1、BRITISH STANDARD BS 6556-3: 1985 Incorporating Amendment Nos. 1 and 2 Low speed digital signals for use in coal mines Part 3: Specification for message protocols UDC 622.489:621.398.037.37:681.327.8BS6556-3:1985 This British Standard, having been prepared under the directionof the Mining and Quarryi
2、ng Requisites Standards Committee, was published underthe authority of the BoardofBSI and comes into effect on 31 January 1985 BSI 09-1999 The following BSI references relate to the work on this standard: Committee reference MQE/25 Draft for comment 82/76936 DC ISBN 0 580 14232 9 Committees responsi
3、ble for this British Standard The preparation of this British Standard was entrusted by the Mining and Quarrying Requisites Standards Committee (MQE/-) to Technical Committee MQE/25 upon which the following bodies were represented: Association of British Mining Equipment Companies Council for Electr
4、ical Equipment for Flammable Atmospheres (BEAMA) Health and Safety Executive National Coal Board National Union of Mineworkers Amendments issued since publication Amd. No. Date of issue Comments 6206 July 1990 6328 May 1991 Indicated by a sideline in the marginBS6556-3:1985 BSI 09-1999 i Contents Pa
5、ge Committees responsible Inside front cover Foreword ii 0 Introduction 1 1 Scope 2 2 Definitions and abbreviations 2 3 Link configurations 3 4 Transmissions 4 5 Checks 4 6 Data only protocol (DOP) 4 7 Simple asynchronous protocol (SAP) 4 8 Marking 8 Appendix A Method for the generation and verifica
6、tion of check field (CF) 16 Figure 1 The components of the byte transmission 9 Figure 2 Operation using DOP 9 Figure 3 Operation for 1 to 15 slaves using SAP 10 Figure 4 Application data message (ADM) and broadcast message (BRO) format 11 Figure 5 Link control message (LCM) and initialization messag
7、e (IM) format 12 Figure 6 A 2 byte transmission 12 Figure 7 Transmission envelope from slave with MODEM 12 Figure 8 Slave response to poll (current loop link) 13 Figure 9 Slave response to poll (MODEM link) 14 Figure 10 Composition of message address byte (AB) 15 Figure 11 Composition of application
8、 data description byte (ADD) 15 Table 1 Start of message (SMB) codes 5 Table 2 Valid address bytes 6 Table 3 Stuffing codes 6 Publications referred to Inside back coverBS6556-3:1985 ii BSI 09-1999 Foreword This Part of BS6556 has been prepared under the direction of the Mining and Quarrying Requisit
9、es Standards Committee. Control and monitoring activities below ground in coal mines require the transmission of data between items of electrical equipment supplied by different manufacturers. Data can be transmitted in analogue or alternatively digital form. For analogue, BS5754:1980 “Specification
10、 for electrical analogue and state signals for use in coal mines” was produced with the purpose of promoting compatibility between transducers, recorders, indicators and data transmission systems of different manufacture and having inputs or outputs, as appropriate, in the form of d.c. voltage analo
11、gue signals or signals derived from relay contacts or a mechanical switch. This standard deals with the digital form. A major disadvantage with control and monitoring systems employing the direct transmission of analogue signals is that since a galvanic connection is necessary between the transmitti
12、ng and receiving circuits, faults, at different points of the system, particularly to earth, can seriously affect overall performance. The avoidance of such faults, on pit-wide systems, is extremely difficult. The generation of analogue signals within high voltage switchgear, and subsequent transmis
13、sion for monitoring purposes, also presents difficulty in achieving the required degree of segregation between intrinsically safe and power circuits. A further disadvantage with analogue signals is that the simultaneous transmission of several signals requires either a multicore cable or separate ca
14、bles. These disadvantages can be largely overcome if the analogue or state information is transmitted in the form of serially coded digital signals. With such signals complete galvanic isolation between the transmitting and receiving circuits is possible and, within wide limits, only two wires are r
15、equired for transmission in each direction irrespective of the volume of data. If full benefit of digital signal data transmission is to be realized it is important that transmission systems of different manufacture conform to a common electrical standard and operate in a uniform manner with identic
16、al procedure (or protocol) for handling the data. This standard aims to promote such conformity. It specifies relevant requirements for a low speed digital signal link comprising a master and one or more slaves which interconnects the intrinsically safe circuits of one item of apparatus and the intr
