User manual
Publication CNET-UM001C-EN-P - November 2005
B-4 ControlNet Overview
Network Update Time (NUT)
The network update time (NUT) is the smallest repetitive time interval in
which data can be sent on the ControlNet network. It represents the fastest
possible update rate for scheduled data transfers on that network. For
example, a network that runs with a 5ms NUT cannot send scheduled data at a
rate faster than 5ms. It can, however, send data at a slower rate. The minimum
NUT you can specify is 2ms. The NUT is divided into three parts:
Table B.1 NUT(Network Update Time) Structure
Figure B.1 NUT Structure
Requested Packet Interval (RPI)
The RPI is the update rate specified for a particular piece of data on the
network. The RPI can be specified for an entire rack of I/O (using a
rack-optimized connection), for a particular module (using a direct
connection) or peer-to-peer data. When you add a module to the I/O
configuration of a controller, you must configure the RPI. This value specifies
This part of the NUT Lets
Scheduled Every scheduled node (on a rotating basis in sequential order) one guaranteed opportunity to transmit per NUT.
Information that is time-critical is sent during this part of the interval.
Unscheduled All nodes transmit on a rotating basis in sequential order. This rotation repeats until the time allotted for this
portion is used up.
The amount of time available for the unscheduled portion is determined by the traffic load of the scheduled
portion. ControlNet guarantees at least 1 node will have the opportunity to transmit unscheduled data every
NUT.
Information that can be delivered without time constraints is sent during this part of the interval.
Maintenance The node with the lowest address transmits information to keep the other nodes synchronized. This time is
automatically subtracted from your NUT. However, the time required for network maintenance is small
(microseconds) when compared to that used for the scheduled and unscheduled portions of the NUT.
Start
• Scheduled Traffic
• Each Device
Transmits Only
Once
• Unscheduled
Traffic
• Network
Maintenance
Boundary Moves According to
Scheduled Traffic Load
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