The two equivalent length bytes in the telegram header of the variable format contain the number of information bytes in the body of the telegram. These include: DA, SA, FC and the PDU. The value ranges from 4 to 249, so that no more than 246 bytes can be transferred to the PDU of a telegram. No value <4 is allowed, because a telegram must comprise at least one DA, SA ,FC and a data byte. The longest telegram therefore comprises 255 bytes overall.
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
LE and LEr: length byte |
4 to 249 |
Length byte |