Measurement Bus
Characteristics and Areas of ApplicationThe Measurement Bus arose out of
cooperation between manufacturers in the field of
manufacturing quality measurement, users from the car
industry and the "Physikalisch-Technische
Bundesanstalt" (Federal German Authority for
Testing, Calibration and Certification) as a typical user
standard. It was standardized in September 1989 as DIN
66348 Section 2: "Interfaces and Control Procedures
for Serial Measurement-Data Transfer, Start-Stop
Transfer, 4-Wire Bus". Range of Applications
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Reliability and Failure Tolerance: The design of the physical characteristics and the data link layer of the Measurement Bus ensure high transmission reliability, even in cases of severe electromagnetic disturbances; it can also detect faulty data blocks. |
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Availability: This is proven by the failure probability of a bus system. The Measurement Bus, which is a four-wire bus, is the only field bus to have separate transmit and receive lines. |
| - Costs: Installation and interface costs for measurement-bus users are low. Both the hardware costs and the development expenses are in line with those of the widespread serial interface in accordance with RS-232-C or V.24, decoupled. Any commercially available PC can be used as a master station, and simple signal level converters and gateways enable the user to connect older equipment already in use. |
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Flexibility and User Friendliness: It is simple to install and to put into operation, and in addition maintenance and fault removal are available in cases of failure or breakdown - these factors are particularly important to the user. The demands made of staff are not any higher than for other tasks in industrial electronics systems. Planning and implementing a system with Measurement Bus users can be done and modified quickly; the master station controls the entire bus centrally and can intervene at any time. At the same time the master station is the network service access point for higher-level networks (MAP [manufacturing automation protocol], MMI), which can access the measurement bus network at any time. Long branch lines and the possibility of extending the network as much as is desired allow the user to arrange the equipment without any spatial restrictions. |
Table 1: Application characteristics of Measurement Bus systems
Criteria for manufacturers or users
to decide to use the Measurement Bus are, in addition to
the capacity to effectively transmit measurement data and
parameters in the areas of application mentioned
previously, further basic requirements of industrial
practice which the Measurement Bus takes into account. In
particular medium-sized companies in the fields of
manufacturing and process engineering, but large
manufacturing companies as well, such as those in the car
industry with man-machine-interface (MMI) networks these
form the Measurement Bus's target group.
Description
The Measurement Bus is
distinguished from other field buses by its use of full
duplex transmission. This results in very high bus
availability and fault tolerance. Users with physical
defects or with protocol errors not only block the entire
bus system, but also the user transmission line to the
master station. Using the free reception line, broadcast
messages can be sent arranging emergency services for all
those users not disturbed, and maintenance can be
requested via special signalling stations. An advantage
of full duplex technology is the low loading of the
processor and the simple construction of line repeaters
and coupling devices for other transmission media
(infrared channels, optical fibres).
| Areas of
Use: -
manufacturing quality measurement, quality
assurance, process control |
| Physical
Characteristics: -
transmission with a differential voltage signal
in accordance with EIA RS 485 |
| Measurement
Bus protocol: -
centrally controlled medium access procedure
(master-slave) for up to 31 users |
Table 2: The most important characteristics of the Measurement Bus
Users can be switched on or off the bus without feedback; an interrupted data transfer is continued without any problems after restoring the connection. Flexible bus management detects those users who have joined or who are absent; no disturbance is caused, nor is there any need to re-initialize the system.
One particular feature of the transmission protocol is the very brief status interrogation check, which the master station can use to determine whether a device has data to be transmitted.
This is particularly useful for fast event processing (fast reaction for example to limits being exceeded). The Measurement Bus thus has a disproportionately high interrogation rate.
The standard master-slave structure of the Measurement Bus (Figure 1) results in very clear applications, as the application programs are only run in the master. A further advantage of this structure is the particular options provided by bus management. The bus users can be served at different frequencies during polling, thus yielding large degrees of freedom when setting reaction times.
Figure 1: Bus structure as in DIN 66348 Part 2
The hierarchical construction linked with the master-slave structure allows easy inclusion of a Measurement Bus network into other networks (LAN, WAN). The master acts as a gateway, a feature which can be particularly easily implemented for networks with MMS (Manufacturing Message Specification), since the application layer to the Measurement Bus, which is scheduled to appear as DIN 66348 Part3 in 1994, is based on MMS. In addition to seven so-called basic services (setting up a connection, clearing a connection, breaking off a connection, job processing, job interruption, event and sequence error reporting), three user service groups (variable services, memory area oriented services and services for program control) and the three general services, "status", "identification" and "transmit list of names" are described. As only three of these lists are obligatory, both extensive applications and simple sensors can be achieved with only one application service, eg "read variable".
The coding is also in ASCII format.
Part 4 of DIN 66348, on which work
is in progress, will define metrological variables,
commands and functions which allow easy control of
measurement functions, the transfer of measurement
parameters and the acquisition and representation of data
to be transmitted for various applications. Several
companies have already managed to include Measurement Bus
equipment in standard measuring programs such as
LabWindows or TestPoint.
Distinction to other bus systems
The Measurement Bus was designed for the reliable and low-cost communication of equipment for the measurement, control and acquisition of process and operational data. Like many other field buses, it is less suited for the time-equidistant detection of highly dynamic processes ("scanning") with very short data records. This task can only be carried out in an optimum way by means of special sensor/actor bus systems. The Measurement Bus should also be distinguished from more costly but fast processor-processor networks, in which the random access of all users and the transmission of large amounts of data are required. But the transition to networks of this kind (eg MAP/MMS) can be achieved in a simple way using the master station as a gateway.
Relevant topics such as conformity testing, the MMS application layer, Measurement Bus chips, the producing of tutorials, and public relations are discussed in five working groups in the ADM. There is close cooperation with the DIN standards committee responsible. Further developments and research on the Measurement Bus are being carried out at the WZL of the RWTH Aachen, at the Chemnitz-Zwickau Technical College and at the University of Hannover.
Contact:
Association of Measurement Bus Users (ADM e.V.)Appelstr. 9A
D-30167 Hannover
Germany
Tel: +44-511-762-4673
Fax: +44-511-762-3917
e-mail: wagner@geml.uni-hannover.de
Internet Informations:
http://www.tu-chemnitz.de/home/uku/FAK/FertMessTech/dmb.html
http://www.infoside.de
