Rockwell Automation DAG6.5.8 APPLICATION GUIDE SCADA SYSTEM - Manual (Page 212)

Publication AG-UM008C-EN-P - February 2005
6-8 Configuring MicroLogix 1000 Controllers
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 command packet is sent
to the master. The master recognizes that the command packet is not
intended for it but for another slave, so the master immediately
rebroadcasts the command packet to the intended slave. When the
intended slave is polled, it sends a reply packet to the master with the
data the first slave requested. The master immediately rebroadcasts
the reply packet to that 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 controllers on the DF1 half-duplex
link.
DF1 half-duplex supports up to 255 devices (address 0 to 254) with
address 255 reserved for master broadcasts. The MicroLogix supports
broadcast reception but cannot initiate a broadcast command. The
MicroLogix supports half-duplex modems using RTS/CTS hardware
handshaking.
DF1 Slave on a Multi-drop Link
When communication is between either your programming software
and a MicroLogix controller or between two MicroLogix controllers via
a slave-to-slave connection on a larger multi-drop link, the devices
depend on a DF1 Master to give each of them polling permission to
transmit in a timely manner. As the number of slaves increases on the
link (up to 254), the time between when your programming software
or the MicroLogix controller is polled also increases. This increase in
time becomes larger is you are using low baud rates.
As these time periods grow, the following values may need to be
changed to avoid loss of communication:
•
programming software - increase poll timeout value and reply
timeout values
•
MicroLogix controller - increase poll timeout