2) or subroutine files.