Page 141 - Programmable Logic Controllers, Fifth Edition

3. Package moves to the position of the limit switch
Figure 6-64 shows an I/O connection diagram for a
and automatically stops. programmed version of the sequential process. Each input
and output device is represented by its symbol and asso-
Other auxiliary features include:
ciated address. These addresses will indicate what PLC
• a stop button that will stop the table, for any reason, input is connected to what input device and what PLC
before the package reaches the limit switch position; output will drive what output device. The address code, of
• a red pilot light to indicate the table is stopped; and course, will depend on the PLC model used. This example
• a green pilot light to indicate the table is running. uses SLC 500 addressing for the process. Note that the
electromagnetic control relay CR is not needed because
A relay schematic for the sequential process is shown its function is replaced by an internal PLC control relay.
in Figure 6-63. The operation of this hardwired circuit can The hardwired relay schematic for the sequential pro-
be summarized as follows: cess can be converted to the PLC ladder logic program

• Start button is actuated; CR is energized if stop but- shown in Figure 6-65. In converting the process to a pro-
ton and limit switch are not actuated. gram the operation of each rung must be understood. The
• Contact CR-1 closes, sealing in CR when the start pushbuttons PB1, PB2 as well as limit switch LS are all
button is released. programmed using the examine-closed (–] [–) instruction
• Contact CR-2 opens, switching the red pilot light to produce the desired logic control. Also, internal relay
from on to off. B3:1/0 is used to replace control relay CR. To obtain the
desired control logic, all internal relay contacts are pro-
• Contact CR-3 closes, switching the green pilot light grammed using the PLC contact instruction that matches
from off to on. the coil de-energized state. The internal relay imple-
• Contact CR-4 closes to energize the motor starter mented in software requires one coil address the contacts
coil, starting the motor and moving the package to- of which can be examined for an ON or OFF condition as
ward the limit switch. many times as you like.
• Limit switch is actuated, de-energizing relay There is more than one method to correctly design the
coil CR. ladder logic program for a given control process. In some
• Contact CR-1 opens, opening the seal-in circuit. cases one arrangement may be more efficient in terms of
• Contact CR-2 closes, switching the red pilot light the amount of memory used and the time required to scan
from off to on. the program. Figure 6-66 illustrates an example of an ar-
• Contact CR-3 opens, switching the green pilot light rangement of series instructions of a rung programmed
from on to off. for optimum scan time. The series instructions are pro-
• Contact CR-4 opens, de-energizing the motor starter grammed from the most likely to be false (far left) to
the least likely to be false (far right). Once the processor
coil to stop the motor and end the sequence.
sees a false input instruction in series, the processor stops
checking the rung at the false condition and sets the out-
put false.
L1 L2
PB1 LS Figure 6-67 illustrates an example of an arrangement
Start PB2 Limit Control of parallel instructions of a rung programmed for opti-
Stop switch relay
mum scan time. The parallel path that is most often true is
CR

Inputs Outputs
CR-1 PL1 L1 L2 L1 L2
Stop Motor starter
R PB1 - Start coil
CR-2 O:4/1 M
PL2 I:3/0
Run PL1 - Stop
PB1 - Stop
G O:4/2 R
CR-3 Motor I:3/1
starter coil PL2 - Run
LS - Limit switch
M I:3/2 O:4/3 G
CR-4 OL
Figure 6-63 Relay schematic for the sequential process. Figure 6-64 I/O connection diagram.

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

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