DIN measuring bus
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DIN 66348 |
|---|---|
| Area | Data transfer |
| title | Interfaces and control methods for the serial transmission of measurement data; Start-stop transmission, part 1: point-to-point connection, part 2: measuring bus, part 3: application services, telegrams and protocols |
| Latest edition | 1986-09 / 2005-07 / 1998-04 |
| ISO | - |
The DIN measurement bus is one of the field bus systems . The intelligent measuring and automation devices of the fieldbus are controlled centrally ( master-slave system as opposed to multi-master).
If you divide an industrial network into three levels, there is firstly a higher-level bus structure to ensure monitoring functions of the process flows, secondly a process-oriented bus structure such as the DIN measurement bus and, as the third level, a special sensor / actuator bus in which the real-time behavior in is in the lower millisecond range.
General
The DIN measuring bus was created in 1989 as a result of cooperation between the areas of production engineering, the automotive industry and the physical-technical federal institute and was standardized in DIN 66348-2 for bit transmission and security. In 1995 the standard was expanded to include a third part for application services, telegrams and protocols. Part 1 defines basic properties for the serial transmission of measurement data that do not directly affect the DIN measurement bus.
It was designed out of the need for an inexpensive and process-oriented bus solution for measuring and testing technology in industrial companies. In addition, the system is also successful in manufacturing technology such. B. used for production and machine data acquisition, in computer-controlled quality assurance , in statistical process control and also in the networking of programmable logic controllers .
Due to the technical properties of the DIN measuring bus (e.g. separate transmission and reception lines and wiring), it can be used in systems and devices that are subject to legal metrology (calibration obligation by the calibration office ). This includes B. use in tank systems (petrol stations, tank trucks, etc.). This bus system can also be used in flow measurement systems and in weighing technology.
Bus characteristics
The DIN measuring bus system is designed as a 4-wire bus and is based on the EIA-485 interface. The RS 485 is a serial interface . This means that the bits are sent one after the other on one line and not simultaneously over several lines as would be the case with a parallel interface. The RS 485 only defines the electrical properties. B. no pin assignments etc. can be defined. Protocols and pin assignments are used by other standards such as B. directly defined by the standard for the DIN measurement bus. With the DIN measurement bus, the RS 485 interface is a 15-pin D-Sub connector.
Due to the 4-wire design, there is a separate send and receive line, which results in full duplex capability (simultaneous transfer between master and slave in both directions, so that each participant has its own transmitter and receiver). This has the following advantages, among others:
- High bus availability
- High fault tolerance
- Low processor load on the master / control station
- Simple structure of power amplifiers (repeaters) and coupling circuits z. B. for fiber optics or infrared
In the EIA RS 485 standard, the design for the send and receive lines is defined as follows. The data output of the master is wired to all data inputs of the slaves. Conversely, all data outputs from the slaves are sent to the master. According to the standard for the DIN measurement bus, it is necessary to galvanically separate the RS 485 interface from the rest of the circuit in order to prevent effects from potential differences. Shielding prevents interference from magnetic or electric fields.
Bus access procedure
The bus access procedure works exclusively in a controlled manner (deterministic) on the basis of a master / slave system, i.e. This means that the master or the control station as the sole participant controls the communication in the entire system. The prerequisite for this procedure is that each participant has a unique address in the network. The bus availability is further increased by the fact that a defective participant does not block the entire system, but only the receiving line from the master. Should this occur, the master can continue to inform the functioning participants via its transmission line via a broadcast telegram (see DIN measurement bus protocol), whereupon emergency services such as B. the disconnection of the subscriber concerned or, if necessary, maintenance services can be activated or requested. If a data transfer should break off, it can be continued from the last position after a new connection is established. In an emergency, the master can terminate the transmission with the aid of the control character EOT ( E nd O f T ransmission, ASCII character 04).
New or missing participants are recognized by the bus system without interference and do not require a new system initialization. The standard structure of this bus system takes place in the line topology .
Network properties
The system is physically designed for 31 slaves as well as an adjustable data transmission from 110 bit / s to 1 Mbit / s. By linking several modules (cascading), the number of participants can be expanded up to 961 (31 * 31). In order to use the full transmission speed, the main connection line must not exceed a length of 500 m (terminating resistors required) and the supply lines to the participants must not exceed 5 m. The use of repeaters makes it possible to extend the transmission links.
DIN measurement bus protocol
The communication basically takes place in three phases:
- request
- transmission
- graduation
The protocol for the DIN measurement bus enables a high level of data security by checking the data sent by means of a block check character or also referred to as a check or parity bit , confirmation of the message transmission (repeated request in the event of an error) and time monitoring of the arrival of the telegrams.
What is to be emphasized with this protocol is the very short status request to query whether there is readiness to receive or whether data is available for transmission to the master. This means that events can be reacted to in real time (maximum 100 ms). The cyclical query for data that is available for transmission from the slave to the master is called "polling". A participant does not release any data without an explicit request from the master.
A further distinction is made between four transmission options:
- Master to slave (parameterization)
- Slave to master (data)
- Cross traffic under the control of the master
- Broadcast from the master
The master can use a broadcast to inform all participants at the same time.
Bit transmission and data link layer
The transmission takes place using ASCII characters (7-bit code), which means that the usual representation of the interfaces on the measuring devices has been adhered to. A parity bit is also appended to the ASCII code. This additional bit enables error detection during transmission. The code body thus consists of 8 bits. Bits 1–5 assign the transmission to an address, bit 6 indicates whether the address (bits 1– 5) is a send or receive address, bit 7 takes on the same status as bit 6 and bit P represents parity - or check bit. The receiver can now use the parity bit to check whether individual bits were corrupted during transmission. It checks the status of the individual bits in 7-bit code and sets the parity to 1 if the number of ones is even (even parity) and to 0 if the number is odd. After this prompt procedure, the data is transmitted using various control characters that are defined in the standard. The information itself is sent in packets of 128 characters. The connection is then terminated by the final transfer.
Application layer
The application layer of the DIN measurement bus is called QMS (Quality Message Specification) and is based on the established MMS ( Manufacturing Messaging Specification , ISO / IEC 9506 Part 1 and 2) standard . MMS provides the communication / application protocol in industrial automation systems and is responsible, on the one hand, for basic services such as connection establishment, termination, cancellation, order processing, event messages, etc. and, on the other hand, offers a further number of optional services such as program control services, variable services. Communication takes place through a client-server construction. QMS maps this application layer for the DIN measuring bus, which means that higher networks with MMS services can also access the DIN measuring bus directly.