Versatile Automation Random Access Network
Versatile Automation Random Access Network ( VARAN ) is a wired data network technology for local data networks (LAN) with the main area of application in the field of automation technology . It enables the exchange of data in the form of data frames between all devices connected to a LAN (controls, input / output devices, drives, etc.).
VARAN includes the specifications for cable types and connectors, describes the physical signaling and specifies packet formats and protocols. From the point of view of the OSI model , VARAN specifies both the bit transmission layer (OSI Layer 1) and the data link layer (OSI Layer 2). VARAN is a master-slave protocol . The VARAN BUS USER ORGANIZATION (VNO) is responsible for maintaining the log.
aims
The structure of the system ensures that sensors can be connected directly to the bus and that no sub-buses are necessary. It can be implemented in both FPGAs and ASICs . In order to enable operation with a low software overhead, the bus was implemented as a hardware solution.
history
The company Sigmatek has in 1994 a real-time bus system for connecting the I / O modules of the then-new product series developed DIAS (DIAS-BUS). This bus enables access to individual analog values or 16 inputs / outputs in approx. 7 µs. In the course of time this bus system was integrated into some servo amplifiers. Around 2 million such bus connections have been delivered to date. VARAN is the further development of the DIAS bus based on Ethernet. The activities relating to disclosure and standardization of the system were transferred to the VNO when it was founded on July 19, 2006.
Physical basics
The physical layer is compatible with IEEE802.3.
- CAT5 cables are used, the maximum distance between 2 bus participants is 100 m.
- The physical transfer takes place at 125 Mbit / s using 4B / 5B coding, which results in a data stream of 100 Mbit / s.
- The line coupling takes place via transformers (galvanic isolation).
- Ethernet PHYs and transformers are used.
Working principle
Basics
The basis is a procedure to view the entire I / O area as a large RAM in which the bus manager (e.g. control CPU) can read or write data as required. Each client occupies its own address range in this memory. Only two basic operations are therefore necessary for the exchange of information:
- "Write the following data to address x ..."
- "Read the following number of bytes from address y ...".
This corresponds to the conventional read and write commands of a processor, but with the addition that the number of data to be transmitted can be freely selected between 1 and 128 bytes. Every data transfer is always initiated by the bus manager and answered immediately by the client.
All devices in the network are addressed with a 16-bit address, each with a 16-bit linear address space. This results in a theoretical network size of 65,536 participants, with device addresses from 0xFF00 to 0xFFFF being reserved for configuration purposes. This means that a maximum of 65,280 participants are possible.
With each new protocol packet, the manager sends an incrementing message counter. If the client does not respond within the agreed timeout period or if the response is incorrect, the manager repeats this packet immediately without increasing the message counter. The client thus recognizes whether it is a repetition. The number of repetitions is adjustable.
In the start-up phase, the VARAN manager assigns the addresses to the bus participants.
The cross-Ethernet traffic is tunneled through the bus manager. The transmission of the Ethernet packets in the bus cycle takes place during the time intervals with lower priority.
The basic protocol structure consists of
- SOF (1 byte) - start of frame
- LNG (2 bytes) - length of the log content
- CMD (1 byte) - protocol identifier (e.g. read, write) and message counter
- command-dependent content (variable)
- CRC (2 bytes) - 16-bit CRC
- EOF (1 byte) - End of Frame
Commands
Global Write: All bus participants are addressed with a global write. Among other things, this command is used to assign addresses, to reset the bus subscribers globally and to send the sync command. This is used to synchronize all clients and is sent at the beginning of each bus cycle.
Memory Read: This command reads data from the memory of a bus participant. The command contains the start address and the number of bytes to be read. The client then sends the requested data.
Memory Write: This command writes data to the memory of a bus participant. The command contains the start address, the number of bytes to be written and the data to be saved. The client sends an acknowledgment after receiving the data.
Memory Read / Write: This command allows reading and writing within a single command. This has the advantage that the protocol overhead only has to be sent once, which further reduces the cycle time.
Control Read: This command allows read access to the control area of a bus participant. Information in the control area is e.g. B. calibration data or the nameplate of a bus participant.
Control Write: Write access to the control area of a bus participant
Control Read / Write: Combination of the two commands Control Read and Control Write in one protocol packet with the aim of reducing the overhead.
Foreign Package Request / Response: With this command, foreign packages can be transported through the VARAN bus system. In particular, TCP / IP or packets from other protocols can be sent to or picked up from the corresponding target station with this command.
Hot plug and play
While a machine is in operation, it is possible to add or disconnect bus users from the network without affecting the ongoing real-time communication. It can be defined in the application software whether a participant is to be considered mandatory.
The bus manager
The bus manager coordinates the entire data traffic. The interface to the CPU is designed as a RAM, in which data objects for communication with the clients are created. Each data object consists of a descriptor field and a data field. The descriptor field contains, among other things, the address to be addressed, the number of bytes to be transmitted, the identifier for read and write and the permitted number of retries. It is also specified here what should happen in the event of an error. The subsequent data field contains the data to be sent or received. The bus manager has several priority areas for receiving such objects.
Realtime (RT) objects have top priority. The bus manager sends a SYNC command at the beginning of each bus cycle and then processes the list of RT objects. This processing can be interrupted by IRQ objects. After the RT objects have been processed, the objects in the list are processed for cyclical, but not time-critical data transfers.
Connection technology
Standard connector
A standard network connector ( RJ-45 ) or FM45 (field connector) can be used for applications outside or within the IP-protected industrial environment (IP65) .
Industrial connector
There are certified plug connections for applications in the industrial environment ( protection class IP67) with integrated power supply (24 V).
Bus topology
- Star, tree and line structures can be implemented
performance
Writing the setpoint and reading the actual values for a drive takes about 5 microseconds via VARAN with 16 bytes of setpoint and actual value data. For the use of actuators and sensors there is an update time of around 2 microseconds. As with all real-time Ethernet systems, these times increase by around 1 microsecond per distribution node passed through. A tree structure is therefore preferable when designing a line structure.
The bus participants are connected via standard CAT 5 cables. Crossed or not crossed is irrelevant, as the PHY module provides an auto-crossover in every bus participant. With a transmission rate of 100 Mbit / s, the VARAN bus (without the use of amplifiers) enables cable lengths of up to 100 m for real-time applications.
| Amount of data | Cycle time | comment |
|---|---|---|
| 1 byte | 2.45 µs | z. B. 8 digital inputs or outputs |
| 2 bytes | 2.59 µs | z. B. 16 digital inputs, 1 analog measured value or 1 analog setpoint |
| 16 bytes | 4.55 µs | z. B. 8 analog measured values or setpoint and actual values for 1 drive |
| 128 bytes | 20.23 µs | z. B. 64 analog measured values with a resolution of 16 bits |
Web links
- Varan bus user organization
- Compilation and reports on VARAN FH Reutlingen