IEC 60870-5-102 telegram structure

FT 1.2 frame format is used;
Frames with fixed and variable block length and single control characters are used;
Hamming distance = 4;

Frame with variable length

This frame type is used to transmit user data between controlling and controlled station

byte\bit	7	6	5	4	3	2	1	0
0	Start byte 1 (0x68)								Header		LPCI		LPDU
1	Block length
2	Block length (copy)
3	Start byte 2 (0x68)
4	RES	PRM	FCB / ACD	FCV / DFC	Function code				Control field
5	Link address fields
	(0, 1 or 2 octets)
	Type identification								DATA UNIT IDENTIFIER		ASDU
	SQ	Number of object
	T	P/N	Cause of transmission
	ASDU address fields
	(1 or 2 octets)
	Record address
	Information object address								Info-object

	Information elements

n-1	Checksum								Tail		LPCI
n	Stop byte (0x16)										LPCI

Frame with fixed length

This frame type is used for link layer services. Sometimes used as an confirmation.

byte\bit	7	6	5	4	3	2	1	0
0	Start byte 1 (0x10)
1	RES	PRM	FCB / ACD	FCV / DFC	Function code				Control field
2	Link address
	(0, 1 or 2 octets)
n-1	Checksum
n	Stop byte (0x16)

Single character data

Single characters are used to confirm link layer and user data transmissions.

0xE5 (positive acknowledge);