Council Regulation (EEC) No 3821/85Show full title

[F1 [F2 2.2.1. Message structure U.K.

[DDP_002] All the messages exchanged between the IDE and the VU are formatted with a structure consisting of three parts:

• header composed by a format byte (FMT), a target byte (TGT), a source byte (SRC) and possibly a length byte (LEN),

• data field composed by a service identifier byte (SID) and a variable number of data bytes, which can include an optional diagnostic session byte (DS_) or an optional transfer parameter byte (TRTP or TREP).

• checksum composed by a checksum byte (CS).

Header				Data field					Checksum
FMT	TGT	SRC	LEN	SID	DATA	…	…	…	CS
4 bytes				Max 225 bytes					1 byte

The TGT and SRC byte represent the physical address of the recipient and originator of the message. Values are F0 Hex for the IDE and EE Hex for the VU.

The LEN byte is the length of the data field part.

The checksum byte is the 8 bit sum series modulo 256 of all the bytes of the message excluding the CS itself.

FMT, SID, DS_, TRTP and TREP bytes are defined later in this document.

[DDP_003] In the case where the data to be carried by the message is longer than the space available in the data field part, the message is actually sent in several submessages. Each submessage bears a header, the same SID, TREP and a 2-byte submessage counter indicating the submessage number within the total message. To enable error checking and abort the IDE acknowledges every submessage. The IDE can accept the submessage, ask for it to be re-transmitted, request the VU to start again or abort the transmission.

[DDP_004] If the last submessage contains exactly 255 bytes in the data field, a final submessage with an empty (except SID TREP and submessage counter) data field must be appended to show the end of the message.

Example:

Header	SID	TREP	Message	CS
4 Bytes	Longer than 255 Bytes

Will be transmitted as:

Header	SID	TREP	00	01	Submessage 1	CS
4 Bytes	255 Bytes

Header	SID	TREP	00	01	Submessage 2	CS
4 Bytes	255 Bytes

…

Header	SID	TREP	xx	yy	Submessage n	CS
4 Bytes	Less than 255 Bytes

or as:

Header	SID	TREP	00	01	Submessage 1	CS
4 Bytes	255 Bytes

Header	SID	TREP	00	02	Submessage 2	CS
4 Bytes	255 Bytes

…

Header	SID	TREP	xx	yy	Submessage n	CS
4 Bytes	255 Bytes

Header	SID	TREP	xx	yy+1	CS
4 Bytes	4 bytes				] ]

Textual Amendments

F1 Inserted by Council Regulation (EC) No 2135/98 of 24 September 1998 amending Regulation (EEC) No 3821/85 on recording equipment in road transport and Directive 88/599/EEC concerning the application of Regulations (EEC) No 3820/85 and (EEC) No 3821/85.

F2 Substituted by Commission Regulation (EC) No 1360/2002 of 13 June 2002 adapting for the seventh time to technical progress Council Regulation (EEC) No 3821/85 on recording equipment in road transport (Text with EEA relevance).