How to specify Attribute ID in OPC Server DevicenetExplicit

Question

I'm trying to set up an Allen Bradley 842D Devicenet Absolute Encoder
using the OPC server.  The manuals for the device mention that the
data can be retrieved from the various Attribute ID's in each
class.  From the OPC Server Help example below,  it starts to
describe the location of the data desired,  but fails to show me
where I specify the attribute ID.  For example, lets say in the
example below,  I now want Attribute 2 or 3.  How does the
below example tell me how?.
Since I'm trying to get all this data in via the IA OPC server, 
I'm not planning on using the DNET Vi's.  BUT.....I did look at
the "GetIdentityAttrs.vi" and notice that the VI's provided allow for
Class ID, Instance ID, and Attribute ID.  All of which correctly
gives me the information I want when I run this VI with the 842D
encoder.
From OPC Server Online Help:
NIDeviceNetExplicit Example
Suppose you want to get attribute 1 from Class ID 1, Instance 3 of an
object. Class ID 1 is the Identity Class, where all the identification
information for the device is contained.
Enter the background information when you create the
NIDeviceNetExplicit object, setting the Service Code to 14 and the
Class ID to 1. From the manufacturer’s documentation, you know that
GetAttribute only accepts an unsigned integer (USINT) as a parameter.
The baud rate for the object in question is an 8-bit unsigned integer
(USINT). To read the baud rate, access the IID3.USINT0 data member.
From the Above,  this is what how i'm interpreting the configuration:  IID ( Instance ID).USINT (data offset)
To re-iterate my question:  How do I specify the Attribute ID when using the IA OPC Server DeviceNetExplicit Object? 
Regis

Answer 1

Hi Regis,
The general steps to send & receive the explicit message using the NI OPC Server:
Configure an OPC item to write the explicit message, you have to set the attribute ID for the GET request message here.
Configure an OPC item to read the explicit message, you will get the explicit response from it.
Example: Read the Vendor ID (ClassID=1, InstanceID=1, AttributeID=1, DataType=UINT)
(1) Configure the NIDeviceNetExp object with the right MAC ID, ServiceCode=14(GET), ClassID=1
(2) Add an OPC item (Data Member) NIDeviceNetExp1.IID1.USINT0 to write the parameter.
(3) Add an OPC item (Data Member) NIDeviceNetExp1.IID1.UINT0 to read the Vendor ID.
(4) Then you get the alarm message in IA OPC server and get failure message in your OPC client , if you have one started allready, because it reads the item automatically.
The magic action is here: you write the value 1 to NIDeviceNetExp1.IID1.USINT0. (then the attribute ID 1 is set correctly).
You can get the vendor ID then reading NIDeviceNetExp1.IID1.UINT0.
2. Example: If you want to get the Serial Number (ClassID=1, InstanceID=1, AttributeID=6, DataType=UDINT)
Add an OPC item NIDeviceNetExp1.IID1.UDINT0
You write the value 6 to NIDeviceNetExp1.IID1.USINT0
You can get the Serial Number in NIDeviceNetExp1.IID1.UDINT0
So you can conclude that:
(1) if you write to NIDeviceNetExp1.IID1.USINT0, USINT is the data type used to encode the request message parameter.
(2) if you read from NIDeviceNetExp1.IID1.UDINT0, UDINT is the data type used to decode the response message.
Attached you can find an example using LV and  Datasocket to read the Vendor ID.
Hope that helps with your problem.
DirkW
Attachments:
DNET to OPC Item.vi ‏44 KB