254 Rockwell Automation Publication 1766-UM001O-EN-P - September 2021
Appendix F MicroLogix 1400 Distributed Network Protocol
The parameter Channel for Unsolicited Response in the DNP3 Slave
Configuration tab indicates that the Unsolicited Response is reported via
Channel 0 only. In this example, Initial Unsolicited Response is sent on startup
and all events of class 1, 2 and 3 are reported. Since Hold Times are configured
to 5 seconds, generated events are reported after 5 seconds.
Collision Avoidance The controller currently supports the first of the two methods listed below for
collision avoidance.
• Detecting transmitted data (TX/RX line on RS-485 communication).
• Detecting out-of-band carrier (DCD on RS-232C communication).
When the controller is connected to RS-485 network, it monitors all data on the
link. If the controller is preparing to transmit a packet and finds the link busy,
it waits for an interval defined by the Backoff_Time until it is no longer busy.
Backoff_Time = Pre Transmit Delay (x1 ms) + Max Random Delay (x1 ms)
The Pre Transmit Delay (x1 ms) in the Link Layer Channel Configuration file is
a fixed delay and the Max Random Delay (x1 ms) in the Channel Configuration
file is a maximum random delay for Channel 0 and Channel 2. You must
specify those parameters to get the collision avoidance mechanism.
After the Backoff_Time, the controller tries again, either indefinitely, or up to a
configurable maximum number of retries. If a maximum is used, the protocol
considers this as a link failure.
Time Synchronization The time value in the embedded RTC module of the controller is updated by an
RTC Function file every 2 seconds. This resolution is insufficient to log DNP3
events in a DNP3 subsystem. Another timer, incremented by 1 millisecond in
the DNP3 Slave subsystem, serves to provide appropriate resolution.
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