Rockwell Automation 1747-KE,D17476.12 DH-485/RS-232C Interface Module - Manual (Page 29)

Publication 1747-UM005B-EN-P - March 2006
Communicate with the Interface Module 3-3
You may prefer local mode for use in applications where the RS-232C
link is not networked, since it simplifies the polling algorithm.
Remote mode should be used when more than 31 SLC nodes are
required on the DH-485 network. Because the interface module is
transparent to the master device, existing drivers can be used without
rewriting.
Local Mode
Local mode requires an intelligent master device, capable of
specifying both a station address and a destination address. Because
the interface module acts as a slave on a half-duplex network, the
half-duplex master’s access to the DH-485 node is indirect. The
destination address and the station address are generally different.
In local mode, the polling algorithm used by the half-duplex master is
simplified so that the master only needs to poll the single interface
module. The module will respond to messages from the half-duplex
master only if the station address contained in these messages is the
node address of the interface module. The module then forwards the
packet to the appropriate DH-485 node, as defined by the destination
address.
Responses from remote nodes on the DH-485 network contain a
destination address equal to that of the interface module, and not that
of the half-duplex master device. The module responds to poll
packets from the half-duplex master by returning whatever data has
been forwarded to it by the remote nodes under its jurisdiction.
In the Local Mode illustration on page 3-4, the half-duplex master only
polls the interface module at station address 01.
Messages from the half-duplex master to the SLC 500 controllers are
sent using a master message containing both the station address of the
interface module (node 01) and the destination address of the SLC 500
controller (node 03 for example). Responses from the SLC 500
controllers to the half-duplex master contain the destination address
of the interface module (node 01), which then returns all responses to
the half-duplex master station upon being polled.
During configuration of the interface module, the module’s
destination address, or slave address, is selected if Local mode has
been selected.