8.1 Alarm Protocol

The formerly described method for an alarm protocol (see diploma work of Andreas Steffens “Eigenschaften und Anwendungen des M-Bus”) was based on time slices for each of the maximum 64 alarm devices. This alarm protocol has not been standardized.

We now suggest to return to standard alarm protocol which conforms to the standard IEC 870-2:

The master software polls the maximum 250 alarm devices by requesting time critical data (REQ_UD1 to adresses 1 .. 250). A slave can transmit either a single character acknowledgement E5h signalling no alarm or a RSP_UD with the CI-Field 71h to report an alarm state.

Fig. 34 Telegram for an Alarm-Respond

The alarm state is coded with data type D (boolean, in this case 8 bit). Set bits signal alarm bits or alarm codes. The meaning of these bits is manufacturer specific.

The timeout for time critical communication must be set to 11..33 bit periods to ensure a fast poll of all alarm devices. With a baudrate of 9600 Bd and all 250 slaves reporting an alarm just in time before a timeout occurs each slave will be polled in periods of maximum 5.5 seconds. This seems to be fast enough for alarms in building control systems and other applications. For faster alarm systems the number of alarm sensors could be limited to 63 (reducing the worst case overall signal delay to less than 1.5 sec or increase the transmission speed to 38400 Bd (with the new repeater hardware) and achieve the same speed for up to 250 devices.

The functionality of the FCB- and FCV-Bit should be fully implemented in this alarm protocol to ensure that one-time alarms are safely transmitted to the master. If the slave has reported an one-time alarm and the next REQ_UD1 has a toggled FCB (with FCV=1) the slave will answer with an E5h signalling no alarm. Otherwise it will repeat the last alarm frame to avoid that the alarm message gets lost.

This new alarm protocol has the advantages of being standardized in IEC 870-2, simple implementation in slaves and master, fast poll cycles and using almost (with the exception of shorter timeout) the normal protocol. In addition it is allowed under EN1434-3 which allows all other types of communication of IEC-870-5-2.

8.2 Coding of Data Records

The standard IEC 870-5-4 defines the following data types for usage inside the application layer:

Type A = Unsigned Integer BCD := XUI4 [1 to 4] <0 to 9 BCD>

Type B = Binary Integer := I[1..X] <-2X-1 to +2X-1-1>

Type C = Unsigned Integer := UI[1 to X] <0 to 2X-1>

Type D = Boolean (1 bit binary information) := XB1 B1[i] <0 to 1>

Type E = Compound CP16 (types and units information)

The following data types can only be used with the variable data structure:

Type F = Compound CP32: Date and Time

Type G: Compound CP16: Date

Type H: Floating point according to IEEE-standard

"Short floating Point Number IEEE STD 754" = R32IEEESTD754
R32IEEESTD754 := R32.23 {Fraction, Exponent, Sign}

Fraction = F := UI23 [1to 23] <0 to 1-2-23>
Exponent = E := UI8 [24 to 31] <0 to 255>
Sign = S := BS1 [32] S<0> = positive
S <1> = negative

F <0> and E <0> := (-1) S * 0 = ± zero
F <¹ 0> and E <0> := (-1) S * 2E-126(0.F) = denormalized numbers
E <1 to 254> := (-1) S * 2E-127(1.F) = normalized numbers
F <0> and E <255> := (-1) S * ¥ = ± infinite
F <¹ 0> and E <255> := NaN = not a number, regardless of S

The following ranges are specified by IEE Std 754-1985 for floating point arithmetics:

Range: (-2128 + 2104) to (+2128 - 2104), that is -3.4* 1038 to +3.4*1038
smallest negative number: -2-149, that is: -1.4* 10-45
smallest positive number: +2-149, that is: + 1.4* 10-45

8.3 Tables for Fixed Data Structure

8.3.1 Measured Medium Fixed Structure

Notes:

1. Record Medium/Unit is always least significant byte first.

2. H.C.A. = Heat Cost Allocator

3. Media from “Gas Mode2” to “H.C.A. Mode2” are defined additionally to EN1434-3 for some existing meters with CI-Field 73h (intentionally mode1), which transmit the multibyte records with high byte first in contrast to the CI-Field. The master must know that these media codes mean mode 2 or high byte first. Further use of these codes for “pseudo media” is not allowed for new developments.

