FL-net
From SCADApedia
The FL-net specification, also known as the Open PLC Network 2 (OPCN-2). OPCN-1 connects PLC’s to sensors, actuators and other instruments. OPCN-1 is called a device level network. OPCN-2 connects PLC’s to HMI, DCS and other PLC’s and is called a controller level protocol.
While FL-net was developed in Japan, many non-Japanese control system vendors have developed FL-net modules for their PLC’s to service the market. According to a 2006 Control Engineering survey, FL-net comprises 4% of the controller market in the manufacturing sector.
History
The Field Automation (FA) link protocol specification (FL-net) was developed by the Japanese Manufacturing Science Technical Center (MSTC) in 1999. It was based largely on IEC 8802-3.
FL-net was transferred to the Japan Electrical Manufacturers’ Association (JEMA), adjusted to meet JEMA format, and issued as JEM-1479: Protocol Specification for FA Control network [FL-net (OPCN-2)] in 2000. In addition to JEM-1479, JEMA has issued a specification on conformity testing (JEM-1480) and two technical reports to assist with implementation (JEM-TR 213 and 214).
A significant revision of the FL-net protocol occurred in 2002. In Version 1.0, cyclic frame data and the token were sent in the same frame. In Version 2.0, a new token mode was specified and now tokens and cyclic frame data are sent in two separate frames. The compatibility between Version 1.0 and Version 2.0 devices is not assured, and different compatibility modes and considerations are in Version 2.0 of JEM-1479 and 1480.
Protocol Details
The FL-net specification leverages the cost savings and ubiquity of Ethernet, IP, and UDP and adds reliable and deterministic performance through a token sharing protocol. It creates an equivalent of token ring over Ethernet. The key performance value is 50 ms for transmissions that involve the token.
FL-net is typically used on a LAN to connect a small number, less than 100, systems.
There are two types of messages supported in FL-net.
- Cyclic Transmissions – Sent on a scheduled basis when a node has a token. A node can send up to 17,408 bytes while it has a token.
- Message Transmissions – Sent when an event occurs. Message transmissions are request / response and have a lower performance criteria, within 500 ms.
The FL-net protocol is not a traditional master/slave protocol, although it can be used this way. It was designed as an any-to-any protocol. There are other subtle differences in terminology such as message transmissions use transaction codes (TCD) rather than function codes, but they both serve the same purpose.
A FA link frame consists of a header that is between 64 to 96 bytes and a data up to 1024 bytes. This FA link frame is then encapsulated into a UDP frame. This is a demonstration of how OSI layer terminology can be confusing. The FL-net link layer protocol, typically layer 2, gets encapsulated in a transport layer, layer 4.
The use of the connectionless UDP protocol and a static transmission port are relatively unique features of FL-net. The ports are defined as:
- Transmission – Always UDP/55003
- Reception – UDP/55000 to 55002
- 55000 for token frames and cyclic frames
- 55001 for message frames
- 55002 for trigger frame and participation request frame
Note that the concept of master / slave is not present in these port assignments. Sequence numbers are used to insure frames are in order and prevent duplication.
FL-net is also unique in that it assigns a standard subnet for FL-net use, 192.168.250.x. Typically the last octet in the IP address is made to be the same as the FL-net node number.
The UDP port selection and use of a default subnet make it easy to identify FL-net traffic.
