Tag Archives: host link

Implementing the Omron Host Link Protocol (VB.Net)

Previously we used VB6 (Visual Basic 6) to implement the Omron Host Link Protocol to communicate to our programmable logic controller. (PLC) In part 2 we used Visual Basic for Applications (VBA) to accomplish our host link serial communication. We will now look at using VB.Net to establish and retrieve information in and out of the PLC using host link. (C mode commands)

Hostlink communication protocol is a method developed by Omron for communication to PLC’s and other equipment. This ASCII based protocol is used over RS232 or RS422/RS485. It is a many to one implementation which means that you can communicate with up to 32 devices back to a master. (1:N) This communication on the industrial floor can control PLC’s, Temperature Controllers, Panel Meters, etc.

The Omron CP1H plc will be used with a CP1W-CIF11 (RS485) communication serial link adapter. We will also use a USB-485M Universal Serial Bus to RS485 adapter from automation direct. The video at the end of this post will demonstrate the communications by reading 10 registers form the DM area of the PLC. Continue Reading!

How to Implement the Omron PLC Host Link Protocol

Hostlink communication protocol is a method developed by Omron for communication to PLC’s and other equipment. This ASCII based protocol is used over RS232 or RS422/RS485. It is a many to one implementation which means that you can communicate with up to 32 devices back to a master. (1:N) This communication on the industrial floor can control PLC’s, Temperature Controllers, Panel Meters, etc.

Our look at this protocol will include the wiring, setting of RS232 port settings, protocol format and  writing a VB6 program to read information from the PLC. I will also point you links to then store this information into a database and share over an intranet/internet. Lets get started.

Wiring of the communication ports will depend on the equipment purchased. If communicating over 15 meters, it is recommended to switch to RS422 or RS485 connection. However I have seen RS232 runs of 50 meters without an issue. It will depend on your implementation and electrical noise in the plant.
Omron Host Link Cable Connections

The above diagram is the basic communication needed for RS232C. Note that the shield of the communication wire is connected only to one side. This ensures that any noise induced in the communication is filtered to one end.

Settings for RS232C communications are set in a number of ways. Older Omron C**K PLC were set through a series of dip switches. Current Omron SMR1/CPM1 PLC’s are set though data memory locations.
Note: Most of the time, you need to cycle the power or switch to program / run mode for the setting to be activated.
Omron Host Link Settings 1 Omron Host Link Settings 2

I generally tend to leave everything at the default settings: 9600 bps, Even parity, 7 data bits, 1 stop bit. The default host link unit number is 00. (32 max. – 00 – 31)

Protocol Format
Each piece of equipment will have a list of parameters that can be read and written using the HostLink protocol. This can be found in the programming manual of the device. Here are the areas in the CPM1/CPM1A/CPM2A/CPM2C/SRM1(-V2) from the programming manual.

HostLink Areas

Lets take a look at the command to read the DM area. All of the commands and responses will be in an ASCII format.

HostLink DM Area Read

The command format begins with a ‘@‘ sign followed by the Node / Unit number that you wish to communicate. Header code is the command in which you with to execute. (RD) This header code will determine the next series of information. In our case the next four digits will be the beginning word followed by the next four digits to indicate the number of words. The next part of the command is the FCS (checksum) calculation. The comparison to this at each end will ensure that the command/response is correct. FCS is a 8 bit data converted into two ASCII characters. The 8 bits are a result of an Exclusive OR performed on the data from the beginning to the end of the text in the frame. In our case this would be performed on the following:

"@00RD00000010"

The last part of the command is the terminator. This is an ‘*’ followed by the character for the carriage return. (CHR$(13))

The response format begins with a ‘@’ sign followed by the Node / Unit number that you are communicating to. The header code is next (RD) followed by the End Code. The end code is a two digit ASCII code that indicates the message response / errors when executing the action. A normal code of ’00’ indicates that everything is fine. See the operation manual for the entire list of end codes for your equipment. The next part of the response depends on the header code executed. In our case it would contain the data requested. The last two parts of the response is the FCS and terminator just like the command format.

Omron HostLink Frame_Response

The above shows the timing of the command and responses.

Visual Basic VB6 (Example)
Now lets look at an example of reading the first 10 words from the DM area  of an Omron PLC.

HostLink VB6 Program1

The first step is the design the form. You can see that we have our ten DM area words set out to populate with values. We also have a T$ for transmit. This will show what we are sending to the PLC. The RXD$ will show what the response will be from the PLC.

