Rockwell Automation 1761-HHP-B30 MicroLogix 1000 with Hand-Held Programmer (HHP) - Manual (Page 430)

Appendix D
Understanding the Communication Protocols
D–4
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
t, SLC 5/04t
, and SLC 5/05
t, and enhanced
PLC-5 ® processors. Rockwell Software WINtelligent LINX
t 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.
DF1 Half-Duplex Slave
Protocol