Goals and Symptoms

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.

Facts and Changes

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

Causes and Fixes

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.

Using the "Parameter" screens the module is configured for its set points,max value
and mode of operation. To reset the count is a simple matter of sending a command
to the first 4x register of the module's I/O map reference. Prior to doing anything you
should be aware of the terminology involving this module.

Reset: Turning off the output coils associated with the card. Reset has nothing to do
with your count values being set to 0 or max value.
Preset: This can be a hardware input or software command. Preset will set your current
count to zero if your counting up, or to max value if your counting down.

To Preset and Enable counters 1 and 2... Send the value 233H to the module.
To Preset and Enable counter 1 only..... Send the value 203H
To Preset and Enable counter 2 only......Send the value 230H

A hardware Preset is accomplished by pulsing the Preset input terminal on the EHC 202 for counter 1
or counter 2.



Follow the above mentioned command with a 300H command, which is the Read Counter command.
If you want to know what the last count was prior to Preset,send a 400H command. This command
will buffer the last count seen by the counters just before the Preset command arrived.

Remember, you can Preset the module with hard wired inputs to the Preset input of each counter or
you can use the software Preset described here. It is permissible to use both methods simultaneously
if you wish. Follow the wiring diagrams in the Quantum Hardware Reference Guide, they are accurate and
should be relied upon for proper module operation.
 

Using the "Parameter" screens the module is configured for its set points,max value
and mode of operation. To reset the count is a simple matter of sending a command
to the first 4x register of the module's I/O map reference. Prior to doing anything you
should be aware of the terminology involving this module.

Reset: Turning off the output coils associated with the card. Reset has nothing to do
with your count values being set to 0 or max value.
Preset: This can be a hardware input or software command. Preset will set your current
count to zero if your counting up, or to max value if your counting down.

 To perform the initial start up of the module you can send any of the following commands
 based upon the number of counters being used. The value should be sent to the 1st
 4xxxxx or %MW assigned to the module in the IO Map.

Startup
To Preset and Enable counters 1 and 2... Send the value 233H to the module.
To Preset and Enable counter 1 only......... Send the value 203H
To Preset and Enable counter 2 only..........Send the value 230H

 Following the Preset and Enable command you will send the Read command to the 1st
4xxxxx or %MW reference assigned to the module in the IO Map

 Read Counters  1 & 2  ...............................Send the value 300H


Run Time
  Once the PLC application is running and you wish to Preset and Enable counters 1 & 2
 send the following commands to the EHC202.

To Preset and Enable counters 1 and 2... Send the value  0033H to the module.
To Preset and Enable counter 1 only......... Send the value  0003H
To Preset and Enable counter 2 only..........Send the value  0030H


If you want to know what the last count was prior to Preset send a 400H command. This command
will buffer the last count seen by the counters just before the Preset command arrived.

Remember, you can Preset the module with hard wired inputs to the Preset input of each counter or
you can use the software Preset described here. A hardware Preset is accomplished by pulsing the
Preset input terminal on the EHC 202 for counter 1or counter 2. It is permissible to use both methods
simultaneouslyif you wish. Follow the wiring diagrams in the Quantum Hardware Reference Guide
they are accurate and should be relied upon for proper module operation.
 

Goals and Symptoms

Internet users may require to know the current TCP/IP network configuration values for setting up their Ethernet devices for TCP/IP connection or sending/receiving Email. In the example below, the ION meter is required to have its own IP address, Subnet mask, Gateway, DNS and SMTP server etc. Some of this information may be found on the network, or a network administrator may have to provide details. This is a document shows how to find common information and also explains the terminology.

Causes and Fixes

Displaying the current TCP/IP network configuration values:
Go to Start > Run > and type cmd and click ok. In the command line type ipconfig/all

Ipconfig is a command line tool used to control the network connections on Windows NT/2000/XP machines. There are three main commands: "all", "release", and "renew". Ipconfig displays all current TCP/IP network configuration values and refreshes Dynamic Host Configuration Protocol (DHCP) and Domain Name System (DNS) settings. Used without parameters, ipconfig displays the IP address, subnet mask, and default gateway for all adapters.

Understand internet terminology:

DNS server

Short for Domain Name System (or Service or Server), an Internet service that translates domain names into IP addresses. Because domain names are alphabetic, they're easier to remember. The Internet however, is really based on IP addresses. Every time a domain name is used, a DNS service must translate the name into the corresponding IP address. For example, the domain name www.example.com might translate to 198.105.232.4.

The DNS system is, in fact, its own network. If one DNS server doesn't know how to translate a particular domain name, it asks another one, and so on, until the correct IP address is returned.

