SCTM Protocol

SCTM (Serial Coded Tele-Metering) protocol or IEC 60870-5-102 represent a protocol that assures the communication between measuring instruments, (energy meters, water or gas meters) placed in sub-stations or home and a central acquisition station that could be the computer of regional utilities distributor.

Introduced by Landis & Gyr on data concentrators and energy counters, the protocol is base on telegram transmitted between above-mentioned elements placed in a network (often the bus is a RS485/X.27 bus).

Having, like example, the OSI model, SCTM define all the seven layers of the protocol and can transfer data using telegrams with a variable length. In figure we present the structure of this:

Layer 1 (Physical Layer): describe the transmission medium that can be: 2, 4 wires line, dedicated line PSTN (Public Switched Telephone Network) system lines, radio networks, GSM (Groupe Speciale Mobile) /GPRS (General Packer Radio Services) network. The physical net configuration can be: point to point or line (bus).

Layer 2 (Link layer): manage the link between the elements that communicate together. This layer is responsible for serial/parallel conversion, frame synchronization, detection and correction of errors, monitoring of signals quality, recognition of station address, generating of check codes, handling of length telegram, error recovery, labeling of data blocks, channel switching.

Layer 3 (Network layer): implement the handling of messages priority, handling of broadcast messages, routing messages.

Layer 4 (Transport layer): is not use on SCTM protocol.

Layer 5 (Session layer): establish and breakdown data connection if this is realized through the public lines (PSTN, GSM/GPRS).

Layer 6 (Presentation layer): assure the data format that will be delivering for user.

Layer 7 (Application layer): describe the types of information, query strategies, command set for inquiry of data, password for user layer. The protocol manages single commands and special control commands and test telegrams and is think to transport metering data and tariffs.
In the figure is presented the structure of header the central stations telegram:

7	6	5	4	3	2	1	0	Bits
SOH								Start of Header
CS bit, Priority bit, Following bit								Header status bits
								Sub station number between: 000- 999
								Data Block Number
								Acknowledge No.
								Length of data block (0 ... 255)
								Header Check character
								Terminator ETX/STCX

Figure 5.9 Header structure of the central station telegram

Block number is a counter of numbers of data block transmitted after a initialization telegram. This number is calculated that the total block numbers modulo 10.
The header is fixed length that can be: 8, 11, 12 or 16 ASCII characters, exclusive STX or ETX.

Query number represent a digit that is calculated like rest of division of the total numbers of queries make by central station by 10 (Query number modulo 10).The structure of data block includes:

Figure 5.10 The structure of data block

Where STX and ETX are the Start Of Text and End Of Text characters, IFC represents one of the commands accepted by protocol (e.g. TABENQ1, BUFENQ1i, POINTER,NEXT, UPDATEi, TABSETi, BUFSETi, IDENT, SETTIME, TIME), data are the operands of commands, and BCC is the data Block Check Code.

Finally we present the mandatory telegram that is issue by the central station for initialize the communication with sub-station, and the response from sub-station these are INITCOM telegram:

Table 5.6 INITCOM telegram

1	2	3...7	8	9	10..12	13	14
SOH	STA	nnnnn	BLN	?	000	HCC	ETX
		RTU no.			DBL
Header

Table 5.7 The response from sub-station

1	2	3...7	8	9	10..12	13	14
SOH	STA	nnnnn	BLN	0	000	HCC	ETX
		RTU no.			DBL
Header

BLN (Block Number must be 0), Byte 9 represent the number acknowledges and must be 0. HCC represent the Header Check Character.