The CC-Link Data Frame and its format
Data can be used more efficiently with the help of the CC-Link data frame. The standard data length of this frame is 930 bytes. It is important to note that up to 918 bytes of the 930 bytes can be used. With these settings, the CC-Link data frame can reach an efficiency ratio of 98 %, which is very high. This increases the data capacity, the data transfer of the respective stations becomes more efficient, and the transfer speed of the data and information increases.
The message in detail
Looking at a model of a message, it is composed of up to seven elements. Fields marked with F, A1, A2, ST1, ST2, DATEN (data), and CRC are provided for this. The function of each field will be explained in the following:
- Fields with the letter F are found at the beginning and the end of each message. They are designed to show the beginning and the end of a message.
- The field A1 is the address byte of the transferring station. It is the master station sending this message to the receiving station. The master station sends certain data with this message.
- Accordingly, A2 is the address byte of the receiving station. The data here indicates the type of message sent by the slave station.
- ST1 and ST2 (also called status information bytes) indicate the communication status and the data quantity. ST1 is relevant for the communication status between the master station and the slave station. This communication status can be cyclic or acyclic, i.e. it moves with the progress of the system or it indicates special communication between two stations.
- ST2 as the second status information byte indicates the quantity of data to be sent or received between the master station and the slave station.
- Then follows a large field with the name DATEN (data). This field contains all bit and word data transferred during the communication. On the one hand, this is cyclic communication data, but data of the transient communication may also be present.
- The cyclic redundancy check, abbreviated as CRC, can recognize errors in the message, when this is up to 16 bits. With this, the range from the first F-field at the beginning of the data frame to the first bit of the CRC field is checked.
Message of the master station
With a message from the master station, the field DATEN (data) is divided into three main parts:
- RY refers to the stations connected to the master station and can contain up to 256 bytes of output data. With this, it should be noted that each station connected to the network can have 4 bytes in each message, which corresponds to 32 bits. If it should become necessary for each connected station to require more data, it is possible to combine four stations to a single station. This station will then have 128 bits instead of the usual 32 bits.
- The field RWw takes care of the encryption of the data, which takes place according to the same principle as the reading of data in the field RY. 512 bytes is the max. value contained in the data elements as word files. Each station can occupy max. 4 words, which corresponds to 8 bytes.
- The third part of the master station message is relevant for transient communication as well as the acyclic communication. this takes effect when a transient message is to be sent from the master station to a specific slave station. This message also is a part of the normal data frame.
Message of a slave station
When a slave sends a message, the format of this message is similar to that of a message of the master station. Here it is important that the elements of the slave message at the beginning and the end of the message are identical with those of the master message.
- At the beginning is the RX element, which contains up to 16 bytes of bit data reported back. If this message is used by only one station, then only 23 bits or 4 bytes respectively are used as RX information. However, if several stations have been combined, the device again consists of four stations. This device would use 4 times 32 bits per station, i.e. 4 bytes. Overall this would mean that 16 bytes are used per message of the slave station.
- As in the case of the master station, the RWr field takes care of the decryption of the data. It can contain max. 32 bytes of readable data. Of these, a station again can use 16 bytes.
- The differentiation between local and intelligent station has already been mentioned above. With the slave message, there is a third field that offers the opportunity for an answer. This means that when the slave station is an intelligent or a local one, an answer can be added to the message, namely with a transient communication request. This answer can be up to 34 bytes long.