Understanding the Communication Protocols
D–5
DF1 Half-Duplex Slave Protocol
DF1 half-duplex slave protocol provides a multi-drop single master/multiple slave
network. In contrast to DF1 full-duplex, communication takes place in one
direction at a time. You can use the RS-232 port on the MicroLogix as both a
half-duplex programming port, as well as a half-duplex peer-to-peer messaging port.
The master device initiates all communication by “polling” each slave device. The
slave device may only transmit message packets when it is polled by the master. It
is the master’s responsibility to poll each slave on a regular and sequential basis to
allow slaves to send message packets back to the master. During a polling sequence,
the master polls a slave either repeatedly until the slave indicates that it has no more
message packets to transmit or just one time per polling sequence, depending on
how the master is configured.
An additional feature of the DF1 half-duplex protocol is that it is possible for a slave
device to enable a MSG instruction in its ladder program to send or request data
to/from another slave. When the initiating slave is polled, the MSG instruction
command packet is sent to the master. The master recognizes that the command
packet is not intended for it but for another slave, so the master immediately
rebroadcasts the command packet to the intended slave. When the intended slave is
polled, it sends a reply packet to the master with the data the first slave requested.
The master again recognizes that the reply packet is intended for another slave, so
the master immediately rebroadcasts the reply packet to that slave. This
slave-to-slave transfer is a function of the master device and is also used by
programming software to upload and download programs to processors on the DF1
half-duplex link.
Several Allen-Bradley products support DF1 half-duplex master protocol. They
include the SLC 5/03
, SLC 5/04
,
and SLC 5/05
, and enhanced PLC-5
®
processors. Rockwell Software WINtelligent LINX
and RSLinx (version 2.x and
higher) also support DF1 half-duplex master protocol.
Typically, the master maintains an active node table that indicates which slaves are
active (slaves that responded the last time they were polled) and which slaves are
inactive (slaves that did not respond the last time they were polled). The active
slaves are polled on a regular basis. The inactive slaves are only polled occasionally
to check if any have come back online.
DF1 half-duplex supports up to 255 devices (address 0 to 254) with address 255
reserved for master broadcasts. The MicroLogix supports broadcast reception but
cannot initiate a broadcast command. The MicroLogix supports half-duplex
modems using Request-To-Send/Clear-To-Send (RTS/CTS) hardware handshaking.
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