1794-IE8 scaling in PLC-5

Hey guys,

I'm reading in analog values from 1794-IE8 flex IO card to my PLC-5/40 using BTR instructions. Now, I'm doing my scaling in wonderware InTouch 7.1. I believe 4ma=0 and 20ma= 30840. Please confirm that.

Also, when you do the BTW instruction what configuration you write to those cards in the first PLC scan. Only thing I found in the manual was to punch in a decimal value of -256 as the first word of the data file in your BTW instruction (and length=1) showing a configuration of 4-20mA. Is that right or I have to do anything else ? Is there way to get a scaled analog value directly from the cards by configuring the BTW instruction in a certain way ?

Any help is appreciated.
Thank you.

Not sure about the -256, I thought it was positive 256, and for raw counts I get 0-30832. This is from playing around with the module configuration in RSLogix 5, I don't have the manual.

You could do the scaling with a compute instruction CPT using the following expression:
Assume engineering units are 0-300 psig

((Current counts / 30832) * 300)= scaled value (stored in floating point)

Assume engineering units are 50-300 psig

((Current counts / 30832) * 250))+50 = scaled value (stored in floating point)

See attached

0-7878 HEX (0-30840 dec)

Configure BTW Here

Thanks Ken. I'm letting their wonderware do the scaling for now(as it was doing it before) but from Mickey's attached file it looks like my raw max is 30840 for the AI card. As you said, I guess I have to play with it online and tune it up maybe.

The reason I asked about punching in -256(decimal) in the first word of the data file in BTW instruction to configure 1794-IE8 because in the manual it says F0-F7(full range bits) are 0s and C0-C7(configuration bits) are all 1s. F0 starts from 0 decimal in the binary radix of that word in the data file and C0 starts from 8 decimal. Punching in this configuration gives you a decimal value of -256 when you change the radix to decimal from binary for that word in the data file.

Please let me know what you think about it.
Thanks.

Thanks Mickey.

But I can't open that window. Everytime I click on Setup Screen under BTR or BTW instruction it says "No modules exist in the IOConfiguration for this R/G/M". But I do have them added in the IO Configuration window. I have added one rack per chassis. I don't know if I have to wait till I do an auto-configure out in the field and maybe then I can access the setup screen for BTR/BTW.

In the I/O configuration, you need to click on the Flex I/O adapter, and then select the module for the correct location, then verify the program.

I did that too Ken. It still gives me the same message that I wrote to Mickey whenever I click on the Set Up Screen for BTR/BTW instruction. You didn't mention about the -256 configuration thing. So I'm guessing that I'm on the right track.

Thanks.