Copyright © IFAC Distributed Computer Control Systems TaJlinn . U.SS.R. 1982
STANDARDIZATION WORK FOR COMMUNICATION AMONG DISTRIBUTED COMPUTER CONTROL SYSTEMS G. G. Wood European Market R esearch & Produ ct Planning, FoxboTO Yoxall, R edMII. Surrey RH1 2HL, UK
ABSTRACT: The current position of some standardization groups working in local area communications are discussed and compared with the needs of Distributed Computer Control Systems in industry. The standards groups are International Electrotechnical Commission (IEC), International Standards Organization (ISO) and Institute of Electrical and Electronic Engineers (IEEE) • The paper then introduces the following issues which may affect further evolution of the work on these standards : (1)
The need and direction levels.
for
services and
protocols
(2)
The impact of fibre optics which favour a change from the passive "T bus" to a network of point-to-point links for communication.
(3)
The economic arguments for the application areas with numerically small markets to adopt subsets of the standards developed for 1arge markets.
KEYWORDS: Local Area Networks, Communications, PROWAY, 625. Industrial Data Highways, Communication sub-systems.
in the higher
IEEE 802 ,
IEC
Disclaimer. This paper is not an official statement by any standardization body. The information is drawn from individual discussions and working documents which are subject to change, consequently it should not be relied upon for decisions about product design or commercial activity. The opinions are those of the author and do not necessarily represent his employer, any other committee member or their sponsoring organizations. The author has been a member of the IEC Working Group on PROWAY since its inception and Chairman of the Coupler Sub-Group. He is a liaison member of the IEC Working Group on Interface Systems for Programmable Measuring Ins t rument s. I NTRODUCTI ON
of these evol vi ng standards coul d support industrial Distributed Computer Control These applications are Systems (DCCS). characteri sed by up to 100 mi croprocessor based stations separated by distances up to 2 KM. Such systems are the industrial equivalent of a Local Area Network (LAN).
The age of the microprocessor has arrived and microprocessors are being interconnected for many different application purposes and environments. The most critical factor in these multi-micro systems is the communication structure. User concern for this subject is reflected in committees seeking to produce standards for communication among distributed microprocessor based devices. These standards are unusual in that they seek to define a standard before industry usage has created a "de facto" standard which can then be tidied up and documented. Several
This paper covers the author's view of the present status among standards which could support a DCCS or Industrial LAN in the future:- Process Plant Communications (PROWAY 1 and Process Application formats)
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G.G. Wood
- Laboratory Communications: (Serial extension of IEC 625-1)2 - Office Co~munications: (IEEE 802 and Ethernet)4 - Post and Tekegraph Communications (CCITT X 25)
The 625-Serial and IEEE 802 groups are working on several standards with various optional choices in Layers 1 and 2. Some versions will meet all the criteria of Table 1 and others which meet only criteria 1, 2 and 3.
Following a summary of present status among these standards, the paper di scusses some areas which are not covered by the committees and some of the trade-off decisions which must be made.
The X25 standard includes Network Layers of the OSI model and can support criteria 2 to 5 of Table 1. Multi master is not easily provi ded by X25, however a number of manufacturers have developed products using X25 components with non-standard enchancements to provide multi master capabi 1 ity.
PRESENT STATUS OF STANDARDS The I EC PROWAY Commi ttee has defi ned functional requirements for an industrial data highway. Table 1 lists some of these requirements and reasons for their choice. 1.
2.
Mul ti pl e master stati ons sharing the highway, with passive connections from each station to the data highway. Thi s ensures that loss of one station or loss of power in one part of the plant will not interrupt communication among other plant areas. Single wire, serial, multi-drop connection among stations. This gives a simple message path and economi c benefi ts in cabling and connections. Options include redundant paths.
3.
Variable length message frame with no restriction on the bits and bytes contained in a user's application message.
4.
Operation of the communication system must be deterministic to support calculation of worst case access time and guaranteed response figures.
5.