DHCP server

Short for Dynamic Host Configuration Protocol, a protocol for assigning dynamic IP addresses to devices on a network. With dynamic addressing, a device can have a different IP address every time it connects to the network. In some systems, the device's IP address can even change while it is still connected. DHCP also supports a mix of static and dynamic IP addresses.

Dynamic addressing simplifies network administration because the software keeps track of IP addresses rather than requiring an administrator to manage the task. This means that a new computer can be added to a network without the hassle of manually assigning it a unique IP address. Many ISPs use dynamic IP addressing for dial-up users.

WINS server

Short for Windows Internet Naming Service, a system that determines the IP address associated with a particular network computer. This is called name resolution. WINS supports network client and server computers running Windows and can provide name resolution for other computers with special arrangements. Determining the IP address for a computer is a complex process when DHCP servers assign IP addresses dynamically. For example, it is possible for DHCP to assign a different IP address to a client each time the machine logs onto the network.

WINS uses a distributed database that is automatically updated with the names of computers currently available and the IP address assigned to each one.

DNS is an alternative system for name resolution suitable for network computers with fixed IP addresses.

IP Address

An identifier for a computer or device on a TCP/IPnetwork. Networks using the TCP/IP protocol route messages based on the IP address of the destination. The format of an IP address is a 32-bit numeric address written as four numbers separated by periods. Each number can be zero to 255. For example, 1.160.10.240 could be an IP address.

Within an isolated network, IP addresses can be assigned at random as long as each one is unique. However, connecting a private network to the Internet requires using registered IP addresses (called Internet addresses) to avoid duplicates.

Every computer that communicates over the Internet is assigned an IP address that uniquely identifies the device and distinguishes it from other computers on the Internet. An IP address consists of 32 bits, often shown as 4 octets of numbers from 0-255 represented in decimal form instead of binary form. For example, the IP address

168.212.226.204

in binary form is

10101000.11010100.11100010.11001100.

But it is easier for us to remember decimals than it is to remember binary numbers, so we use decimals to represent the IP addresses when describing them. However, the binary number is important because that will determine which class of network the IP address belongs to. An IP address consists of two parts, one identifying the network and one identifying the node, or host. The Class of the address determines which part belongs to the network address and which part belongs to the node address. All nodes on a given network share the same network prefix but must have a unique host number.

Class A Network -- binary address start with 0, therefore the decimal number can be anywhere from 1 to 126. The first 8 bits (the first octet) identify the network and the remaining 24 bits indicate the host within the network. An example of a Class A IP address is 102.168.212.226, where "102" identifies the network and "168.212.226" identifies the host on that network.

Class B Network -- binary addresses start with 10, therefore the decimal number can be anywhere from 128 to 191. (The number 127 is reserved for loopback and is used for internal testing on the local machine.) The first 16 bits (the first two octets) identify the network and the remaining 16 bits indicate the host within the network. An example of a Class B IP address is 168.212.226.204 where "168.212" identifies the network and "226.204" identifies the host on that network.

Class C Network -- binary addresses start with 110, therefore the decimal number can be anywhere from 192 to 223. The first 24 bits (the first three octets) identify the network and the remaining 8 bits indicate the host within the network. An example of a Class C IP address is 200.168.212.226 where "200.168.212" identifies the network and "226" identifies the host on that network.

Class D Network -- binary addresses start with 1110, therefore the decimal number can be anywhere from 224 to 239. Class D networks are used to support multicasting.

Class E Network -- binary addresses start with 1111, therefore the decimal number can be anywhere from 240 to 255. Class E networks are used for experimentation. They have never been documented or utilized in a standard way.

Subnet mask

A mask used to determine what subnet an IP address belongs to. An IP address has two components, the network address and the host address. For example, consider the IP address 150.215.017.009. Assuming this is part of a Class B network, the first two numbers (150.215) represent the Class B network address, and the second two numbers (017.009) identify a particular host on this network.

Subnetting enables the network administrator to further divide the host part of the address into two or more subnets. In this case, a part of the host address is reserved to identify the particular subnet. This is easier to see if we show the IP address in binary format. The full address is:

10010110.11010111.00010001.00001001

The Class B network part is:

10010110.11010111

and the host address is

00010001.00001001

If this network is divided into 14 subnets, however, then the first 4 bits of the host address (0001) are reserved for identifying the subnet.

