Rockwell Automation 1769-SDN Compact I/O DeviceNet Scanner Module User Manual - Manual (Page 134)

134 Publication 1769-UM009E-EN-P - August 2009
Appendix B CompactLogix Backup on the DeviceNet Network
Develop the CompactLogix
Backup Application The CompactLogix backup is enabled from an RSLogix 5000 programming
software user program with a few simple ladder rungs. The following rungs are
used in the CompactLogix backup:
• Backup Heartbeat Configuration Rungs - required
• Reading Backup State Rung - optional
• Reading Backup Status - optional
Backup Heartbeat Configuration Rungs
The first, and most critical, step is to set the backup heartbeat constant in the
DeviceNet scanner. The heartbeat constant enables the backup feature and
determines the switchover time (2 x heartbeat).
By default, the heartbeat is zero; this default value disables the Backup mode.
Your user program must set the heartbeat to a non-zero value to enable
backup.
The heartbeat occurs in multiples of 8 ms (8, 16, 24). We recommend a value
of 16...48 ms for most applications. The recommended heartbeat times result
in switchover times of 32...96 ms. These times do not include controller scan
delays.
IMPORTANT
If multiples of 8 are not used for the requested heartbeat, then
the DeviceNet scanner module uses the next higher supported
heartbeat value that can be read from the scanner. For example,
if you set the heartbeat to 10, the scanner module uses a 16 ms
heartbeat.