Publication 1762-RM001C-EN-P
E-6 Protocol Configuration
DF1 Half-Duplex
Protocol
DF1 Half-Duplex protocol provides 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 port on the
MicroLogix controller as both a Half-Duplex programming port, and a
Half-Duplex peer-to-peer messaging port.
DF1 Half-Duplex Operation
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 slave devices an opportunity to
communicate. 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 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. 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.
The MicroLogix controllers can only act as slave devices. A device that
can act as a master is required. Several Allen-Bradley products support
DF1 Half-Duplex master protocol. They include the SLC 5/03™ and higher
processors, enhanced PLC-5
®
processors, and Rockwell Software RSLinx
(version 2.0 and higher) also support DF1 Half-Duplex master protocol.
DF1 Half-Duplex supports up to 255 devices (address 0 to 254) with
address 255 reserved for master broadcasts. The MicroLogix controllers
support broadcast reception but cannot initiate a broadcast command.
The MicroLogix controllers support Half-Duplex modems using RTS/CTS
hardware handshaking.
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