View source for BACnet MS/TP

BACnet "Multidrop Serial Bus/Token-Passing" is the optional, [[RS-485]] based physical layer for BACnet. It is based on cheap, relatively slow, serial communications.

Note that in addendum 135-2020ce the culturally offensive "Master-Slave/Token-Passing" has been replaced with the new "Multidrop Serial bus/Token Passing"

Starting with [http://www.bacnet.org/Addenda/Add-135-2008ab.pdf Addendum 135-2008ab], 9600 and 38400 are the required baud rates and all of the others (19200, 57600, 76800, and 115200) are optional


{| border="1" cellpadding="10" cellspacing="0"
|+ align="top" | '''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)
|}



[[NSDU]]max = max ( [[APDU]]ms/tp, [[RPDU]] ), and Prior to Revision ** (prior to 135-2006), was 480 bytes. 

[[NPCI]]max = 9 + 6 byte MAC + 6 byte MAC = 21

So [[NPDU]]max = [[NSDU]]max + [[NPCI]]max =  480 + 21 = 501

And so [[MPDU]]max = [[NPDU]]max + 11 (above) = 512







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