Page 138 - Programmable Logic Controllers, Fifth Edition - Mobile version

Input module
Output module
wiring connections Ladder logic program wiring connection
L1 L2
Man/Auto Low sensor switch OFF/ON Motor
1
OFF ON Motor
M
Man/Auto Latch/Unlatch Pump running
Man Auto
G
Low level
Low sensor switch
Man/Auto High sensor switch
R
2 L
High sensor switch Latch coil High level
Man/Auto Low sensor switch Y
3 U
Unlatch coil
Man/Auto

Motor G
4
Low sensor switch R
5
High sensor switch Y
6

Figure 6-57 Program used to implement control of the water level in the storage tank.

• The low-level status light is controlled by the rung relatively small power rating and is used to supply DC
5 Examine-off instruction addressed to the low power to all devices physically mounted in the backplane
s ensor switch. of the PLC rack. In this application a 24 VDC field power
• The high-level status light is controlled by the rung supply is used for the input devices and a 120 VAC field
6 Examine-on instruction addressed to the high power supply for the output devices. This allows a low-
s ensor switch. voltage 24-volt control signal to control 240-volt output
devices. SLC 500 controllers use a rack/slot-based ad-
Figure 6-58 shows a typical I/O module wiring dia- dress system where the slot location of the I/O modules
gram and addressing format for the water level control in the rack establishes the PLC address. The addresses
program implemented using an Allen-Bradley modu- for the field devices of this particular application are
lar SLC 500 controller. The chassis power supply has a shown below:

FIELD DEVICE ADDRESS Signifies
OFF/ON Switch I:2/0 The input module in slot 2 and screw terminal 0
MAN/AUTO Switch I:2/4 The input module in slot 2 and screw terminal 4
LOW SENSOR Switch I:2/8 The input module in slot 2 and screw terminal 8
HIGH SENSOR Switch I:2/12 The input module in slot 2 and screw terminal 12
MOTOR O:3/1 The output module in slot 3 and screw terminal 1
PUMP RUNNING Light O:3/5 The output module in slot 3 and screw terminal 5
LOW LEVEL Light O:3/9 The output module in slot 3 and screw terminal 9
HIGH LEVEL Light O:3/13 The output module in slot 3 and screw terminal 13
B3:0/0 Internal retentive bit instruction that does not drive a real-word device

Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs Chapter 6 119

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