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BACnet MS/TP

Physical and Electrical considerations

Packet structure

Receive Frame State Machine

Slave and Master Node State Machine

Design issues (timing, buffering, MAC address)

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BACnet MS/TP EIA-485

120 120

510 47K 47K

+5

RTSTxRx

510

+5

RTSTxRx RTSTxRx

+ + +

- - -

Node with networkbias resistors

Node with localbias resistors

Node withoutbias resistors

Baud Rates: 9600, 19200, 38400, 57600, 76800Parity: noStart Bits: 1

Stop Bits: 1Least Significant Bit First (same as EIA-232)Uses standard UART

This may not be the ideal circuit for EIA-485. Therehas been much debate in the BACnet MS/TPworking group about it.

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MS/TP Packet Structure0x55

Preamble0xFF

Length

Destination Address

Source Address

Frame Type

Header CRC

Data

Data CRC

pad

2 octets, most significant octet first

1 octet

1 octet

1 octet

2 octets

1 octet

(present if Length is non-zero) Length octets

(present if Length is non-zero) 2 octets

(optional) at most, 1 octet0xFF

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Receive Frame State Machine

IDLEDATACRC

Bad CRC

Good CRC

EatAnOctet

EatAnError

Preamble 1

Timeout

NotPreamble

PREAMBLEError

HEADERCRC

BadCRC

NoData

NotForUs

HEADER

Frame Type

Destination

Source

Length 1

Length 2

DATA

Preamble

2

CRC 1

Data Octet

Data

Tim

eout

Error

HeaderCRC

Timeout

Error

RepeatedPreamble 1

CRC2

FrameTooLong

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Master Node State Machine

POLL

FOR

MASTER

DONE

WITH

TOKEN

INITIALIZE

USE

TOKEN

PASS

TOKEN

WAIT

FOR

REPLY

IDLENO

TOKEN

ANSWER

DATA

REQUEST

SendMaintenancePFMNothingToSend

SendNoWaitSendAnotherFrame

SoleMasterSoleMasterDeclareSoleMaster

SendNextPFM

SendAndWaitR

eceive

dReply

ReplyTim

eOut

Inva

lidFr

ame

Receive

dPostponeSen

dToken

ResetM

ainten

anceP

FM

Receiv

edR

eplyT

oPFM

Don

eWith

PFM

Fin

dN

ewSuccessor

GenerateT

ok

en

SawFrame

LostToken

Receiv

edUnexpectedFram

e

RetrySendToken

SawTo

kenUser

ReceivedU

nexpectedFram

e

ReceivedToken

DeferredReply

Reply

ReceivedDataNeedingReply

DoneInitializing

ReceivedPFM

ReceivedDataNoReplyReceivedInvalidFrame

ReceivedUnwantedFrame

SoleMasterRestartMaintenancePFM

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MS/TP Timing

Tframe_abort

< 60 bit times

55 FF 00 72 0A 00 00 A9 55 FF 00 03 72 00 00 95 55 FF 00 A0 03 00 00 04

Tturnaround

> 40 bit times

Tusage_timeo

ut

< 20ms

Token TokenToken

Tpostdrive

< 15 bit times

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MS/TP Timing

55 FF 01 07 03 00 00 6D 55 FF 00 0A 03 00 00 04

TokenPoll For Master

Tusage_timeo

ut

< 20ms

EF 0F 41 2D 55 FF 00 72 0A 00 00 A9

Treply_timeout

< 255ms

Token

80 00 01 0E 31 00 62 FF01 04 02 05 53 07 C4 0255 FF 05 89 0A 00 12 12

BACnet Data Expecting Reply

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MS/TP MAC Address

Master Node MAC 0-127 (0x00 - 0x7F)

Slave Node MAC 0-254 (0x00-0xFE)

Broadcast 255 (0xFF)

DIP switch, dials, LCD

Non-volatile, same after power cycle or reset

Proposed Auto Addressing (DHCP, ZeroConfig)