Analogue counts

[Conor 4]

Hi all, I was wondering if someone could explain something to me. On our site we use about 95% Bradley PLC's. My question is about the counts that I read back from the PLC, i.e. when I am online. There are differences in the counts that I am reading 4 mA ~ 0 counts 20 mA ~ 31200 counts or 4 mA ~ 6240 counts 20 mA ~ 31200 counts Is this to do with the type of Analogue card that the Input is coming through?

[Mickey 71]

Its usually a configuration choice. In the example above one card is configured for 4 to 20ma, the other is configured for 0 to 20ma. Some cards don't have this feature. Some cards each channel can be configured differently. What is the complete catalog number of your analog card.

[Conor 4]

Hi Mickey, I will have to look at the cards. But, basically you are saying that 0 ~ 31200 counts, means that the card is 0 ~ 20 mA? (I don't mean to put words into your mouth. Also, as far as I can remember I have noticed this on the older platform PLCs, PLC5. Thanks again, Conor

[TimWilborne 108]

As Mickey said it is extremely dependent on the card. For example I'm working with a 1762-IF4 on a Micrologix 1400 right now. It's configuration choices are +-10VDC or 4-20mA. From there you can scale it for PID or raw which you have selected raw. But this is misleading, if you select the 4-20mA option your range is actually 0-21 mA over 15 bit or 0-32767 So to get your raw number for a particular mA you take the mA and multiply by the full scaling and divide by the full mA range or XmA*32767/21 = raw value Always read the manual closely for the particular card and PLC combination! mA=raw 0=0 1 =560 2=3121 3=4681 4=6241 5=7802 6=9362 7=10922 8=12483 9=14043 10=15603 11=17164 12=18724 13=20284 14=21845 15=23405 16=24965 17=26526 18=28086 19=29646 20=31207 21=32767