CC-Link and its Memory Organization
With the memory organization of CC-Link, data can be exchanged more easily between connected stations. With this, there is a division of the stations into a master station and 64 further stations, also called slave stations, which can be connected. The special feature of the master station is the memory area, which is distributed uniformly under the other stations that can be connected. The capacity of the memory area is 16384 bits and 4096 words (8192 bytes), for memory of data from the network, and it is a central station. This means that one of the 64 stations can store 256 bits and 64 words.
Adjustment of the Memory Information
When a station sends new information to the master station, this then updates the area around the station, so that it corresponds to the state of the station that sent the information. This also applies in reverse: When the master station sends information or data to one of the 64 stations, the station updates itself, so that the internal memory of this station finally assumes the state around the master station, so that both sides are adjusted.
The abovementioned memory of the decentralized stations contains an internal CC-Link memory. It is adapted to the requirements of the operands. An operand is a variable to which a memory function is assigned. In this model, the operands are supported by the stations. The requirements of the operands can go up to 1792 bits and 256 words (512 bytes).
In addition to the decentralized stations, there are also local stations. These local stations serve to exchange system data. The adjective “local” is important as the stations can be active in their system, but not to outside of it. As regards the memory organization of CC-Link, local stations can map the entire memory range of the master station. Accordingly, they require a larger memory, namely 16384 bits and 4096 words (8192 bytes).
Finally, the intelligent stations must be named, they look after the management of the decentralized stations, so that the system can maintain its order and can function.
Finally, the three different communication types of CC-Link must be mentioned. They are divided into test communication, cyclic communication, and transient communication.
Test communication serves to determine whether one or more stations really are part of the system. This is done by testing after the system has been switched on, which stations, decentralized, local, or intelligent ones are integrated into the system, i.e. which ones receive data and information and which do not. Here, it is also important to find out if stations connected earlier to the network may have been removed (for whatever reasons) and then were re-integrated into the system. This means that the reconnected stations are able to send and receive data and information, even if earlier they had been removed from the system for a short time.
The data and information transfer between the master station and the local intelligent as well as the decentralized stations is controlled with the aid of cyclic communication. It is executed with each network cycle, whereby the cycle indicates a periodically occurring event of the same kind or a similar one. Logically, this communication type is used most often and contains various elements that can be executed as commands between the stations. These are
- attribute read
- attribute set
- object-oriented services
- batch reading of station information
Finally, the transient communication is listed here. This communication counteracts the cyclic communication by directing certain data and information a cyclically, i.e. against the normal flow, to specific stations. These stations are specified in advance. So-called interrupts control the transient communication, An interrupt is a short-time interruption of a usual program execution in order to perform another processing, and in most cases the time is short. With the aid of this communication, the master station can exchange information or data with individual preselected slave stations. This appears especially practical if these selected stations do not show this information exchange in the normal cycle. Naturally, this process also functions so that a slave station can exchange information or data with a station selected in this way. Here, a distinction is made between three different types of data transfer:
- First, there is the periodic data transfer between the master and the local and the intelligent stations. With this, it is important that this periodic transfer is not cyclic.
- There are also special transfers taking place only to one station.
- And finally, there are batch transmissions including all stations.
The transient communication is used for polling of system and memory information, for run and stop commands, for line test requests, for reading and writing of memory, and for requesting the system level.
Efficiency of the Data Transfer
The communication types are executed in the listed order. With this, local stations can send large data quantities directly to other intelligent stations without having to go via the master. However, the decentralized stations cannot use the transient communication.