Rockwell Automation 1747-L5xx SLC 500 Modular Hardware Style User Manual - Manual (Page 205)

Publication 1747-UM011G-EN-P - June 2008
RS-232 Communication Interface 205
Full-duplex (Point-to-Point)
DF1 Half-duplex Protocol
DF1 half-duplex protocol provides a multi-drop single master/multiple
slave network. In contrast to the DF1 full-duplex protocol,
communication takes place in one direction at a time. You can use
channel 0 as a programming port, or as a peer-to-peer port using the
MSG instruction.
In half-duplex mode, the SLC 5/03, SLC 5/04, and SLC 5/05 processors
can be either master or slave devices. As a master device, the
processor polls each slave on the network on a regular and sequential
basis. The master also supports routing of data packets from one slave
to another, or slave-to-slave communication. As a slave device, the
processor can send data packets when polled by the master device,
which initiates all communication with slave devices.
If the master device has no data to send, it can still receive data from
the slave device. To do this, the master sends out a poll packet
addressed to the slave. If the slave has data to send, it does so in
response to the poll packet. Otherwise, the slave sends a simple
two-byte response, so that the master knows that it is active.
Several Allen-Bradley products support half-duplex master protocol.
They include the Enhanced PLC-5 processors, and SLC 5/03, SLC 5/04,
and SLC 5/05 processors. RSLinx (2.0 or later) software also supports
half-duplex master protocol.
DF1 Half-duplex supports up to 255 slave devices (addresses 0...254)
with address 255 reserved for master broadcasts. The SLC 5/03,
SLC 5/04, and SLC 5/05 processor support broadcast reception. SLC
5/03, SLC 5/04, and SLC 5/05 processors with operating system FRN
C/6 can also initiate broadcast write commands via the MSG
Modem
SLC 5/03
Processor
(1747-L532)
1747-CP3 Cable
Modem
SLC 5/03
Processor
(1747-L532)