Rockwell Automation 1770-HT8 SMART TRANSMITTER - Manual (Page 21)

Introducing the Smart Transmitter
Chapter 1
Interface
1-10
Poll/Response Mode
The HART protocol supports two modes of digital communications,
poll/response and burst. In poll/response mode the host processor requests
information from (polls) the smart device. Both point-to-point and
multidrop networks can employ this mode.
When the host processor sends a request or control information to the
HART field devices, the Smart Transmitter Interface reads the routing
information in the header portion of the data. It then strips the header off
the message and sends the data down the appropriate channel to the HART
device. Responses from the HART devices are returned to the host
processor when the host polls for them.
If there are no more messages to be forwarded, the Smart Transmitter
Interface stays on the last used channel and watches the traffic. This allows
higher throughput if consecutive messages are sent to the same channel.
Burst Mode
In burst mode the HART field device continuously transmits digital data to
the Communications Controller in burst monitor mode without the need for
request messages from the host. This mode cannot be used with multidrop
networks.
In burst monitor mode, the host processor sends the Smart Transmitter
Interface a list of all the channels whose devices are preset to burst mode.
If the list changes, the host must provide a new list. The Smart Transmitter
Interface continuously monitors the channels on the list in order, returning
to the first as soon as the last has been checked. The data collected from
the burst channels is stored in a Burst Data table. The latest information on
any channel is sent to the host processor upon request.
While in burst monitor mode, the Smart Transmitter Interface still
responds to requests from the host to poll any channel. When the polling is
complete, it resumes monitoring the burst channels.
The Smart Transmitter Interface (1770-HT1 and 1770-HT8/16) features
include:
remote I/O port for interface to programmable controllers (RIO
scanners) and DH+ hosts
RS-232C port for interface to serial hosts
7-segment display and push buttons for communications configuration
Features of the Smart
Transmitter Interface