User Manual Owner manual

Communicating with the KF3

Chapter 4

42

HalfDuplex DF1 Protocol  Local Mode

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

Host

Computer

(Master)

KF3

SLC 500

Controller

SLC 500

Controller

SLC 500

Controller

Link Couplers

Node

DH485

Network

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.