Understanding 4-20mA inputs and raw values
Understanding 4-20mA inputs and raw values
(OP)
I am not just a bit rusty, but majorly rusty...
I am using an AB 1756-L55 ControlLogix 5555 Controller.
It has a remote 1794AENT/A Adapter.
I have an Analog Input Module - 1794/IE8
I have an Analog Output Module - 1794/OE4
My Analog Input Module is configured for (0-10V) / (0-20mA)
My Analog Output Module is configured for 4-20mA
For Inputs: I have been told that a 4mA signal will produce a value of 6242 (I figure it is exactly 6241.6). And a 20mA signal will produce a value of 31208.
For Outputs: 0mA = 0 value, and 20mA - 30840.
1. Can someone confirm this?
2. Where did they find this? In the manual for the analog cards, the PLC?
3. I forced an output to 15420 and noticed that the output was not 12mA. Should it have been?
4. For a PID Loop, in RSLogix v11.15, I am setting up a PID loop and am looking at the scaling tab. I have a Pressure Transmitter that is ranged from 0-300psi. I have a Control Valve that opens from 0 to 100%.
Should my Values be:
Process Variable:
Unscaled Max = 31208
Unscaled Min = 6241.6
Engineering Unit Max = 300
Engineering Unit Min = 0
Control Variable:
Max at 100% = 30840
Min at 0% = 0
I am sure this is such basic stuff, I am just trying to gain a better understandning.
I am using an AB 1756-L55 ControlLogix 5555 Controller.
It has a remote 1794AENT/A Adapter.
I have an Analog Input Module - 1794/IE8
I have an Analog Output Module - 1794/OE4
My Analog Input Module is configured for (0-10V) / (0-20mA)
My Analog Output Module is configured for 4-20mA
For Inputs: I have been told that a 4mA signal will produce a value of 6242 (I figure it is exactly 6241.6). And a 20mA signal will produce a value of 31208.
For Outputs: 0mA = 0 value, and 20mA - 30840.
1. Can someone confirm this?
2. Where did they find this? In the manual for the analog cards, the PLC?
3. I forced an output to 15420 and noticed that the output was not 12mA. Should it have been?
4. For a PID Loop, in RSLogix v11.15, I am setting up a PID loop and am looking at the scaling tab. I have a Pressure Transmitter that is ranged from 0-300psi. I have a Control Valve that opens from 0 to 100%.
Should my Values be:
Process Variable:
Unscaled Max = 31208
Unscaled Min = 6241.6
Engineering Unit Max = 300
Engineering Unit Min = 0
Control Variable:
Max at 100% = 30840
Min at 0% = 0
I am sure this is such basic stuff, I am just trying to gain a better understandning.





RE: Understanding 4-20mA inputs and raw values
The caveat is that you must configure all channels, a small price to pay for the conveinence, I would highly recommend it. Now your program will just use Engineering units and you will find it much easier to understand.
One count will be equal to Engineering Units/Resolution. For example lets assume we have a 0 to 10V signal, which represents 0 to 1000 psi, and assuming a 12 bit resolution 1 count = 1000/4095 or .244psi.
4-20MA signals are a little different since the signal never goes to zero, and I have not done this in 20+ years. But maybe this will help.
RE: Understanding 4-20mA inputs and raw values
There is an analog data format table in the appendix of the manual which should help explain things.
http:/
RE: Understanding 4-20mA inputs and raw values
"My Analog Input Module is configured for (0-10V) / (0-20mA)"
while you were using 4mA as min value of the range,
"For Inputs: I have been told that a 4mA signal will produce a value of 6242 (I figure it is exactly 6241.6). And a 20mA signal will produce a value of 31208."
Similarly, in your AO you config 4-20, but you used 0-20mA
"My Analog Output Module is configured for 4-20mA
-For Outputs: 0mA = 0 value, and 20mA - 30840."
RE: Understanding 4-20mA inputs and raw values
NWBeaver states that the values are 12-bit and shifted left, I would assume 4-bits in witch case 40950 would be the maximum count.
4mA is 25% of full scale for a count of 1024, 10240 shifted to the left. I can't get on Rockwells website right now, so I am making assumptions.
The information is in the manual for the particular module.
As far as Engineering units I would use 3000 or even 30000 for better resolution, unscaled max would be 4095 and 0 for min assuming the value will alwasy be positive.
The analog output scaling would be the same.
RE: Understanding 4-20mA inputs and raw values
RE: Understanding 4-20mA inputs and raw values
The value 30840 comes from the Data Table in the AB manual that I posted a link to above.
AB used a range of 4 to 21ma for the 4 to 20 mode, and 0 to 21ma for the 0 to 20ma mode...
So, for the 4 to 20ma mode, 20ma turns out to be about 30840.
The math goes something like this...
With 12 bits, 4095 represents full scale.
The value is shifted left 3 bits (multiplied by 8)
So 4095 becomes 32760
For a span of 0 to 21 ma, 32760/21 = 1560
20ma is then 1560 x 20 = 31200 or 79E0h
For a span of 4 to 21 ma, 32760/17 = 1927
20ma is then 1927 x 16 = 30832 or 7870h
RE: Understanding 4-20mA inputs and raw values
I tried to get to your link, but was unable to.