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336 Rockwell Automation Publication 2080-UM002N-EN-E - November 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.