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.
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.
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.
CC-LINK IE (INDUSTRIAL ETHERNET)
This is a group of open networks that provide and transmit information and data to the control unit and in doing so, provide an integrated solution for automation on many levels. These networks offer a high transmission speed and deterministic communication via single or multiple lines for data exchange with a multitude of products from various manufacturers. Many systems with CC-Link/IE networks are well suited for the controlling of individual machines, production islands or processes in all fields of industry. CC-Link/IE also is successfully used in the controlling of entire installations or factories.
Control and communications link as known as CC-Link was provided by Mitsubishi as its own network for connecting own products with each other for the automation of installations. Because of the large demand, CC-Link was marketed as open networks. The CLPA (CC-Link Partner Association) was founded in order to continuously develop the network technology in an open environment and to further expand it as a successful solution for industrial application sectors.
Currently, there are four versions of CC-Link networks, which supplement each other and cover different aspects of communication. Standard CC-Link is an industrial network that transmits control data as well as information at a high speed and in doing so, ensures successful installation or process automation. This network on the control and manufacturing level connects a wide range of devices with high transmission speed and deterministic exchange of data via a single line. CC-Link is available in version 1.1 and version 2.0. These mainly differ from each other in the number of data that can be exchanged between two stations. The versions can be operated together in a network. CC-Link/LT is a bit-oriented system that was created for the use of actuators and sensors. It reduces and simplifies the work involved in connection of devices with the switch cabinet. Errors during connection are prevented by profiled lines that prevent incorrect polarity, and the installation time is minimized. 3-wire lines can also be used. CC-Link/LT is based on CC-Link technology. CC-Link Safety meets the requirement of recognising errors that could lead to a fault. This function simply brings machines immediately to a safe condition when a communication error is recognized. This is realized by the addition of a level with the safety function to the top level of the CC-Link protocol, which recognizes each error during the transmission. CC-Link Safety is an expansion of the normal CC-Link and it enables the connection of standard CC-Link devices together with CC-Link Safety devices to the same networks. A CC-Link safety master module must be used as the master module. CC-Link Industrial Ethernet (IE) uses Ethernet technology for seamless exchange of data on all levels of an installation, from the top level down to the manufacturing level. CC-Link IE exists in different versions as a field bus or as a control network, but frequently is based on CC-Link. CC-Link IE was developed for the frequent exchange of many quantities of data in an industrial or production installation. In contrast, CC-Link IE Field was improved for linking various devices and their connections to different, existing networks, e.g. Standard CC-Link.
Characteristics of CC-Link/IE
Exchanging of devices during operation using CC-/IE: Networks can be added quickly into the existing network, without having to stop or reset the original network. This reduces repair and maintenance down-times in the CC-/IE of production areas / sections to a minimum. Furthermore, high speeds are possible. The CC-/IE network enables the simple exchange of data and communication with inputs and outputs that is independent of transmission times. This is provided by the transmission of one gigabit per second and the real-time protocol. This is currently the fastest possible speed for the industrial sector. Transmission within media over the network in order to manage devices is also possible, as is the simple transmission of control data. In most cases, existing knowledge concerning the connection of an Ethernet network, equipment, and tools can be used for a CC-Link/IE network. The simple setting and diagnosis can be done without prior knowledge of the internal processes of the Ethernet. This minimizes the total costs from the initial set up, through to commissioning and repair. Furthermore, because of the different available structures, such as bus topology or mixed forms of these two network forms, it is always possible to provide a very good network for diverse configurations. The next characteristic to be mentioned is the fact that the diverse devices can be operated simply and without network knowledge as a result of the common storage technology. In the case of the CC-Link/IE network, the configuration software can access systems on the manufacturing level across the entire CC-Link hierarchy. Devices can be added to the network or maintained from any place. This characteristic maximizes efficiency during planning. A star-shaped network does not necessarily require complex switches with management characteristics, but cheaper switches without management functions also can be used. A bus or ring characteristic does not use hubs.