17、insically safe circuits of other apparatus in such a way that units of different manufacture can be interchangeably coupled at the transmission line terminals. This standard is published in three Parts as follows: Part 1: Specification for optical coupling; Part 2: Specification for transformer coup
18、ling; Part 3: Specification for message protocols. Parts 1 and 2 of this standard offer alternative methods of achieving galvanic isolation between the intrinsically safe circuits of interconnected apparatus, but the two methods are not compatible. Part 1 of this standard specifies a600bits/s transm
19、ission system which employs optical coupling as the means of achieving galvanic isolation. A system will comprise a master and from1to8slaves connected in point-to-point or multi-drop modes, with two cable conductors being used for each direction of transmission. Transmission from the master to a sl
20、ave is achieved by the master acting as a switched current source which activates a light emitter at each slave. Transmission from slave to master is achieved by the master acting as both a current source, which is switched at the slave by a light dependent device, and a current monitor which senses
21、 the switched current.BS6556-3:1985 BSI 09-1999 iii Part 2 of this standard specifies a600bits/s transmission system which employs transformer coupling as the means of achieving galvanic isolation. A system will comprise a master and from1to15slaves connected in point-to-point or multi-drop modes. T
22、ransmission between master and slaves is achieved by frequency shift keying (FSK) techniques, one pair of cable conductors being used for each direction of transmission. This Part of this standard defines the message protocols to be used by systems complying with Parts1or2. It does not, however, pla
23、ce any restrictions on the application data contained in transmitted messages, although industry standards may exist to regulate this. It is envisaged that for high voltage applications optical coupling will be employed due to the relative ease of meeting segregation requirements. The optical system
24、, however, has a limit of8slaves and a range of2km while the transformer method has a greater range and can handle up to15slaves. For intermediate applications either system may be suitable. In Parts 1 and 2 of this standard slaves are the physical interface between the transmission line and one or
25、more addressable logical slaves specified in Part 3 of this standard. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immu
26、nity from legal obligations. Attention is drawn to the Health and Safety at Work etc. Act1974, the Mines and Quarries Act1954, the Regulations made under these Acts, and also any other appropriate statutory requirements or byelaws. These place responsibility for complying with certain specific safet
27、y requirements on the manufacturer and the user. The address of the recognized certification authority in the United Kingdom for Group1(coal mining) apparatus for intrinsic safety purposes is as follows: Health and Safety Executive HSE (M) Certification Support Unit Harpur Hill, Buxton, Derbyshire S
28、K17 9JN. Summary of pages This document comprises a front cover, an inside front cover, pages i to iv, pages1to 18, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on th
29、e inside front cover.iv blankBS6556-3:1985 BSI 09-1999 1 0 Introduction 0.1 General Two protocols are covered by this Part of this standard; data only protocol (DOP) and simple asynchronous protocol (SAP). Both protocols use the same element of transmission which consists of11bits as follows (seeFig
30、ure 1): a) 1start bit; b) 8data bits (a byte); c) an even parity bit; d) a stop bit. The even parity bit contributes to error checking of the data. Within each protocol between1and128bytes of application data, each complete with its start, stop and parity bits, can be transmitted. 0.2 Data only prot
31、ocol (DOP) DOP is a very simple protocol limited to point-to-point applications (seeFigure 2). Transmissions consist purely of application data. Separate messages are identified by a short gap. Error detection is limited to parity, framing and, in the case of MODEM systems, carrier checks. 0.3 Simpl
32、e asynchronous protocol (SAP) 0.3.1 General description of SAP. SAP is a set of rules which ensures that order is maintained on the data link between the master and its slaves. The master controls all link activity. There is only one master and from1 to15slaves. The slaves are connected in multi-dro
33、p as shown inFigure 3. The master can send a message to one or all slaves. Slaves may only send messages to the master. SAP allows the use of four message types as follows: Each message begins with a start of message byte (SMB) by which it is identified (seeTable 1). The second byte is always the ad
34、dress byte (AB) which contains the slave address. The LCM and IM consist simply of the SMB and AB and are used for link control purposes only. The ADM and BRO are data messages and contain the application data field (ADF) which may be between1and128bytes in length. The ADF may exceed this length whe
