76 Chapter 5 Basics of PLC Programming User defined 0 Output file Input file Status file Bit file Timer file Counter file Control file Integer file Floating point file 1 2 3 4 5 6 0 1 2 3 to 255 program files 3 to 255 Program file 2 System file 1 System file 0 7 8 Designated 9 to 255 Main program Subroutines B T C R N F ST A Bit Timer Counter Control Integer Floating Pt. String ASCII Data files Program files – Program Files SYS 0 – SYS 1 – LAD 2 – Figure 5-1 Program and data file organization for the SLC 500 controller. Figure 5-2 Program file tree. Part 2: The module slot number and a forward slash to separate the slot from the terminal screw. Part 3: The screw terminal number. The SLC 500 stores data in data tables that are based on 16-bit words. The input image table file is that part of the program memory allocated to storing the on/off status of connected discrete inputs. Figure 5-5 shows the connection of an open and closed switch to the input image table file through the input module. Its operation can be summarized as follows. • For the switch that is closed, the processor detects a voltage at the input terminal and records that information by storing a binary 1 in its bit location. • For the switch that is open, the processor detects no voltage at the input terminal and records that information by storing a binary 0 in its bit location. • Each connected input has a bit in the input image table file that corresponds exactly to the terminal to which the input is connected. • The input image table file is changed to reflect the current status of the switch during the I/O scan phase of operation. • If the input is on (switch closed), its corresponding bit in the table is set to 1. • If the input is off (switch open), the corresponding bit is cleared, or reset to 0.
Petruzella_ProgrammableLogicControllers__5e
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