WOODWARD 8701-758 5601-1126 backlight/backlight assembly

Description

The PLC network is composed of several levels of subnets, and the communication process of each level of subnet is determined by the communication protocol, and the communication method is the core content of the communication protocol. Communication methods include access control methods and data transmission methods. The so-called access control (also known as access control) method refers to how to obtain the right to use shared communication media, while the data transmission method refers to how a station transfers data after obtaining the right to use communication media.

1. Periodic I/O communication method
The periodic I/O communication method is commonly used in the remote I/O link of PLC. The remote I/O link operates in a master-slave manner, with the PLC remote I/O master unit as the master station and all other remote I/O units as slaves. Set up a “remote I/O buffer” in the main station, using a mailbox structure, divided into several sub boxes corresponding to each slave station -1. Each sub box is further divided into two compartments, one for sending and the other for receiving. The communication processor in the master station adopts a periodic scanning method to exchange data with each slave station in sequence, and sends the data sent in the corresponding sub boxes to the slave station. The data read from the slave station is placed in the receiving sub boxes of its corresponding sub boxes. This cycle repeats, causing the “remote I/O buffer” in the main station to be periodically refreshed.

In the main station, the CPU unit of the PLC is responsible for scanning the user program. It processes it in a cyclic scanning manner, and each cycle has a period of time for centralized I/O processing. At this time, it performs read and write operations on the local I/O unit and remote I/O buffer. The CPU unit of the PLC periodically scans the user program, while the communication processor of the PLC asynchronously scans each remote I/O unit. Although the PLC’s CPU unit does not directly manipulate the remote I/O unit, due to the periodic refresh of the remote I/O buffer, the PLC’s CPU unit’s read and write manipulation of the remote I/O buffer is equivalent to directly accessing the remote I/O unit. This communication method is simple and convenient, but it occupies the I/O area of the PLC, so it is only suitable for communication with a small amount of data.

2. Global I/O communication mode
Global I/O communication is a serial shared storage area communication method, mainly used for communication between PLCs with link areas.
Draw a link area in the I/O area of each PLC in the PLC network, and each link area adopts a mailbox structure. The sending area and receiving area with the same number are of the same size and occupy the same address range. One is the sending area, and the other is the receiving area. Communicate through broadcast. PLC1 broadcasts the data from the 1 # sending area on the PLC network, and PLC2 and PLC3 receive it and store it in their respective 1 # receiving areas. PLC2 broadcasts the data from the 2 # sending area on the PLC network, while PLC1 and PLC3 receive it and store it in their respective 2 # receiving areas. PLC3 broadcasts the data from the 3 # sending area on the PLC network, while PLC1 and PLC2 receive it and store it in their respective 3 # receiving areas. Obviously, through the above broadcast communication process, the data in each link area of PLC1, PLC2, and PLC3 is the same, which is called the equivalence process. By using equivalence communication, the data in the link area of each PLC in the PLC network is kept consistent. It contains both the data sent by itself and the data sent by other PLCs. Due to the fact that each PLC has the same size of the link area and occupies the same address segment, as long as each PLC accesses its own link area, it is equivalent to accessing the link area of other PLCs, which is equivalent to exchanging data with other PLCs. In this way, the link area becomes a named shared storage area, and the shared area becomes an intermediary for various PLCs to exchange data.

The link area can be refreshed asynchronously (equivalently) or synchronously. Asynchronous refresh is independent of the user program in the PLC, and is broadcast and communicated sequentially by the communication processors of each PLC, repeating the process to maintain equivalence in all link areas; Synchronous refresh is initiated by sending instructions to the link area in the user program, which only refreshes when the data in the link area changes.

In the global I/O communication mode, the PLC directly uses read and write instructions to perform read and write operations on the link area, which is simple, convenient, and fast. However, it should be noted that writing operations on a certain address in one PLC can only be performed on the same address in other PLCs. Like the periodic I/O method, the global I/O method also occupies the I/O area of the PLC, so it is only suitable for communication with a small amount of data.

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