Ladder Diagram - GE VersaMax PLC User Manual

Relay
Power
Rail
%I0001
%R0123
CONST
00002
106 GFK-1503E
For public disclosure

8.3.2 Ladder Diagram

This traditional PLC programming language, with its rung-like structure, executes from
top to bottom. The logic execution is thought of as power flow, which proceeds down
along the left rail of the ladder, and from left to right along each rung in sequence.
Power flow into function
MUL_
INT
%R0124
I1 Q
Multiplication function
I2
The flow of logical power through each rung is controlled by a set of simple program
functions that work like mechanical relays and output coils. Whether or not a relay passes
logical power flow along the rung depends on the content of a memory location with
which the relay has been associated in the program. For instance, a relay might pass
power flow if its associated memory location contained the value 1. The same relay
would not pass power flow if the memory location contained the value 0.
If a relay or other function in a rung does not pass logical power flow, the rest of that rung
is not executed. Power then flows down along the left rail to the next rung.
Within a rung, there are many complex functions that can be used for operations like
moving data stored in memory, performing math operations, and controlling
communications between the CPU and other devices in the system.
Some program functions, such as the Jump function and Master Control Relay, can be
used to control the execution of the program itself.
Together, this large group of Ladder Diagram relays, coils, and functions is called the
Instruction Set of the CPU.
Power flow out of function
VersaMax PLC User Manual
%Q0001
Coil