Publication 1763-UM001E-EN-P - June 2015
Connecting to Networks via RS-232/RS-485 Interface 201
DF1 Half-Duplex Protocol
DF1 Half-Duplex protocol is a multi-drop single master/multiple slave
network. DF1 Half-Duplex protocol supports data transparency (American
National Standards Institute ANSI - X3.28-1976 specification subcategory
D1). In contrast to DF1 Full-Duplex, communication takes place in one
direction at a time. You can use the RS-232/485 port on the MicroLogix™ as
both a Half-Duplex programming port and a Half-Duplex peer-to-peer
messaging port.
DF1 Half-Duplex Operation
A DF1 Half-Duplex master device initiates all communication by “polling”
each slave device. The slave device may only transmit 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 slave devices an opportunity to communicate.
An additional feature of the DF1 Half-Duplex protocol is that it is possible for
a slave device to enable a MSG write or read to/from another slave. When the
initiating slave is polled, the MSG is sent to the master. The master recognizes
that the message is not intended for it, but for another slave, so the master
immediately forwards the message to the intended slave. The master does this
automatically; you do not need to program the master to move data between
slave nodes. This slave-to-slave transfer can also be used by programming
software to allow slave-to-slave upload and download of programs to
processors (including the master) on the DF1 Half-Duplex link.
MicroLogix 1100 can act as the master or as a slave on a Half-Duplex network.
When the MicroLogix 1100 is a slave device, a master device is required to
“run” the network. Several other Allen-Bradley products support DF1
Half-Duplex master protocol. They include the SLC 5/03™ and higher
processors, enhanced PLC-5 processors, MicroLogix 1200/1500 and Rockwell
Software RSLinx (version 2.x and higher).
DF1 Half-Duplex supports up to 255 devices (address 0 to 254) with address
255 reserved for master broadcasts. As a DF1 Half-Duplex slave device, the
MicroLogix™ supports broadcast reception. As a DF1 Half-Duplex master,
the MicroLogix 1100 supports both the reception and initiation of broadcast
write commands (via the MSG instruction). The MicroLogix™ also supports
Half-Duplex modems using RTS/CTS hardware handshaking.
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