Allen-Bradley MicroLogix 1200 Reference Manual page 400

E-8 Protocol Configuration
Publication 1762-RM001C-EN-P
Considerations When Communicating as a DF1 Slave on a Multi-drop
Link
When communication is between either your programming software and
a controller or between two 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 controller is polled also increases. This increase
in time may become larger if 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 and reply timeout values
• MicroLogix controller: increase poll timeout
Ownership Timeout
When a program download sequence is started by a software package to
download a ladder logic program to the controller, the software takes
"program ownership" of the controller. Program ownership prevents other
devices from reading from or writing to the controller while the download
is in process. Once the download is completed, the programming
software returns the program ownership to the controller, so other devices
can communicate with it again.
The controller clears the program ownership if no supported commands
are received from the owner within the timeout period. If the program
ownership were not cleared after a download sequence interruption, the
controller would not accept commands from any other device because it
would assume another device still had program ownership.
If a download sequence is interrupted, due to
IMPORTANT
electromagnetic interference or other events, discontinue
communications to the controller for the ownership
timeout period and then restart the program download.
The ownership timeout period is 60 seconds. After the
timeout, you can re-establish communications with the
controller and try the program download again. The only
other way to remove program ownership is to cycle
power to the controller.