Since the structure and working principle of the PLC are different from those of the relay contactor control circuit, there are some differences between the ladder diagram control program and the relay contactor control circuit. There are roughly five differences.
What is the difference between PLC ladder diagram and contactor control circuit?
Although the plc ladder control program and the relay contactor control circuit have similarities, they are not absolute one-to-one correspondence. Due to the difference in the structure and working principle of the PLC and the control circuit of the relay contactor, there are some differences between the ladder control program and the relay contactor control circuit.
(1) The ladder programming of PLC is the representation method of the analog relay contactor control system. Therefore, the components in the ladder diagram also use the name in the relay contactor control system, which is called “(soft) relayâ€. . 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. There are two opposite states. When the status of the corresponding bit is "1", it means The relay coil is "powered". When the state is "0", it indicates that the relay coil is "powered out", so it is called "soft relay". By using "relay" to represent the components in the PLC, the ladder program can be designed in the form of a relay contactor control system.
(2) The "current" flowing in the ladder program is not a real physical current, but a "energy flow". It can only flow according to the rules of "from left to right" and "from top to bottom". "Energy flow" does not allow backflow. When the "energy flow" arrives, the corresponding coil is electrically connected. In fact, "energy flow" is just an image representation of the user program operation that satisfies the output execution condition. The rule of "energy flow" is specified in order to comply with the order in which the PLC scan is "from left to right" and "from top to bottom". However, the current in the relay contactor control system is the real physical current, which can be measured by an ammeter, and its flow direction can also flow freely according to the actual situation of the external power supply.
(3) The normally open, normally closed contacts in the ladder program are not actual physical contacts. They simply reflect the state of the corresponding bits in the input, output map registers, or data registers that correspond to the state of the physical switch in the field. In the PLC, the normally open contact is considered to be a "read" operation of the register status, while normally closed. The contact is a "reverse" operation on the bit register.
(4) The coil in the ladder program is not the actual physical coil, and it cannot be used to directly drive the actuator of the field component. The state in the output coil is directly transferred to the corresponding bit in the output image register, and then the status is "1" (high) or "0" (low) in the output image register bit to control the output circuit. The corresponding circuit, after power amplification, controls the PLC's output device (relay, transistor or thyristor), and then its contacts are turned on and off to control the actuator of the external field component.
(5) When programming the ladder program, the contacts of the PLC internal relay can be called infinitely in principle, because the bit status in the memory unit can be read repeatedly; and the number of relay contacts in the relay contactor control circuit is determined by the relay The structural form is determined and thus fixed as the structural form is determined, and the number is limited. It should be especially emphasized that in the PLC, the coil can usually only be called once in the same ladder program, so it should be avoided to reuse the coil with the same address number (repetition coil will lead to uncertainty of the output result).
What is the difference between PLC ladder diagram and contactor control circuit?
Although the plc ladder control program and the relay contactor control circuit have similarities, they are not absolute one-to-one correspondence. Due to the difference in the structure and working principle of the PLC and the control circuit of the relay contactor, there are some differences between the ladder control program and the relay contactor control circuit.
(1) The ladder programming of PLC is the representation method of the analog relay contactor control system. Therefore, the components in the ladder diagram also use the name in the relay contactor control system, which is called “(soft) relayâ€. . 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. There are two opposite states. When the status of the corresponding bit is "1", it means The relay coil is "powered". When the state is "0", it indicates that the relay coil is "powered out", so it is called "soft relay". By using "relay" to represent the components in the PLC, the ladder program can be designed in the form of a relay contactor control system.
(2) The "current" flowing in the ladder program is not a real physical current, but a "energy flow". It can only flow according to the rules of "from left to right" and "from top to bottom". "Energy flow" does not allow backflow. When the "energy flow" arrives, the corresponding coil is electrically connected. In fact, "energy flow" is just an image representation of the user program operation that satisfies the output execution condition. The rule of "energy flow" is specified in order to comply with the order in which the PLC scan is "from left to right" and "from top to bottom". However, the current in the relay contactor control system is the real physical current, which can be measured by an ammeter, and its flow direction can also flow freely according to the actual situation of the external power supply.
(3) The normally open, normally closed contacts in the ladder program are not actual physical contacts. They simply reflect the state of the corresponding bits in the input, output map registers, or data registers that correspond to the state of the physical switch in the field. In the PLC, the normally open contact is considered to be a "read" operation of the register status, while normally closed. The contact is a "reverse" operation on the bit register.
(4) The coil in the ladder program is not the actual physical coil, and it cannot be used to directly drive the actuator of the field component. The state in the output coil is directly transferred to the corresponding bit in the output image register, and then the status is "1" (high) or "0" (low) in the output image register bit to control the output circuit. The corresponding circuit, after power amplification, controls the PLC's output device (relay, transistor or thyristor), and then its contacts are turned on and off to control the actuator of the external field component.
(5) When programming the ladder program, the contacts of the PLC internal relay can be called infinitely in principle, because the bit status in the memory unit can be read repeatedly; and the number of relay contacts in the relay contactor control circuit is determined by the relay The structural form is determined and thus fixed as the structural form is determined, and the number is limited. It should be especially emphasized that in the PLC, the coil can usually only be called once in the same ladder program, so it should be avoided to reuse the coil with the same address number (repetition coil will lead to uncertainty of the output result).
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