Automatic acknowledge, within the communication sub-system, for each message sent. Thi s is a safety feature to support fast faul t recovery, ensure messages are not lost and keep them in sequence. TABL E 1. GU IDELl NES FOR GOOD INDUSTRIAL DCCS
The PROWAY standards group is working to meet the Table 1 criteria by defining a standgrd for Layers 1 and 2 of the OSI model •
AVAILABILITY PROWAY and 625-Serial are still in the definition stage and products meeting these standards are not expected in the market place until 1985 or later. IEEE 802 is in the final stages of definition and products for some of its subsets are now available in the market place. DEVELOPMENT FEATURES
OF
UPPER
LAYER
The application of small DCCS projects will involve a single, local bus. In such a case the functi ons of a network 1 ayer are not needed. Al so for small systems the maximum frame length and task or session connections are usually fixed at installation time and absorbed into the appl i cati on programs. The small DCCS system effectively has its application layer directly interfaced to the Link Layer. larger projects with multiple For independent busses, Network and transport layers are needed to free the application from concern about message length, absolute addresses and connections. For ease of system expansion from a small DCCS to a 1arge Network DCCS the interface logic or state machines should be the same for each interface above the Network layer. If this goal is achieved, then low cost single bus applications can be built without the intermediate layers (345). However, they can easi ly be expanded into larger networks by inserting the extra layers and changing the application address references from absolute to logical. PASSIVE "T" AND FORWARD
CONNECTION
OR
STORE
The industrial community have requested passive "T" junctions to increase the integrity of their DCCS installations.
Communication Among Distributed Computer Control Systems
PROWAY, Serial-625 and IEEE 802 each embody versions which meet this aim. However, this requirement will cause problems when optical data links are applied in industry. The technology of optical point-to-point data links is now well established and offers significant opportunity to eliminate such traditional problems as: electrical interference lightning effects ground loops spark hazard in explosive environments
In some situations a DCCS requires links through the PTT for a remote sub-system or management computer. The use of X25 adaptations should simplify such interaction with PTT systems. REFERENCES AND NOTES 1.
Passive "T" optical connections are not expected to be economic in the near future so other methods must be used to ensure link integrity when power fails at a station. Three solutions can be considered:i)
Battery bac kup to sustain the active retransmission elements in each station.
ii)
Optical switching relays which bypass the station active elements when power fails.
iii)
Multiple pOint-to-point links glvlng a network solution for rerouting and bypassing a failed station.
The choi ce among these wi 11 depend on a ba1 ance of fi rst cost and installed mai ntenance cost. Future development of LSI components for Networking Protoco1s will probably favour the third solution.
Industrial Uses (e.g. PROWAY) 10 5 Units per year Laboratory and Li g~t Industry (e.g. Serial-625) 10 units per year. Office Automation (e.g. IEEE 802) very 1arge PTT and CCIlT (e.g. X25) very large. TABLE 2.
MARKET ESTIMATE FOR DCCS STATIONS
For economic reasons the development of PROWAY and Serial 625 standards must consider adaptions of X25 or IEEE 802. This allows the economics of large scale productions.
PROWAY is being developed by the International E1ectrotechnica1 Commission Technical Committee 65C, Working Group 6. This group was previously named TC65A, WG6 and drafts parts of PROWAY were circulated for national comment by TC65A. Another working group TC65C WG1 is working on standards for message formats in industrial data highways.
2.
International E1ectrotechnica1 Commission Technical Committee 65C, Working Group 3. This group was previously named TC66 WG 3 and defined a short distance, bit serial, byte parallel data bus which IEC has been pub1 ished as an standard number 625. This group is now investigating a long distance, fully serial bus which will have maximum compatabi1ity with 625. This paper refers to the proposed serial extension as "625-Seria1 ".
3.
Institute of Electrical and Electronic Engineers, Project 802 . Draft. documents were pub1 i shed for comment at the end of 1981. The drafts include many sub options some of which are very close to the Ethernet standard.
4.
The Ethernet, A Local Area Network: Data Li nk Layer and Physical Layer Specification. Jointly published by DEC, Inte1 and Xerox Corporations. A1 so referred to as the DIX Data Highway.
5.
International Telephone and Telephony Consultative Committee. Recommendation X25 covers physical, Data Link and Network functions.
6.
Internat i ona 1 Organization for Standa rdi zat ion ISO TC97 SC16 "Reference Model for Open Systems Interconnection". ISO Draft Proposal 7598.
ECONOMICS Table 2 gives an approximate estimate of the market for station couplers and logic in DCCS applications.
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