The MSComm is used to communicate through the serial ports of the computer. The following is the settings for the communication port.

HostLink VB6 Program2

Here is the VB6 code for the program:
When the form loads the Date/Time will get updated and Timer1 is enabled. This timer controls the interval in which the commands get executed. (Set to 1 second)

Private Sub Form_Load()
 Label2.Caption = Format(Date, "YYYY/MM/DD") + "    " + Format(Time, "HH:MM:SS")
 Timer1.Enabled = True
 End Sub

The following code will open the communication port, set the command format, send the command through the port, receive the response through the port and display the information. It will then close the communication port.

Private Sub Timer1_Timer()
 Timer1.Enabled = False
 MSComm1.PortOpen = True
 Label2.Caption = Format(Date, "YYYY/MM/DD") + "    " + Format(Time, "HH:MM:SS")
'Check DM AREA DM0000 to DM0009 data update
 T$ = "@00RD00000010"
 charreturn = 51
 GoSub FCS
 GoSub communicate
'Show Transmit information
 Label24.Caption = Buffer
 'Show Returned information
 Label26.Caption = rxd$
If Mid(rxd$, 6, 2) = "00" And (Len(rxd$)) >= charreturn Then
 Label4.Caption = Mid(rxd$, 8, 4)
 Label6.Caption = Mid(rxd$, 12, 4)
 Label8.Caption = Mid(rxd$, 16, 4)
 Label10.Caption = Mid(rxd$, 20, 4)
 Label12.Caption = Mid(rxd$, 24, 4)
 Label14.Caption = Mid(rxd$, 28, 4)
 Label16.Caption = Mid(rxd$, 32, 4)
 Label18.Caption = Mid(rxd$, 36, 4)
 Label20.Caption = Mid(rxd$, 40, 4)
 Label22.Caption = Mid(rxd$, 44, 4)
 End If
 Timer1.Enabled = True
 MSComm1.PortOpen = False
 Exit Sub

The following is the subroutine to communicate. Timer2 is the amount of time to wait before expecting an answer on the communication port. Once the command has been sent a maximum of two seconds is waited for an response. If no response nothing is returned. When the response is obtained, the FCS is checked and if correct the information is returned.

communicate:
 rxd$ = ""
 Buffer = T$ + FCS$ + "*" + Chr$(13)
 MSComm1.Output = Buffer
 Timer2.Enabled = True
 Do
 DoEvents
 Loop Until Timer2.Enabled = False
 If Time > #11:59:50 PM# Then
 timeout = #12:00:02 AM#
 Else
 timeout = DateAdd("s", 2, Time)
 End If
 MSComm1.InputLen = 0
 Do
 If timeout <= Time Then GoTo timeoutcom
 DoEvents
 Loop Until MSComm1.InBufferCount >= charreturn
 rxd$ = MSComm1.Input
 fcs_rxd$ = Left((Right(rxd$, 4)), 2)
 If Left(rxd$, 1) = "@" Then
 T$ = Mid(rxd$, 1, (Len(rxd$) - 4))
 ElseIf Mid(rxd$, 2, 1) = "@" Then
 T$ = Mid(rxd$, 2, (Len(rxd$) - 5))
 rxd$ = Mid(rxd$, 2, (Len(rxd$) - 1))
 End If
 GoSub FCS
 If FCS <> fcs_rxd$ Then
 rxd$ = ""
 End If
 clearbuffer$ = MSComm1.Input
 Return

This is the FCS (checksum) calculation routine.

FCS:
 L = Len(T$)
 A = 0
 For J = 1 To L
 TJ$ = Mid$(T$, J, 1)
 A = Asc(TJ$) Xor A
 Next J
 FCS$ = Hex$(A)
 If Len(FCS$) = 1 Then FCS$ = "0" + FCS$
 Return

This is the routine that will execute if the response is not received within the time period expected.

timeoutcom:
 clearbuffer$ = MSComm1.Input
 rxd$ = ""
 Return
End Sub

Timer2 was used as a delay before looking for a response after sending the command.

Private Sub Timer2_Timer()
 Timer2.Enabled = False
 End Sub

Here is the code running:

HostLink VB6 Program3

Helpful Tips/Links:
– When troubleshooting serial communications it is sometimes helpful to use HyperTerminal. This program will send and receive information in/out of the serial ports.
HostLink Command Generator
HostLink Command Format

Watch on YouTube : How to Implement the Omron PLC Host Link Protocol

If you have any questions, need further information or would like a copy of this program please contact me.
Thank you,
Garry



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