Page 95 - Programmable Logic Controllers, Fifth Edition

0
1
2
Output file Input file Status file 3 4 5 Designated

Bit file
Timer file Counter file Control file 6 Integer file 7 9 to 255

8

Floating point file
B
Bit
T
Timer
C
Counter
R
Control
N
Integer
F
Floating Pt.
ST String User defined
Data
files A ASCII
Program
files

0
1 2 Main program – Program Files
System file 0 System file 1 Program file 2 3 to 255 SYS 0 –

program files
3 to 255
SYS 1 –
Subroutines

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 • For the switch that is open, the processor detects no
to separate the slot from the terminal screw. voltage at the input terminal and records that infor-
mation by storing a binary 0 in its bit location.
Part 3: The screw terminal number.
• Each connected input has a bit in the input image
The SLC 500 stores data in data tables that are based table file that corresponds exactly to the terminal to
on 16-bit words. The input image table file is that part of the which the input is connected.
program memory allocated to storing the on/off status of con- • The input image table file is changed to reflect the
nected discrete inputs. Figure 5-5 shows the connection of an current status of the switch during the I/O scan
open and closed switch to the input image table file through phase of operation.
the input module. Its operation can be summarized as follows. • If the input is on (switch closed), its corresponding

• For the switch that is closed, the processor detects a bit in the table is set to 1.
voltage at the input terminal and records that infor- • If the input is off (switch open), the corresponding
mation by storing a binary 1 in its bit location. bit is cleared, or reset to 0.

76 Chapter 5 Basics of PLC Programming

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