326 Rockwell Automation Publication 2080-UM002M-EN-E - April 2022
Appendix G Connect to Networks using DF1
Example DF1 Full-Duplex Connections
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/RS-485 port on the controller 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
controllers (including the master) on the DF1 Half-Duplex link.
The controller can act as the master or as a slave on a Half-Duplex network.
When the controller is a slave device, a master device is required to run the
network. Several other Allen-Bradley products support DF1 Half-Duplex
master protocol.
DF1 Half-Duplex supports up to 255 devices (address 0…254) with address 255
reserved for master broadcasts. As a DF1 Half-Duplex slave device, the
controller supports broadcast reception. As a DF1 Half-Duplex master, the
controller supports both the reception and initiation of broadcast write
commands (via the MSG instruction). The controller also supports Half-
Duplex modems using RTS/CTS hardware handshaking.
Computer
CH0
Micro870 controller
Micro870 controller
USB
Computer
Modem cable
Modem
Modem 1761-CBL-PM02 with null modem adapter
CH0
IMPORTANT Firmware revision 20 for Micro850 and Micro870 controllers does not
support the broadcast function in Half-Duplex Master. This feature will
be supported in a future firmware revision.
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