Although PLC ladder diagram control programs and relay contactor control circuits have similarities, they are not absolutely one-to-one. Due to the differences in the structure and working principle of PLCs and relay contactor control circuits, there are some differences between the two.
(1) PLC ladder diagram programming is a representation method for simulating relay contactor control systems. Therefore, the components in the ladder diagram also use the terminology used in relay contactor control systems, which is "(soft) relay". However, the "soft relay" in the ladder diagram is not a real physical relay. Each "soft relay" is a "bit register" in the PLC memory, with two opposite states. When the state of the corresponding bit is "1", it means that the relay coil is "energized", and when the state is "0", it means that the relay coil is "de-energized". Therefore, it is called "soft relay". By using "relay" to represent the components in the PLC, the ladder diagram program can be designed in the form of a relay contactor control system.
(2) The "current" flowing through the ladder diagram program is not a real physical current, but an "energy flow," which can only flow according to the rules of "from left to right" and "from top to bottom." The "energy flow" is not allowed to flow backward. When the "energy flow" arrives, the corresponding coil is energized and connected. In fact, the "energy flow" is just a visual representation of the output execution conditions met in the user program operation. The rules for the direction of the "energy flow" are specified to conform to the PLC scanning sequence of "from left to right" and "from top to bottom." However, the current in the relay contactor control system is a real physical current, which can be measured with an ammeter, and its flow direction can also flow freely according to the actual situation of the external power supply.
(3) The normally open and normally closed contacts in the ladder diagram program are not actual physical contacts. They only reflect the state of the corresponding bits in the input and output image registers or data registers corresponding to the state of the physical switches on site. In the PLC, normally open contacts are considered to "read" the state of the bit register, while normally closed contacts are considered to "invert" the state of the bit register.
(4) The coils in the ladder diagram program are not actual physical coils and cannot be used to directly drive the actuators of field components. The state in the output coil is directly transmitted to the corresponding bit in the output image register. Then, the state "1" (high level) or "0" (low level) in the output image register bit is used to control the corresponding circuit in the output circuit. After power amplification, it controls the output device (relay, transistor or thyristor) of the PLC, thereby opening and closing its contacts to control the actuators of external field components.
(5) When programming a ladder diagram, the contacts of the relays inside the PLC can be called an unlimited number of times in principle, because the bit states in the storage unit can be read repeatedly; however, the number of relay contacts in the relay contactor control circuit is determined by the structure of the relay, and therefore will be fixed as the structure is determined, and its number is limited. It should be particularly emphasized that in the PLC, under normal circumstances, the coil can only be called once in the same ladder diagram program, so the reuse of the same address number of the coil should be avoided as much as possible (repeated coils will lead to uncertainty in the output results).
