1756-OF8 Scaling

1756-L71
RS5000 V20
Analog inputs: 1756-IF16, 4-20mA,Float data single ended mode.
Analog outputs: 1756-OF8, Current, Float data, Output range 0-20mA, High signal=20.0mA Low signal=4.0mA

A customer has requested that we resend our analog inputs (flows, psi, temp etc..) back to their DCS via analog outputs.
I'm guessing there's two ways to do this?
1. Match the AO scaling High/Low signal and High/Low engineering to that of the AI scaling along with matching the AO limits to the High/Low engineering.
Or
2. Use the SCL to scale the AI 4mA back to 0 for low engineering and 20mA back to 100 for high engineering. This would then match the default settings of the 1756-OF8.

Is there some other faster/easier way to do this?

Thanks

I see that I was wrong. The IF16 module in Floating Point mode and set for 4-20 mA uses all 16 bits, but the OF8 module set the same way only uses 15 bits, so each count of the IF16 = 0.32 microAmps, where each count of the OF8 = 0.65 microAmps. You will need to multiply each input by 0.34/0.65 or 0.523077, before sending it to the output.

Does the DCS communicate with the PLC? If so why not just poll the info.

My previous messsage should have read "...so each count of the IF16 = 0.34 microAmps,"

MATT116 said:

1. Match the AO scaling High/Low signal and High/Low engineering to that of the AI scaling along with matching the AO limits to the High/Low engineering.

Click to expand...

Just match the Scaling on the AI and AO channels.

AI High Engineering = AO High Engineering
AI Low Engineering = AO Low Engineering
Then MOV AI_ChData AO_ChData

Lancie1 said:

The IF16 module in Floating Point mode and set for 4-20 mA uses all 16 bits, but the OF8 module set the same way only uses 15 bits, so each count of the IF16 = 0.32 microAmps, where each count of the OF8 = 0.65 microAmps. You will need to multiply each input by 0.34/0.65 or 0.523077, before sending it to the output.

Click to expand...

You don't need to scale it before sending it out when your Modules are set to use Float data. The module will interpret it the best that it can.

However, this is a good caution to MATT116:
You will lose resolution when you consume the data from the field, and then output it again.
Your input: 0.32uA
Your output: 0.65uA
(from Publication 1756-UM009C-EN-P - December 2010, pg. 49)

I think Mickey has the right approach. Why can't you send it in the clear over other communications? That way your DCS has the exact same info as your PLC.
Otherwise, your output will drop resolution, and you will see discrepancies between the PLC and DCS.

Mickey said:

Does the DCS communicate with the PLC? If so why not just poll the info.

Click to expand...

Because their old school I guess? they don't trust anything that is not "hard wired" nor will they use analog splitters.

devo.xx said:

Just match the Scaling on the AI and AO channels.

AI High Engineering = AO High Engineering
AI Low Engineering = AO Low Engineering
Then MOV AI_ChData AO_ChData

Click to expand...

Won't I also need to adjust the High and Low limits also to match the High and low engineering

devo.xx said:

You don't need to scale it before sending it out when your Modules are set to use Float data. The module will interpret it the best that it can.

Click to expand...

I was just thinking if I scaled it I would not have to copy the AI high/low scaling to AO high/low scaling

devo.xx said:

However, this is a good caution to MATT116:
You will lose resolution when you consume the data from the field, and then output it again.
Your input: 0.32uA
Your output: 0.65uA
(from Publication 1756-UM009C-EN-P - December 2010, pg. 49)

I think Mickey has the right approach. Why can't you send it in the clear over other communications? That way your DCS has the exact same info as your PLC.
Otherwise, your output will drop resolution, and you will see discrepancies between the PLC and DCS.

Click to expand...

They refuse to do anything over ethernet or MODBUS, "It's not robust enough"...that was their reasoning. I did bring the resolution issue up to them but they were ok with it since they will only be displaying the data on their end.


I guess ultimately I was looking for a simpler way other than coping the AI scaling over to the AO scaling, atleast its only 24points so I can just hammer through them


Thanks for the help guys

Look at a signal splitter going into the PLC, split it in two one to the PLC and one to DCS.

MATT116 said:

They refuse to do anything over ethernet or MODBUS, "It's not robust enough"...that was their reasoning. I did bring the resolution issue up to them but they were ok with it since they will only be displaying the data on their end.

Click to expand...

It happens. Clients want what they want.

MATT116 said:

I guess ultimately I was looking for a simpler way other than coping the AI scaling over to the AO scaling, atleast its only 24points so I can just hammer through them

Click to expand...

Yeah, otherwise you'd have to write code in the PLC to do all the scaling. Which would just increase your scan time. Also, I find the best practice is to keep my AI and AO High/Low Engineering the same (4,000.0-20,000.0 for 4-20mA) otherwise my eyes tend to get crossed.

cornbread said:

Look at a signal splitter going into the PLC, split it in two one to the PLC and one to DCS.

Click to expand...

They shot that idea down, Why I don't know they didn't give me a reason.

Are you actually providing Analog Output Modules just to re-transmit the Analog Inputs back to the DCS ?

Why can't you just loop the DCS inputs with the PLCs Analog Input... ?

Yup that's how they want to do it. They shot down any other idea and they don't care about the price.