Rockwell Automation 1770-HT8 SMART TRANSMITTER - Manual (Page 101)

Communicating with the Smart
Transmitter Interface
Chapter 4
Communicating with the Smart Transmitter
Interface
4-36
Half duplex protocol can be used on a point-to-point link, but more usually
operates on a link with all nodes interfaced through half duplex modems.
There may be from 0 to 63 decimal nodes simultaneously connected to a
single link.
Figure 4.12
Half Duplex Network
90035
Programmable Controller
or Host Computer
Modem
Modem Modem Modem
Smart Transmitter Interface Smart Transmitter Interface Smart Transmitter Interface
Hart Field Devices Hart Field Devices Hart Field Devices
RSĆ232C Link
RSĆ232C Link
Ć
One node is designated as the master, and controls which node has access
to the link. All other nodes are slaves and must wait for permission from
the master before transmitting. In the Smart Transmitter Interface network,
the host computer is the master and the Smart Transmitter Interface is the
slave.
With half duplex protocol, you can use:
a two-circuit system - the master sends and slaves receive on one circuit,
slave send and master receives on the other
a one-circuit system - master and slaves send and receive on the same
circuit