13-3
13 Communications Performance and Communications Load
NJ-series CPU Unit Built-in EtherNet/IP Port User’s Manual (W506)
13-1 Communications System
13
13-1-1 Tag Data Link Communications Method
The number of packets transferred each second is called the used bandwidth or PPS (packets per sec-
ond).
The PPS is calculated from the RPI and heartbeat as follows for each connection:
PPS used in a connection (pps) = (1,000 ÷ RPI (ms)) + (1,000 ÷ Heartbeat transmission period (ms))
Use the following equation to calculate the total number of packets transferred by each built-in Ether-
Net/IP port (Unit) in 1 second.
Built-in EtherNet/IP port’s total PPS = Total PPS of originator connections + Total PPS of target connec-
tions*
* Connections set as target connections must be added, too.
The maximum number of packets that the CPU Unit can transfer in 1 second (called the allowed Unit
bandwidth) is 1,000 pps, so set the connection below this maximum value.
Example:
Node 1 has both originator and target connections, with send RPI of 200 ms and 2 ms, and receive RPI
of 500 ms.
Node 2 has originator connections only, with receive RPIs of 200 ms, 2 ms, and 5 ms.
Node 3 has target connections only, with send RPIs of 5 ms and 1 ms.
Each node’s total PPS is calculated as follows:
• Total PPS of node 1’s Unit
= 1000 / 200 ms + 1000 / 2 ms + 1000 / 500 ms (for data)
+ 1000 / 200 ms + 1000 / 100 ms + 1000 / 500 ms (for heartbeat)
= 524 pps
• Total PPS of node 2’s Unit
= 1000 / 200ms + 1000 / 2 ms + 1000 / 5 ms (for data)
+ 1000 / 200ms + 1000 / 100 ms + 1000 / 100 ms (for heartbeat)
= 730 pps
• Total PPS of node 3’s Unit
= 1000 / 5 ms + 1000 / 500 ms (for data)
+ 1000 / 100 ms + 1000 / 500 ms (for heartbeat)
= 214pps
All of the Units are within the allowed Unit bandwidth of 1,000 pps, so they can transfer data.
Requested Packet Interval (RPI) and Bandwidth Usage (PPS)
O: Originator
T: Target
HB: Heartbeat
O
O
O
O
TT
T
T
RPI: 200 ms
HB: 200 ms
RPI: 2 ms
HB: 100 ms
RPI: 500 ms
HB: 500 ms
RPI: 5 ms
HB: 100 ms
Node 1
Node 2 Node 3
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