8.3.2 Table of Physical Units

8.4 Tables for Variable Data Structure

8.4.1 Measured Medium Variable Structure

8.4.2 Data Field Codes

Variable Length:

With data field = 1101b several data types with variable length can be used. The length of the data is given with the first byte of data, which is here called LVAR.

LVAR = 00h .. BFh : ASCII string with LVAR characters
LVAR = C0h .. CFh : positive BCD number with (LVAR - C0h) · 2 digits
LVAR = D0h .. DFH : negative BCD number with (LVAR - D0h) · 2 digits
LVAR = E0h .. EFh : binary number with (LVAR - E0h) bytes
LVAR = F0h .. FAh : floating point number with (LVAR - F0h) bytes [to be defined]
LVAR = FBh .. FFh : Reserved

Special Functions (data field = 1111b):

8.4.3 Codes for Value Information Field (VIF)

The first block of the table contains integral values, the second typically averaged values, the third typically instantaneous values and the fourth block contains parameters (E: extension bit).

VIF-Codes for special purposes:

Note:

• Coding the VIF in an ASCII-String in combination with the data in an ASCII-String (datafield in DIF = 1101 b) allows the representation of data in a free user defined form.

8.4.4 Extension of primary VIF-Codes

If the VIF contains an extension indicator (VIF = $FD or $FB) the true VIF is contained in the first VIFE.

a) Codes used with extension indicator $FD

Notes:

• ¹ nn = 00 second(s)
• 01 minute(s)
• 10 hour(s)
• 11 day(s)
• ² The information about usage of data type F (date and time) or data type G (date) can be derived from the datafield (0010b: type G / 0100: type F).
• Ž pp = 00 hour(s)
• 01 day(s)
• 10 month(s)
• 11 year(s)

b) Codes used with extension indicator $FB

8.4.5 Codes for Value Information Field Extension (VIFE)

The following values for VIFE are defined for an enhancement of VIF other than $FD and $FB:

Notes:

• ¹ “Date(/time) of” or “Duration of” relates to the information which the whole data record header contains.
• ² The information about usage of data type F (date and time) or data type G (date) can be derived from the datafield (0010b: type G / 0100: type F).

VIFE-Codes for reports of record errors (slave to master):

VIFE-Codes for object actions (master to slave):

Note:

• The object action “write / replace” (VIFE = E000 0000) is the default and is assumed if there is no VIFE with an object action for this record.

8.5 References

• [1] Färber, G. : Bussysteme, R.Oldenbourg Verlag München Wien, 1987
• [2] Gabele, E., Kroll, M., Kreft, W. : Kommunikation in Rechnernetzen, Springer Verlag Heidelberg, 1991
• [3] Steffens, Andreas : Diplomarbeit “ Der M-Bus - Eigenschaften und Anwendungen”, University of Paderborn, Department of Physics, 1992
• [4] Texas Instruments Deutschland GmbH : Data Sheet TSS 721, 1993
• [5] Texas Instruments Deutschland GmbH : Seminar Material, M-Bus Workshop, 1992
• [6] Ziegler, Horst : Seminar Material, M-Bus Workshop, 1992
• [7] IEC 870-5-1 : Telecontrol Equipment and Systems, Part 5 Transmission Protocols, Section One - Transmission Frame Formats, 1990
• [8] IEC 870-5-2 : Telecontrol Equipment and Systems, Part 5 Transmission Protocols, Section Two - Link Transmission Procedures, 1992
• [9] EN1434-3: Heat Meters, Part 3 Data Exchange and Interface, 1997 §
• [10] Aquametro AG Therwil : M-Bus Automatic Slave Recognition with Wildcard Algorithm, 1992
• [11] Papenheim, Andreas: Diplomarbeit “ Anwendungsbeispiele für den M-Bus”, University of Paderborn, Department of Physics, 1993
• [12] Texas Instruments Deutschland GmbH: Applications Report “Designing Applications for the Meter-Bus”, 1994 (translation of reference [11])
• [13] Ziegler, Horst; Froschermeier Günther: “M-Bus: Die Meßbus-Alternative”, Elektronik 16/1993