Rockwell Automation 1761-NET-DNI DeviceNet Interface (DNI) - Manual (Page 44)

Publication 1761-UM005B-EN-P - October 2006
44 DNI Configuration Parameters and Programming Notes
DF1 Heartbeat Rotation Example
The value entered for the heartbeat determines how often the DNI
will rotate the heartbeat bit. The table below illustrates the heartbeat
rotation with the default setting of two (heartbeat checked on every
other read).
A. DNI gets (reads) data from controller
B. DNI checks heartbeat and toggles bit
C. DNI sets (writes) data to controller
D. Data Scan Delay period
E. DNI gets (reads) data from controller
F. Data Scan Delay (period)
Consider the following with the above list.
• B is done inside the DNI and is completed extremely fast,
typically less than a millisecond.
• D is the only consistent time variable; the DNI waits for the
amount of time specified by the Data Scan Delay parameter.
• A, C, and E are variable; the time associated with each is
dependent on a number of factors: baud rate, communication
errors/retries.
• If you are monitoring the heartbeat bit in the controller to
determine communications integrity, as a general rule, multiply
the Data Scan Delay by the DF1 Heartbeat plus 1 to determine
the preset time value for the heartbeat timer.
• DF1 Heartbeat determines how many input reads are done
between heartbeat checks and toggles (step B). When the DF1
Heartbeat is set to 2, this occurs on every other read; when the
DF1 Heartbeat is set to 3, this occurs on every third read.
See the application example DF1 Heartbeat (Bit 7) on page 51 and
MicroLogix as I/O on the DeviceNet Network on page 69 for more
information about using the DF1 Heartbeat.
DF1 (RS-232) Port A D E F A
DNI’s Read Sequence
B C B C
DNI’s Write Sequence
IMPORTANT
The example above does not represent accurate
timing relationships.