12-3
12 Communications Performance and Communications Load
CJ-series EtherNet/IP Units Operation Manual for NJ-series CPU Unit (W495)
12-1 Communications System
12
12-1-1 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 EtherNet/IP Unit
(Unit) in 1 second.
EtherNet/IP Unit’s total PPS = Total PPS of originator connections + Total PPS of target connections
Note Connections set as target connections must be added to the total PPS of the target connections.
The maximum number of packets that the CPU Unit can transfer in 1 second (called the allowed Unit
bandwidth) is 6,000 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 6,000 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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