138 Programmable Logic Controllers: Hardware and Programming
Summary
• Input and output device symbols are used to create relay logic diagrams.
• Once a relay logic diagram is constructed, addresses are assigned to each input and output
device, those devices are connected to PLC ports, and the PLC ladder diagram is created.
• Ladder logic diagrams are created in the offline mode.
• Latch and unlatch instructions were used to create PLC ladder logic diagrams and the
program was tested on a PLC trainer.
• When converting relay logic diagrams to the ladder logic diagrams, you must follow the
following six rules:
1. Place a contact in the upper-left corner of the ladder logic diagram.
2. Place the coil at the end of the rung.
3. All contacts must be placed horizontally.
4. Outputs cannot be connected in series with other outputs.
5. Program execution flow must be from left to right.
6. Place the rung numbers on the left side of each rung.
• After constructing the relay logic diagram for a control system, assign addresses to each
input and output device, connect the input/output devices to the PLC ports, and then
create the PLC ladder diagram.
• When creating a new PLC file, the first step is to select the proper PLC processor.
• PLC diagrams can be evaluated with normally open and normally closed switches and
contacts.
• The PLC program execution flow, called program scan, on each rung is from left to right.
The program scan is from the top rung to the bottom rung moving through the entire
PLC ladder logic diagram.
• The operational scan rate is the time required to execute the PLC ladder diagram once.
Therefore, instructions in the PLC ladder logic diagram are scanned starting from the
instruction in the upper-left corner and ending with the instruction on the lower- right
corner. After the last instruction in the lower-right corner is executed, the process of
scanning restarts.
• Each PLC operational cycle has three separate scan parts: input scan, program scan, and
output scan.
• Since scanning takes place from left to right across each rung, and from the top to bottom
rungs, sometimes specific inputs or outputs may not be updated in time.
• A programmable logic controller project consists of the ladder logic diagram file and its
associated data files.
• The
SYS0
program file is used by the PLC processor to keep track of its internal register
settings and the
SYS1
program file is reserved. Therefore, these two system files are not
available to the RSLogix 500 software user.
• The main program file (LAD2) holds the main PLC ladder logic diagram.
• In a PLC project, there may be more than one program file which also means that there
might be several subroutine program files in addition to the main program file.