Rockwell Automation 1770-HT8 SMART TRANSMITTER - Manual (Page 68)

Chapter 4
C Communicating with the Smart
Transmitter Interface
4-3
Figure 4.2
Layered Nature of Protocols
Layer 3
90066
DLE
STX
DST
SRC
CMD
STS
TNS
FNC
LEN
BCC/CRC
ETX
DLE
DF1 Packet
Smart Transmitter Interface Command
Control
Parameter
Channel Number
Preamble
Delimiter
Address
HART Command
Byte Count
Data
Check Byte
Smart Transmitter Interface Packet
HART Packet
Layer 2
Layer 1
Protocol Conversion
The Smart Transmitter Interface receives data in one of three protocols:
HART, Smart Transmitter Interface and DF1. Figure 4.2 illustrates the
layered nature of these three protocols as transmitted from a host computer.
Here the DF1 packet contains the Smart Transmitter Interface packet,
which in turn, contains the HART packet. The Smart Transmitter Interface
converts all received packets to the required format before transmitting
them to their destination. As mentioned previously, when sending a packet
to a HART field device, it adds preamble bytes. When sending a packet to
the host processor the Smart Transmitter Interface attaches a four byte
header. The contents of these bytes are explained in the sections describing
the various packets.