The subnet mask is the network address plus the bits reserved for identifying the subnetwork. (By convention, the bits for the network address are all set to 1, though it would also work if the bits were set exactly as in the network address.) In this case, therefore, the subnet mask would be 11111111.11111111.11110000.00000000. It's called a mask because it can be used to identify the subnet to which an IP address belongs by performing a bitwiseAND operation on the mask and the IP address. The result is the subnetwork address:

Gateway

Gateway is a node on a network that serves as an entrance to another network. In enterprises, the gateway is the computer that routes the traffic from a workstation to the outside network that is serving the Web pages. In homes, the gateway is the ISP that connects the user to the internet.

In enterprises, the gateway node often acts as a proxy server and a firewall. The gateway is also associated with both a router, which use headers and forwarding tables to determine where packets are sent, and a switch, which provides the actual path for the packet in and out of the gateway.

SMTP

SMTP is short for Simple Mail Transfer Protocol, a protocol for sending e-mail messages between servers. Most e-mail systems that send mail over the Internet use SMTP to send messages from one server to another; the messages can then be retrieved with an e-mail client using either POP or IMAP. In addition, SMTP is generally used to send messages from a mail client to a mail server. This is why both the POP or IMAP server must be specified and the SMTP server when the e-mail application is configured.

POP

POP is short for Post Office Protocol, a protocol used to retrieve e-mail from a mail server. Most e-mail applications (sometimes called an e-mail client) use the POP protocol, although some can use the newer IMAP (Internet Message Access Protocol). There are two versions of POP. The first, called POP2, became a standard in the mid-80's and requires SMTP to send messages. The newer version, POP3, can be used with or without SMTP.

IMAP

IMAP is short for Internet Message Access Protocol, a protocol for retrieving e-mail messages. The latest version, IMAP4, is similar to POP3 but supports some additional features. For example, with IMAP4, a search can be done through the e-mail messages for keywords while the messages are still on mail server. The messages found can then be downloaded to the client machine.

Setting up the gateway radio

• In the Access Point radio, or whichever radio which you are directly connecting your PC to (this is called the eDiags Gateway radio), go into Setup, then eDiag Setup and enable eDiags. Enter the IP address of your computer (must match the address entered above), then click the Activate Changes button.

Setting up TView diagnostics database

• Open TView Management Suite, then select the Diagnostics software package.
• Click No if asked you whether you want to open a previously-used database. (unless you DO want to open that previously-created database!)
• Go to the Settings menu then to eDiags Port Settings. Enter the IP address used by the LAN port of your computer again here. Some computers have more than one LAN card, so we must specify the appropriate port. Note the IP port number is 1040, same as in the radio. Leave this as-is. Click Apply Changes then Exit.

• Go to the File menu, then New. Select a location to save your radio database and give it a name that means something to you and the system being tested. Then click Save.
• This opens the Database Setup dialog where you enter all your radios.
• Enter the Access Point first. Type its name and serial number. You MUST know the serial numbers. Each radio’s serial number is printed on the label on its under-side. If the radios are already installed, you can find the serial numbers by opening the saved cfg files with a program such as WordPad or Excel. Each radio's serial number is in there. (you DID save each radio’s config file in a safe place didn’t you?)
• Specify the radio model and configuration eg J Series Access Point. For other radios it'll be J Series Remote or Bridge. If a Bridge, it may be a single or dual antenna type.
• Select the eDiags (Ethernet) communication port. Enter the IP address of the radio you're directly connected to, and on which you have enabled the eDiags feature.

• If desired you may change any of the Alarm limits. For example you may be using a 24 volt power supply, so the range might be 23 to 25 volts.
• Click the Apply Changes To Database button, then click Add New to add more radios.
• Change the radio type to Remote or Bridge as needed. Also ensure you leave the eDiags IP address the same for all radios. Do NOT enter each radio's own IP address. In all cases, leave it at the IP address of the radio that has eDiags configured, and to which your computer is directly connected.
• When done adding radios, click Exit.

Monitoring the diagnostics

• Click the Group Poll button. (looks like three people standing in a small group) This will poll each radio in the system one after another. Any alarms show up at the bottom. Alarms can be acknowledged/cleared with buttons at the top. To change polling interval, go to the Settings menu, then Polling Setup.

• Disable Group Polling by de-selecting the same button. Then click Individual Poll. Select a Remote radio to poll. then click Toggle to turn on polling. You'll get a nice bar graph of values. When done, toggle polling off and exit.

• Go into the Tools menu then Statistical Performance. You'll see the second tab is the packet error test. Select a remote radio and start polling. It will run until you click Finish.

• You can then click the Commissioning Report button to get a nice report you can print.

• Go to the Data Logging menu and select View Trend – J and K Series. Select the desired radio. Drag the pointer at the bottom across to see parameters recorded at specific times and dates.