Rockwell Automation 1761-HHP-B30 MicroLogix 1000 with Hand-Held Programmer (HHP) - Manual (Page 432)

Appendix D
Understanding the Communication Protocols
D–6
Modem
Modem Modem Modem Modem Modem
RS-232
(DF1 Protocol)
MicroLogix 1000
Programmable
Controller (Series D) SLC 5/03 Processor
Modular Controller
Rockwell Software WINtelligent LINX, RSLinx 2.0 (or higher),
SLC 5/03, SLC 5/04 and SLC 5/05, or PLC-5 processors
configured for DF1 Half-Duplex Master
MicroLogix 1000
Programmable
Controller (Series D) MicroLogix 1000
Programmable
Controller (Series D) SLC 500 t
Fixed I/O Controller
with 1747-KE
Interface Module
Considerations When Communicating as a DF1 Slave on a Multi-drop
Link
When communication is between either your programming software and a
MicroLogix 1000 Programmable Controller or between two MicroLogix
Programmable 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 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 Programmable Controller - increase poll timeout
Ownership Timeout
When a program download sequence is started by a software package to
download a ladder logic program to a MicroLogix controller, the software
takes “file ownership” of the processor. File ownership prevents other
devices from reading from or writing to the processor while the download is
in process. If the controller were to respond to a device’s read commands
during the download, the processor could respond with incorrect information.
Similarly, if the controller were to accept information from other devices, the
information could be lost because the program download sequence could
immediately overwrite the information. Once the download is completed,
the programming software returns the file ownership to the controller, so
other devices can communicate with it again.