From a device on the same network:
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
81 0b 00 18 01 20 ff ff 00 ff 10 00 c4 02 00 00 f5 22 05 c4 91 00 21 11
ll ll ll ll nn nn nn nn nn nn aa aa aa aa aa aa aa aa aa aa aa aa aa aa
|--7--|--6--|--5--|--4--|--3--|--2--|--1--|--0--|
+-----+-----+-----+-----+-----+-----+-----+-----+
0: | APDU type | PDU Flags | 10 = Unconfirmed-REQ, no more segments follow
+-----+-----+-----+-----+-----+-----+-----+-----+
1: | Unconfirmed Service Choice | 00 = iAm
+-----+-----+-----+-----+-----+-----+-----+-----+
2: | Application Tag | cls | len/val/type | C4 = ojbect id, class = application tag, len = 4
+-----+-----+-----+-----+-----+-----+-----+-----+
3: | Application Tag Data | 02
4: | | 00
5: | | 00
6: | | f5 object = 8 (device), id = 245
+-----+-----+-----+-----+-----+-----+-----+-----+
7: | Application Tag | cls | len/val/type | 22 = unsigned integer, class = application tag, length = 2
+-----+-----+-----+-----+-----+-----+-----+-----+
8: | Application Tag Data | 05
9: | | c4 = Maximum APDU Length Accepted = 1476
+-----+-----+-----+-----+-----+-----+-----+-----+
10: | Application Tag | cls | len/val/type | 91 = Application Tag: Enumerated Value, length = 1
+-----+-----+-----+-----+-----+-----+-----+-----+
11: | Application Tag Data | 00 = Segmentation supported: both
+-----+-----+-----+-----+-----+-----+-----+-----+
12: | Application Tag | cls | len/val/type | 21 = unsigned integer, class = application tag, length = 1
+-----+-----+-----+-----+-----+-----+-----+-----+
13: | Application Tag Data | 11 = Vendor Identifier = 17 = Honeywell
+-----+-----+-----+-----+-----+-----+-----+-----+
Application Tags:| # | Type | Description |
| 1 | Object Id | Device Object Id |
| 2 | unsigned integer | Maximum APDU Length Accepted |
| 3 | Enumeration | Segmentation supported |
| 4 | unsigned integer | Vender Identifier Official List |
Values for Maximum APDU Length Accepted:| 0 | 50 |
| 1 | 128 |
| 2 | 206 |
| 3 | 480 |
| 4 | 1024 |
| 5 | 1476 |
Resources
Bacnet wiki - i-am