LIAN 98(en) : Protocol L&G SCTM, Telegram structure

SCTM : Telegram structure

Each telegram contains one so called header that includes the information needed for the controlling of the communication. If necessary a data block including the process information is added to the header. Header and data block are each for itself with a block check character ( BCC/ HCC ) protected against transmission errors.

Telegram header

The Header telegram without data block is used from the central station ( ZS ) for the initialization of the communication or from the ZS as well as from the substation ( US ) for the acknowledge of data blocks.

There are header telegrams with four different lengths depending on the number of US-NO bytes.

Header length = 8, number of US-NO bytes = 0
Header length = 11, number of US-NO bytes = 3
Header length = 13, number of US-NO bytes = 5
Header length = 16, number of US-NO bytes = 8 ( US-No. + reflection )

The number of station fields is a system parameter. A dynamic change over between the different headers is not allowed.

Start of Header ( SOH ) :

Each SCTM telegram starts with SOH ( 01H ). This is always the first sign.

Status :

AUZ	Number of station characters : Status ( binary ) = 0011 0000 -> US no. has an odd number of characters ( 3 or 5 ) Status ( binary ) = 0110 0000 -> US no. has an even number of characters ( 0 or 8 )
PR	Priority bit : 0 = Normal telegram 1 = Telegram with high priority Telegrams with high priority are used for short urgent information especially for the time synchronization provided that it is not transmitted as broadcast command.
F	Following bit : 0 = Single telegram 1 = A further telegram follows to the same data block. Data blocks that would exceed the max, data block length are distributed on several telegrams. For the identification of belonging together telegrams will, except in the last telegram, the F-bit set.
Z / U	Direction bit : 0 = Telegrams from the substation ( monitoring direction ). 1 = Telegrams from the central station ( control direction ).

The status character can assume the following values.

Control direction :

a c e g

for number of US characters = 0 or number of US characters = 8

1 3 5 7

for number of US characters = 3 or number of US characters = 5

Monitoring direction :

'   b d f

for number of US characters = 0    or number of US characters = 8

0 2 4 6

for number of US characters = 3    or number of US characters = 5

Station number and reflection bytes :

A device number is assigned to each substation unit as device address. With this address the single substations can be called and allocated from the central station.

The station field consists by default of 3 numbers ( 000...999 ). Optionally station fields with 5 or 8 numbers are used. The station number can be omitted for US telegrams and/ or ZS telegrams whereby the lengths of the header is reduced to 8 characters ( without ETX or STX ).

US-No. = 0 indicates a "Broadcast command" ( substations ).

Block number ( BL no. ) :

Data block sequence number : ( modulo 10 ) 0... 9 or " > "

The BL-No is always increased by 1 when a data block telegram is transmitted. Data blocks in each send direction are counted continuously independent one from the other. When a header is transmitted without data block the BL-No remains unchanged. The BL-No are set to ZERO with the initialization command ( INITCOM ) of the central station and so the communication with the US is re-initialized.

The BL-No can also be used as "WAIT-FLAG" and includes then the character " > " ( 3EH ).

Quittance number ( Q no. ) :

Data block acknowledge : ( Modulo 10 ) 0... 9 or " ? "

The Q-No contains the number of the data block to be acknowledged and is like the BL-No set to ZERO with the initialization command ( INITCOM ) of the central station.

The Q-No includes the character " ? " ( 3EH ) in the initialization command.

Data block length ( DBL ) :

The DBL indicates the length of the data block in the telegram. The characters STX, ETX and the BCC are counted. The maximum length of the DBL is 256 characters.

Header Check Code ( HCC ) :

The HCC is the length parity completed to even over all header bytes exclusive the control characters SOH, ETX respectively STX and without the HCC byte. The HCC character accepts values in the range ( binary ) of 0011 XXXX and 0101 XXXX which do not effect any control function and therefore do not disturb the procedure.

End of Text ( ETX ) :

The control character ETX ( 03H ) concludes the header, if no data section is available. If a data block is available the ETX will be replaced by STX and initiates the begin of the data block.

Telegram with header and data block

The telegram with data block is used for the transmission of the user data between the controlling ( ZS ) and the controlled ( US ) station and in the opposite direction.

