Df1 Half-Duplex Communications; Df1 Half-Duplex Master Broadcast; Df1 Half-Duplex Slave Broadcast - Allen-Bradley SLC 500 Series Reference Manual
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Table of Contents
DF1 Half-duplex
Communications
DF1 Half-duplex Master/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.
The master device initiates all communication by "polling" each slave device.
The slave device may only transmit data 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 collect data. During a polling sequence, the master polls a
slave repeatedly until the slave indicates that it has no more data packets to
transmit. The master then transmits the data packets for that slave.
Many Allen-Bradley products support half-duplex master protocol. They
include the enhanced PLC-5 processors, SLC 5/03 (OS301 and higher), SLC
5/04, and SLC 5/05 processors. RSLinx (V2.0 and higher) software also
supports DF1 half-duplex master protocol.
Typically, the master keeps two separate tables - one for online slaves and one
for offline slaves. The online slaves are polled on a regular basis. The offline
slaves are polled occasionally to see if they have come back online.
A master device supports routing of data packets from one slave to another.
DF1 half-duplex supports up to 255 slave devices (address 0 to 254) with
address 255 reserved for master broadcasts. Either half-duplex or full-duplex
modem types can be used for DF1 half-duplex network. All SLC 5/03, SLC
5/04, and SLC 5/05 processors support broadcast reception, but only
processors running OS Series C, FRN 6 or higher can initiate broadcast write
commands.
DF1 Half-duplex Master Broadcast
A broadcast write command initiated by the DF1 half-duplex master is
received and executed by all DF1 half-duplex slaves. A broadcast write
command received by the DF1 half-duplex master after polling a DF1
half-duplex slave is received, acknowledged and re-broadcast without being
executed by the DF1 half-duplex master. It is treated like any other
slave-to-slave command, except that no acknowledgement is expected after
the re-broadcast.
DF1 Half-duplex Slave Broadcast
When a broadcast write command is initiated by a DF1 half-duplex slave, it is
queued up just like any other MSG command until it receives a poll from the
DF1 half-duplex master. After transmitting the broadcast write command, the
DF1 half-duplex slave receives an acknowledgement that the DF1 half-duplex
SLC Communication Channels
Publication 1747-RM001G-EN-P - November 2008
13-55
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