Rockwell Automation 1770-HT8 SMART TRANSMITTER - Manual (Page 20)

Introducing the Smart Transmitter
Interface
Chapter 1
1-9
The HART Protocol and the Smart Transmitter Interface
Each Communications Controller can communicate with a maximum of 32
channels via the 1770-HT8 and 1770-HT16 Terminal Blocks. As all
channels are multiplexed, communications can only occur over one
channel at a time. Each channel can have one HART field device
connected to it in point-to-point mode, or up to 15 devices in a multidrop
network. The host addresses these channels using channel numbers 0–31
(decimal).
The Communications Controller in the Smart Transmitter Interface
receives HART protocol commands from the host processor (via Remote
I/O or RS-232C connection) and routes these commands (via the Terminal
Blocks) to the HART field devices. The Smart Transmitter Interface
receives responses from the HART field devices and transmits the
responses to the host when it polls for them.
In both multidrop and point-to-point networks each device has a unique
address that is included in every HART message. A device picks up
messages destined to it via this unique address.
When the Terminal Block receives the composite digital/analog signal
from the HART field devices, it filters out the digital portion of the signal
(see Appendix A for more information on the filtering circuitry), and
passes the analog portion on to an Allen-Bradley Analog I/O module, such
as the 1771-IFE. The Analog I/O module decodes the analog data and
passes it along to the programmable controller.
At the same time, the Terminal Block reads the digital portion of the signal
and multiplexes it to the Communications Controller. The Communications
Controller embeds the digital data into messages conforming to the RIO or
DF1 protocol format, and passes it to the appropriate host processor and its
application program in one of three ways:
to a programmable controller via the remote I/O link
to a computer via the RS-232C port
to a host on the Data Highway Plus network via the pass-through feature
of an Allen-Bradley PLC-5 family programmable controller
Important: You cannot have more than one DH+ host computer using the
pass-through feature or more than one RS-232C host.