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The 1785-KE module automatically takes care of the data link layers on
the Data Highway Plus network.
This means that your application programs at the PLCs and computers do
not need to bother with inter-node protocol, handshaking, or control of the
Data Highway Plus link.
You must program these layers, however, for the asynchronous link
between your 1785-KE and its RS-232-C device. This programming
allows your computer to communicate with the Data Highway Plus
network, through the asynchronous link.
For more information on the commands, protocols, and diagnostics used
on your Data Highway Plus network, refer to the Data Highway/Data
Highway Plus Protocol and Command Set User’s Manual (publication
1770-6.5.16). This manual contains the information you need to program
the software layers for your computer and troubleshoot your network.
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The Data Highway Plus link has a communication rate of 57,600 bits per
second. The Data Highway Plus implements peer-to-peer communication
with a token passing scheme to rotate link mastership among its stations.
Since this method does not require polling, it is time efficient. The Data
Highway Plus uses factory set timeouts to rebuild the token pass network if
a token is lost because of a defective station. This timeout is not adjustable.
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The 1785-KE’s RS-232 asynchronous link has a selectable communication
rate from 110 bits per second to 19,200 bits per second. It uses a selectable
protocol of half-duplex (master-slave, polled) or full-duplex (peer-to-peer,
unpolled).
You can select the half-duplex protocol for a multidrop or point-to-point
communication link. You can select the full-duplex protocol only for a
point-to-point communication link.