35、re byte stuffing is necessary (see0.3.6). 0.3.2 Normal link operation. A link control message (LCM) is used by the master to poll each active slave in a designated polling sequence. A slave polled by an LCM may respond by transmitting one of the following: a) an application data message (ADM) if the
36、re is application data to send; b) an LCM if there is no application data to send; c) an initialization message (IM) if the slave requires initialization. Reception of a poll is the only condition which allows a slave to transmit a message. Only one slave is allowed to transmit over the link at any
37、one time. The master sends application data to a single slave by means of an ADM or to all slaves by means of a broadcast message (BRO). 0.3.3 Initialization. Initialization of stations is achieved by the exchange of initialization messages (IM). An IM from the master is a poll that is substituted i
38、n the polling sequence in place of an LCM where a slave requires initialization. 0.3.4 Message loss. The protocol includes facilities which detect when an application data message (ADM) has been lost. Each ADM to and from each slave is designated as either ODD or EVEN by assigning unique start of me
39、ssage bytes (SMB) to each type (seeTable 1 ). A master or slave receiving an ADM acknowledges its reception and type (ODD or EVEN) by the use of an acknowledge bit (ACK-BIT) contained in the address byte (AB). The ACK-BIT acts as a toggle which is set and reset only by the reception of a valid in-se
40、quence ADM. Failure of the ACK-BIT to toggle as expected will cause a re-transmission. To ensure that a datum point is established after initialization, messages before the reception of a valid ADM will have the acknowledge bits (ACK-BITs) equal to one and the first ADM to and from each slave will b
41、e EVEN. 0.3.5 Broadcast message. A broadcast facility is provided to allow the master to transmit to every slave. An address is reserved for this purpose. This message is received simultaneously by all slaves and is not acknowledged. a) an application data message (ADM) (seeFigure 4); b) a link cont
42、rol message (LCM) (seeFigure 5); c) a broadcast message (BRO) (seeFigure 4); d) an initialization message (IM) (seeFigure 5).BS6556-3:1985 2 BSI 09-1999 0.3.6 Byte stuffing. As the codes80H,81H,83H,85H and87Hare reserved for link control, it is necessary to ensure that these codes are not transmitte
43、d when they occur in application data. To prevent this, data having the same code as a reserved code is transmitted as two bytes, a stuff byte (STF), reserved value80H, and a byte derived by subtracting80H from the data byte. The receiving station on recognizing80H adds this to the following byte to
44、 restore the original data (seeTable 3). 0.3.7 Message priority and length. Facilities are provided for assigning higher priority to selected ADMs and BROs. This is achieved by a priority bit contained in the applications data description byte (ADD). The remaining bits in this byte are used to repre
45、sent the number of bytes in the application data field (ADF). 0.3.8 Error checking. Additional to the parity, framing and, in the case of the MODEM system, carrier checks within the transmission element, there are two further checks used within the protocol. The address byte (AB) contains its own er
46、ror detection bits so that a slave recognizing its address (or a broadcast) is assured of its validity and can prepare to receive the remainder of the message. The check field (CF) at the end of each ADM and BRO allows the integrity of data to be checked before it is used. This is a16bit cyclic redu
47、ndancy check. 0.3.9 Link failure. Link failures are detected by stations being unable to gain correct responses after re-transmissions. Slaves considered by the master to have failed are polled at less regular intervals defined by the polling sequence. 0.3.10 Poll insertion. To improve link efficien
48、cy, a master transmitting an ADM may insert a poll within the ADM. This facility is used when there is no reply outstanding. An ADM may have many polls inserted. Such polls override the normal order of transmission and may appear anywhere in an ADM including immediately following the SMB or an STF o
49、r between the two bytes of the CF. 1 Scope This Part of BS6556 specifies two message protocols employed with particular low speed digital signals systems for use in coal mines, using either optical coupling (see BS6556-1) or transformer coupling (see BS6556-2). Either protocol can be employed in either transmission system. The protocols require only limited station intelligence for successful implementation. One protocol, known as simple asynchronous protocol, is designed to work in point-to-point or multi-drop configurations. The other protocol,