Start of Text ( STX ) :

The control character STX ( 02H ) concludes the header, respectively indicates the begin of the data section in the SCTM telegram. The STX is counted in the data block length ( DBL ).

End of Text ( ETX ) :

The data section of a SCTM telegram or the header of a telegram without data block is ended with the control character ETX ( 03H ).

Block Check Code ( BCC ) :

The BCC is the length parity over all data block bytes completed to even, exclusive the control character STX and inclusive ETX. The BCC character accepts values in the range ( binary ) of 0011 XXXX and 0101 XXXX, which do not effect any control function and therefore do not disturb the procedure.

Data block structure in control direction

Information type character ( IAC ) :

A telegram in control direction is identified by 1 or 2 IAC characters.
Possible values of an IAC : E1... E8, D, P, I, N, S1... S3, T1... T4

Ring buffer enquiry ( BUFENQi, UPDATEi ) :

buffer no.	description
01	period 1 ( MP 1 )
02	period 2 ( MP 2 )
03	reserved for period 3 ( MP 3 )
04	reserved for period 4 ( MP 4 )
05	daily buffer
06	reserved
07	weekly buffer
08... 10	reserved
11	monthly buffer
...	reserved
21	PPR buffer ( data of the last period )
...	reserved
51	spontaneous buffer 1
52...	reserved for further spontaneous buffers

Data block structure in monitoring direction

The telegram circulation is controlled by the master station ( ZS ), i.e. the substation ( US ) only replies on inquiry. The analysis of data blocks in monitoring direction is only possible concerning the command sent before in control direction.

Type identification ( TypeID )

IAC	direction	function : parameter, ... - description	TypeID
--	con	INITCOM	1
--	con	ZS quittance	2
E1	con	TABENQ1 : table-address	3
E2	con	TABENQ2 : begin-address, end-address	4
E3	con	TABENQ3 : special IAC ( IACAi )	5
E4	con	BUFENQ1 : buffer number, time-pointer	6
E5	con	NEXTi : buffer number	7
E6	con	BUFENQ2 : buffer number, begin-pointer, end-pointer	8
E7	con	BUFENQ3 : IACAi, Measure-period-No., Measure-period, counter-No	9
E8	con	UPDATEi : buffer number	10
N	con	NEXT	11
S1	con	TABSET1 : table-address, table-value	12
S2	con	TABSET2 : begin-address, end-address, table-value, ...	13
S3	con	BUFSET : data ( length and format device specific )	14
I	con	IDENT	15
D , P	con	IACPASSi : password	16
B0.. B9	con	IACCMD : command number, copy of command number	17
T1	con	SETTIME : date, day of week, time The SETTIME command sets a time register in the substation that will be taken over from the device clock with the following sync command "SSYNC".	18
T2	con	SSYNC The time synchronization is only accepted from the substation if a SETTIME was received.	19
T3	con	HSYNC The hour synchronization command is accepted from the substation without an additional SETTIME command but only to device specific set hours. Is the difference between the device clock and the HSYNC command more than +/- 29 seconds this command is discarded.	20
T4	con	MSYNC The minute synchronization command is accepted from the substation without an additional SETTIME command. With the receipt of the command from the substation the seconds register is set to ZERO. If the seconds register in the. Is the second register in the field from 30 to 59 seconds, the minutes register increases additionally by 1.	21

--	mon	Request INITCOM	129
--	mon	US quittance	130
--	mon	Response TABENQ1 : table-value	131
--	mon	Response TABENQ2 : table-values, ...	132
--	mon	Response TABENQ3 : data ( format pre-programmed under IACAi )	133
--	mon	Response BUFENQ1 : pointer, device-state, counter-no, cnt-value, cnt-status	134
--	mon	Response BUFENQ2 : begin-pointer, end-pointer, device-state, counter-no., cnt-value, cnt-status, counter-no, ...	135
--	mon	Response BUFENQ3 : pointer, device-state, counter-no, cnt-value, cnt-status	136
--	mon	Response UPDATEi : begin-pointer, end-pointer, device-state, counter-no, cnt-value, cnt-status, counter-no, ...	137
--	mon	Response IDENT : ident text	138
--	mon	Response SPOINF1 :	139
--	mon	Response SPOINF2 :	140

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Manual LIAN 98