AB 825P DeviceNet

Hi All,

I am using an AB 825P modular system with RTD scanner module, It is connected to an L35e Via Device Net scanner card 1769-SDN. I want to display and trend my RTD temps on SCADA. They are parameters 383 through to 394, I can read the RTD values Individually by using an explicit message single attribute. My question is can I read all 12 RTD values using 1 message instruction?

Thanks for your help in advance.

The two methods I've seen used to read a group of parameters with a single message request over an A-B CIP network (like DeviceNet) are the Assembly Object, the PCCC Object, and the Scattered Parameter Read command.

Devices like the A-B PowerMonitor 3000 use Assembly Objects to group similar parameters together and you can read them using a single request to read the Assembly.

Devices like PowerFlex drives support the Scattered Parameter Read, in which the request contains a list of parameter numbers, and the reply contains the same list of parameter numbers plus the values of those parameters.

But I don't see either of those supported by the 825P DeviceNet module for RTD data.

The PCCC object emulates PLC-5/SLC-500 data tables. I don't see any reference to that object in the 825P.

There are a handful of Assembly objects that will deliver several related parameters as a single block, but those don't cover the RTD module.

I didn't see any mention in the user manual of the Scattered Parameter Read function. It might still exist, but because the 825P doesn't share the same technical origin as PowerFlex drives, it's unlikely.

Hi Ken,

Thanks for the feed back. I guess I will just have to read them back by individual
messages.

Re-reading that, I meant *three* methods.

Since you've already got a string of messages that works, stick with that method.

Have you tried setting the produced assembly to 100 (custom based), assembly words to the RTD parameter ID's and messaging the assembly?

Johnny Boy said:

...My question is can I read all 12 RTD values using 1 message instruction?

Click to expand...

No, I don't think so?

Appendix E of the 825-P User Manual covers all the supported information which may be Produced, or Consumed using the DeviceNet Communication Card(825-PDN).

strata202 said:

Have you tried setting the produced assembly to 100 (custom based), assembly words to the RTD parameter ID's and messaging the assembly?

Click to expand...

There is indeed a 'Custom Parameter Based Input (Produced) Assembly Instance 100' for the 825-PDN.

Johnny Boy,

The only drawback to using this Custom Assembly is that it only provides 8 words (16 bytes) of configurable Produced data. You require 12 words (24 bytes). So you still would not be able to read all 12 RTD values in 1 message, but you could achieve it in 5 messages.

1 MSG x 8 word
4 MSG x 1 word

To use the Custom Assembly Object, you have to assign 8 of the RTD Data Parameter numbers that you require, say 383-390, to 'PROD ASSY WORD 0' - 'PROD ASSY WORD 7'. These are Parameters 11-18 respectively. Note: You can assign any read, or read/write Parameters to these Assembly words. Then, once you read from Instance 100, it will pull in the Parameter values assigned to the 8 'PROD ASSY WORD's.

Then use Get_Attribute_Single to MSG the remaining 4 RTD values at 391-394.

As far as I can see, this configuration uses the least amount of explicit messages possible.

Or, indeed, you could continue to use 12 single explicit messages.

I've pulled the relevant data together from the manual. See attachment.

George.

Hi George,

Thanks for your reply, Can you please go through the message set up.I had a go and it comes up as object dose not exist.

Thanks Johnny

Johnny,

Sorry for the delay in getting back to you. I am allowed take holidays but once in a blue moon.

Just to confirm what I thought...

47756 - How do you use a single message to read all 12 RTDs on a 825-P?
Access Level: Everyone

So, to get that Assembly 100 working...

Johnny Boy said:

...it comes up as object dose not exist...

Click to expand...

RSLogix 5000 Help said:

CIP Error 16h -
Object does not exist - The object specified does not exist in the device.

Click to expand...

The CIP Error Codes are specified by the ODVA. They more specifically define error code 16h as "The class/instance numbers are wrong"

What message settings did you enter on the MSG 'Configuration' tab when you got the error?

Use the attached screenshot to enter your settings and try again. In my example, 'RTD_Data' is an array of 8 x INT.

EDIT: Set Parameter 10 = 100 (Produced Assembly)

George

Hi George,

Thanks for the reply I had my class number wrong, I changed it to 4 and its working. Thanks again for your help.

Determine Class Instance Attribute

You're welcome.

For future reference...

As per the CIP specification, on any CIP device, the Assembly Object should always be Object Class ID: 0x04 (hex), no matter which Instance/Attribute of an Assembly you are servicing.

If you don't know the CIP settings of a parameter in a device to enter into your message, you can usually find it in the user manual of the device.

Another way is to upload the EDS file from the device and decipher the Class/Instance/Attribute for the parameter in the [PARAMS] section...

47397 - Using an EDS file to find the Class, Instance and Attribute for a parameter
Access Level: Everyone

George