CC-Link Data Transfer
When the network is a CC-Link network, 16,384 bits and a total of 4096 words can be used for the exchange of data. The latter amount corresponds to 8162 bytes. One word is composed of 16 bits, and one bit has one of two possible conditions: the value 0 and the value 1. The value 0 of a bit means “OFF” and the value 1 means “ON”. Depending on the system, the value range of a word is from -32768 to 32767 (for systems with sign) or from 0 to 65535 (for systems without sign). Hexadecimal values in the range of 0 to FFFFH can be stored. There are 64 slaves overall, and the bits and the words are distributed uniformly to them. This means that 256 bits and thus 64 words are available to a slave station.
For expansion of the system
In some cases it is necessary that more bits or words are made available to a device. This can be done in two ways. On one hand, the cycle setting can be multiplied. In many cases a doubling is sufficient, but when needed, it also can be increased by a factor of four or eight. These expansions enable additional bits and words per station. The answer data is divided in the various data telegrams, so that each individual station can send more information in a single cycle. In this way, it is possible that one station uses the full 256 bits or 64 words. On the other hand, it is possible that one device can very easily use up to four stations. This permits use of up to 265 bits and 32 words.
A combination of both variants is possible and can lead to an allocation of a total of 792 bits and 256 words.
The various terms with CC-Link
There are two different designation types for bits in such a system. The input bits are designated RX, the output bits with CC-Link are designated RY. Furthermore, there are designations for words: RWr designates words where the contents can be read. CC-Link uses the designation RWw for words where the contents can also be changed. Furthermore, there are additional words and internal bits in each master station, which can be used for diagnosis of a network. The bits there are called link special flag (SB) and the words are called link special registers (SW).
The various station types with CC-Link Data Transfer
The data transfer of CC-Link has different stations.
On one hand, there is the master station. The master station is in charge of the complete management and control of the CC-Link system. Each network has exactly one CC-Link master, which among other things, is responsible for the bits and words being sent to the individual devices in the network.
The local stations are devices sending the bit and word data, as well as messages, to the master stations. The local stations can also send messages directly to other local stations. There are different devices that can serve as local stations. On one hand, they are PCs, and on the other hand it is possible that a local station has a SPS or a device with greater functionality.
A decentralized station is above all responsible for the processing of small word data or bit data. Overall, it corresponds to an I/O module or a special module and exchanges data with the master station. This can be bit as well as word data.
Decentralized I/O stations can only exchange bit data with the master station. They have digital inputs and outputs and are in charge of communication with the master.
The last station type with CC-Link is the intelligent station. This is located between the local station on one side and the decentralized station on the other side. Intelligent stations process information and convert it to analog values. An example of this is the RS232 interface or the positioning module. In the same way as other stations, the intelligent station can exchange bit and word data with the master.It can also send messages to the master.
Link Cycle Times with CC-Link
The cycle times vary with CC-Link. The cycle time changes depending on the amount of required data and the number of connected stations. As a rule, intelligent stations require more data and thus take more time. Simple connections with digital inputs and outputs need more time for the processing of the data. An I/O station with eight digital inputs or outputs requires, for example, 1.2 ms for eight decentralized stations and 3.9 ms for 64 decentralized stations.
With intelligent stations with 32 bits each and a word count of four, the requirements for the input data and the output data on average are: 2.4 ms for eight local stations, 4.8 ms for 24 local stations, and 5.2 ms for 26 local stations.