Difference between revisions of "BACnet MS/TP"

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{| border="1" cellpadding="10" cellspacing="0"
 
{| border="1" cellpadding="10" cellspacing="0"
 
|+ align="top" | '''MS/TP Frame Format'''
 
|+ align="top" | '''MS/TP Frame Format'''
|0x55 || Framing 1
+
| Octet || Function || Comment
 
|-
 
|-
|0xFF || Framing 2
+
|0x55 || || Framing 1
 
|-
 
|-
|Frame Type ||  
+
|0xFF || || Framing 2
 
|-
 
|-
|Dest MAC ||
+
|Frame Type ||  ||
 
|-
 
|-
|Src MAC ||  
+
|Dest MAC || ||
 
|-
 
|-
|Len MSByte ||  
+
|Src MAC ||  ||
 
|-
 
|-
|Len LSByte ||  
+
|Len MSByte ||  ||
 
|-
 
|-
|Header CRC ||  
+
| Len LSByte ||  ||
 
|-
 
|-
|Data || This is the [[NPDU]], if len non-zero
+
| Header CRC ||  ||
 
|-
 
|-
| Data CRC MSByte || if len non-zero
+
|                || Data || This is the [[NPDU]], if len non-zero
 
|-
 
|-
| Data CRC LSByte || if len non-zero
+
| Data CRC MSByte || || if len non-zero
 
|-
 
|-
| Optional EOF (0xFF) || if USART cannot indicate outgoing shift register clear)
+
| Data CRC LSByte || || if len non-zero
 +
|-
 +
| Optional EOF (0xFF) || || if USART cannot indicate outgoing shift register clear)
 
|}
 
|}
  

Latest revision as of 11:29, 15 December 2018

BACnet "Master-Slave/Token-Passing" is the optional, RS-485 based physical layer for BACnet. It is based on cheap, relatively slow, serial communications.


Starting with Addendum 135-2008ab, 9600 and 38400 are the required baud rates and all of the others (19200, 57600, 76800, and 115200) are optional


MS/TP Frame Format
Octet Function Comment
0x55 Framing 1
0xFF Framing 2
Frame Type
Dest MAC
Src MAC
Len MSByte
Len LSByte
Header CRC
Data This is the NPDU, if len non-zero
Data CRC MSByte if len non-zero
Data CRC LSByte if len non-zero
Optional EOF (0xFF) if USART cannot indicate outgoing shift register clear)


NSDUmax = max ( APDUms/tp, RPDU ), and Prior to Revision ** (prior to 135-2006), was 480 bytes.

NPCImax = 9 + 6 byte MAC + 6 byte MAC = 21

So NPDUmax = NSDUmax + NPCImax = 480 + 21 = 501

And so MPDUmax = NPDUmax + 11 (above) = 512




Keyword List
APDU BBMD BVLC BVLL DADR DNET Encoding I-Am LSAP MAC NPCI NPDU NSAP NSDU SA SADR SNET

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Glossary