Rockwell Automation Ethernet Design Considerations Reference Manual - Manual (Page 99)

Rockwell Automation Publication ENET-RM002C-EN-P - May 2013 99
Predict System Performance Chapter 5
Refine Estimates
To further increase the accuracy of these times, include considerations for system
delays.
For a Rack-optimized Connection, to the RPI, add these delays:
T
MAX (Rack-optimized)
= input filter + RPI + transmission + switch
+ queue
See the table for descriptions of delay types with a rack-optimized connection.
For the Direct Connect Analog Modules, you add the same transmission,
switch, and queue delays as for rack-optimized data:
T
MAX (Analog Non-Isolated)
= 2RTS + transmission + switch + queue
For a Produced or Consumed Tag, you add the same transmission, switch, and
queue delays as for rack-optimized data:
T
MAX (Produced/Consumed Tag)
= 1RPI + transmission + switch + queue
Type of Delay Description
Input Filter Discrete input modules have filters. The default for a 1756 discrete I/O module is 1 ms. For The default for a FLEX
discrete I/O module is 0.25 ms.
input delay = 1 ms
There are no filters for outputs, so there is no additional delay for outputs. Outputs are always sent via an RPI timer.
Transmissions The transmission delay is the interval of time that it takes a packet to be transmitted at a specific bit rate, for example,
100 Mbps. For example, in a 7-slot ControlLogix chassis, the size of the entire packet is approximately 122 bytes
(including header, all protocols, all data, and CRC). At 100 Mbps, this packet takes approximately 10 microseconds.
(0.01 ms) on the wire.
transmission delay = 0.01 ms x (number of CIP connections)
Switch Switch latency is the delay between reception of the first bit and transmission of the first bit. This delay depends on the
type of switch. It is typically 0.1 ms.
switch delay = 0.1 ms
Queue Input data is sent from the remote rack (adapter), through a switch, through a communication module in the controller
rack, and finally to a controller (scanner). If two or more input CIP connections are simultaneously ready to be
transmitted, they must be transmitted sequentially.
It takes 0.2 ms for a 1756-ENBT to process 1 implicit packet. Note that 0.2 ms is equal to the reciprocal of 5000 (pps).
The total queue delay is 0.2ms times the number of CIP connections through the module.
queue delay = 0.2 ms X (number of CIP connections)