User manual

Rockwell Automation Publication 1756-UM001O-EN-P - October 2014 139
I/O Modules Chapter 8
• Requested Packet Intervals (RPIs) faster than 25 ms for unscheduled
modules can overload the 1756-CNB or 1756-CNBR communication
module. To avoid the overload, make these considerations:
– Use a NUT of 10 ms or more.
– Keep the SMAX and UMAX values as small as possible.
• If the module has a Real Time Sample (RTS), disable it or set it to a rate
that is greater than the RPI.
• You can add I/O modules until you reach these limits:
– 75% of CPU utilization of the 1756-CNB or 1756-CNBR
communication module.
– Plan for a CPU-use increase of 1…4% of the 1756-CNB or
1756-CNBR module for each I/O module you add, depending on the
RPI.
– 48 connections on the 1756-CNB or 1756-CNBR communication
module.
– Less than 400,000 unscheduled bytes per second are displayed in
RSNetWorx for ControlNet software after the network has been
scheduled.
1756-CN2, 1756-CN2R, 1756-CN2RXT Modules
The use of 1756-CN2/B, 1756-CN2R/B, and 1756-CN2RXT modules
provides increased capacity for adding I/O while online compared to 1756-CNB
or 1756-CNBR modules. With this increased capacity, you can easily add I/O
and increase ControlNet connections that are used with less impact on the
overall system.
Table 35
demonstrates the performance factors of the 1756-CN2/B,
1756-CN2R/B, and 1756-CN2RXT modules when adding I/O online.
Table 35 - 1756-CN2, 1756-CN2R, and 1756-CN2RXT Performance Example
(1)
No. of Direct
Analog I/O
Connections
Added Online
RPI = 2 ms RPI = 4 ms RPI = 10 ms RPI = 20 ms RPI = 50 ms RPI = 100 ms
CPU %
(2)
Avg.
API
(3)
CPU %
(2)
Avg.
API
(3)
CPU %
(2)
Avg.
API
(3)
CPU %
(2)
Avg.
API
(3)
CPU %
(2)
Avg.
API
(3)
CPU %
(2)
Avg.
API
(3)
0 1.50% N/A 1.50% N/A 1.50% N/A 1.50% N/A 1.50% N/A 1.50% N/A
1 4.80% 2.0 3.70% 4.0 2.50% 10.0 2.30% 20.0 1.90% 50.0 1.70% 100.0
2 7.00% 2.0 5.00% 4.0 3.30% 10.0 2.70% 20.0 2.10% 50.0 1.90% 100.0
3 9.00% 2.0 6.10% 4.0 3.80% 10.0 3.00% 20.0 2.20% 50.0 2.00% 100.0
4 11.20% 2.2 7.40% 4.0 4.40% 10.0 3.40% 20.0 2.40% 50.0 2.10% 100.0
5 11.50% 3.3 8.70% 4.0 5.00% 10.0 3.70% 20.0 2.60% 50.0 2.20% 100.0
6 12.80% 3.3 9.70% 4.0 5.50% 10.0 4.00% 20.0 2.70% 50.0 2.30% 100.0
7 13.80% 3.4 10.80% 4.0 5.90% 10.0 4.30% 20.0 2.90% 50.0 2.30% 100.0
8 15.10% 3.4 11.90% 4.0 6.40% 10.0 4.50% 20.0 3.00% 50.0 2.50% 100.0