Allen-Bradley DH-485 User Manual page 35

Chapter 4
Communicating with the KF3
Host Computer
(Master)
4 2
Half Duplex DF1 Protocol
Local mode requires an intelligent master device, capable of specifying
both a station address and a destination address. Because the KF3 acts as
a slave on a DF1 half-duplex network, the half-duplex master's access to
the DH-485 nodes is indirect: i.e., 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 KF3. The KF3
will respond to messages from the half-duplex master only if the station
address contained in these messages is the node address of the KF3. The
KF3 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 will contain a destination address equal to that of the KF3, and
not that of the half-duplex master device. The KF3 will respond to poll
packets from the half-duplex master by returning whatever data has been
forwarded to it by the remote nodes under its jurisdiction.
Figure 4.1
Local Mode Addressing
RS 232C
Cable
KF3
In Figure 4.1, the half-duplex master running on the computer only polls
the KF3 at station address 01. Messages from the half-duplex master to
the SLC 500s are sent using a master message containing both the station
address of the KF3 (01) and the destination address of the SLC 500 (03, for
example). Responses from the SLC500s to the half-duplex master contain
the destination address of the KF3 (01), which then returns all responses to
the half-duplex master station upon being polled.
Local Mode
Link Couplers
Node
02
Node Node
01 03
Node
00
SLC 500
Controller
SLC 500
Controller
SLC 500
Controller