Rockwell Automation 1764-xxxx MICROLOGIX 1500 PROGRAMMABLE CONTROLLERS - Manual (Page 134)

Publication 1764-UM001B-EN-P - April 2002
E-2 Understanding Communication Protocols
MicroLogix 1500 controllers support the DF1 Full-Duplex protocol via
RS-232 connection to external devices such as computers, controllers,
and other interface devices that support DF1 Full-Duplex.
For information about required network connecting equipment and
examples of DF1 Full-Duplex connections, see Chapter 4.
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. With an active Half-Duplex
Master, you can use the RS-232 port on the MicroLogix 1500 as 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.
The MicroLogix 1500 can only act as a slave device. A device that can
act as a master is required to “run” the network. 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.x and higher).
DF1 Half-Duplex supports up to 255 devices (address 0 to 254) with
address 255 reserved for master broadcasts. The MicroLogix 1500
supports broadcast reception.