Issue

This document will outline factors related to performance including, PLC CPU scan time, Modbus data types and Ethernet Line bandwidth. It will become apparent that in an automation network the speed of the wire is not the hindering factor when it comes to performance.

Product Line
Quantum Platform, SCADA system, 10/100 MB Ethernet cables, Unity Pro, Concept, Proworx NxT, Proworx 32

Environment
Windows 7

Resolution
The number of Modbus requests serviced by the NOE module will vary by the NOE module Exec version and the CPU that it is attached to. But in a maximum case the module can service 12 Modbus requests per PLC scan.
In the example below, we calculate the time it takes to send 1 Modbus query to read 100 holding registers (4x) and receive 1 Modbus response to that query on a 10 Mbs Ethernet link.

As a reminder, in this resolution the actual Ethernet cables speed is
At 10 Mb/s transfer rate it can carry 10 * 1,000,000 = 10,000,000 bits/second
At 100 Mb/s data transfer rate, it can carry 100 * 1,000,000 = 100,000,000 bits/second


Modbus query of reading 100 4x holding registers is 82 bytes
MAC 18 bytes (6 Destination MAC, 6 Source MAC, 2 Ether type, 4 FCS)
IP Header 20 bytes
TCP Header 20 bytes
MB Header 7 bytes (2 XID, 2 Protocol version, 2 length, 1 unit ID,)
MB PDU 5 bytes (1 byte function code, 2 start reference, 2 no of words to read)
IPG 12 bytes
Query Total 82 bytes
656 bits of data

On a 10 Mb/s link 656 bits/10,000,000 bits/s = 0.0656 ms
On a 100 Mb/s link 656 bits/100,000,000 bits/s = 0.00656 ms

Sending 12 of such messages on a 10 Mb/s link will only take about 0.7872 ms and the same message sent on a 100 Mb link will take 0.07872 ms. As you can see the time difference is insignificant when compared to the actual CPU scan.

Modbus response to that query is 279 bytes
MAC 18 bytes (6 Destination MAC, 6 Source MAC, 2 Ether type, 4 FCS)
IP 20 bytes
TCP 20 bytes
MB Header 7 bytes (2 XID, 2 Protocol version, 2 length, 1 unit ID,)
MB PDU 202 bytes (1 function code, 1 byte count, 100 words response data)
IPG 12 bytes
Response Total 279 bytes
2232 bits

On a 10 Mb/s link 2232 bits/10,000,000 = 0.2232 ms
On a 100 Mb/s link 2232 bits/100,000,000 = 0.02232 ms

Note: Message size does have bearing on overall transmission time and it takes longer to transmit the response than to send a query.

Sending 12 of such message on a 10 Mb/s link will take 2.6784 ms and the same message sent on a 100 Mb/s link will take 0.26784 ms.

For an average CPU you will have a 35 ms scan time. Therefore any delay to link speed being either 10 MB or 100 MB is reasonably insignificant compare to CPU scan.

Application Example for Network utilization:

In this example the CPU is executing 10 Modbus Query/Response transactions as outlined above to simulate IO Scanning 100 registers at a 100ms Repetition Rate.

0.000656 % TX Load @ 10 Mbs
0.002232 % RX Load @ 10 Mbs

0.0000656 % TX Load @ 100 Mbs
0.0002232 % RX Load @ 100 Mbs

The link speed comparison is insignificant on Application Response Time compared to other factors:

o CPU scan
o Datatypes requested (4x register vs. none 4x)
o Half duplex operation
o Slave device response time delay

Note: Half duplex can be a significant factor at either 10 or 100 Mb/s due to collisions which will result in communication delays. Typically devices operating at 10 Mb are usually operating at half duplex and therefore subject to collision.



I/O Scanner
I/O Scanner messages are serviced at the end of CPU scan. Each IO Scanner entry requires 0.3 ms to process. IO Scanners response time also depends on the response time of the remote device being IO Scanned.

For example one I/O scanner entry that reads/writes 10 words (~115 bytes or 920 bits) takes approximately 0.092 ms to send on 10 Mbs link, versus 0.0092 ms to process on a 100 Mbs link. The actual time difference related to the connection speed in this example is less than 1 millisecond at 0.0828 ms.

In summary the CPU scan and remote devices response time have more impact on I/O Scanner response time than the link speed. Regardless of the link speed the difference is minute.