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Communications System Section 10-1
3. Switching Hub
Delay
The switching hub delay is the delay time between the arrival of the packet at
the switching hub and the output of the packet from the hub’s transmission
port. This delay depends on the total number of connections used for recep-
tion and data sizes used in the tag data links. In addition, this delay depends
on the switching hub maker and model, but the delay can be approximated
with the following table. (For a precise estimate, contact the switching hub
manufacturer.)
The following values are the delays when cascade connections are not being
used. If cascade connections are used, more nodes can be connected, but
the switching hub delays will increase.
4. Receive Processing
Delay
The receive processing delay is the delay that occurs within the EtherNet/IP
Unit or built-in EtherNet/IP port from the reception of the data packet at the
Unit until the completion of reception processing in the Unit. This delay
depends on the size of the connections used in the tag data links and the
number of connections. In practice, the delay depends on the number of con-
nections used in tag data links with less than 200 words. If the number of con-
nections is “n”, the maximum delay can be calculated with the following
equation.
Maximum reception processing delay = 1 + (n × 0.043) ms
The size of the connections may cause a delay when the data sizes are
smaller and a large number of packets may be received in a fixed interval,
because the data may wait for receive processing.
Example Calculation
of the Tag Data Link
Delay
This example shows how to calculate the tag data link delay when the follow-
ing tag data link connection settings have been made.
In this case, 17 EtherNet/IP Units or built-in EtherNet/IP ports are being used,
and one Unit is receiving 200 words of data from each of the other Units at a
packet interval (RPI) of 5 ms. Thus, 16 tag data link connections are used.
The length of the cables between the Units is 50 m for all connections.
Send processing delay = 5 ms × (15 − 5/100)% = 0.7475 ms
Cable delay = 545 ns × 50 m/100 = 272.5 ns
Switching hub delay = 0.7 ms
Receive processing delay = 1 + (16 × 0.043) ms = 1.688 ms
Tag data link delay = 0.7475 ms + 0.0002725 ms + 0.7 ms + 1.688 ms
≈ 3.14 ms
Words per connection Number of connections used for reception
16 32 64 128 256
2 words 0.2 ms 0.3 ms 0.5 ms 1.0 ms 1.9 ms
200 words 0.7 ms 1.3 ms 2.5 ms 5.0 ms 10.0 ms
400 words 1.2 ms 2.3 ms 4.6 ms 9.1 ms 18.2 ms
600 words 1.7 ms 3.3 ms 6.6 ms 13.2 ms 26.4 ms
722 words 2.0 ms 4.0 ms 7.9 ms 15.7 ms 31.4 ms