I'm late to this discussion, but I'll tell you how I prefer to handle analog channels. In most cases, the analog resolution is higher than required (for example 32767 counts). I set scaling in the module to 0 - 10000. So for example, it you are reading a 4- 20 mA signal from a 5000 PSI pressure transducer, 4mA = 0 PSI and 20 mA = 5000 PSI. So then you can handle the math in your SCP or just use a Compute Statement. Example: Raw Input * Full Scale (5000 PSI)/10000 = 5000 PSI. If more accuracy is desired, you can divide by 1000 and get 50000. Then in your MMI display, you convert it to floating point and divide the tag by 10 and get 5000.0 (or divide by 100 in you math statement and then divide the tag by 100 in the MMI) to get 5000.00 PSI.
Pay attention to where the 250 ohm resistors are, if there is already one or more in the system on some existing displays or recording devices. You can usually connect your differential input right across that existing device using the 1 -5 VDC input mode instead of 4 -20 mA and it will act the same. This is useful when there are a few devices already in the 4-20 mA loop and you are running out of open circuit voltage to add another 250 Ohm resistor. So you just piggy back on to one that is already present using a 1-5 VDC input.
If you are going to do much of this work, 4-20 mA tools are important. Measuring is easy, but sourcing takes a bit more effort. Also, there are clamp on measuring devices that will permit you to make a quick check on a 4 - 20 mA current without disconnecting it. If your not familiar with this, you could Google "Fluke 771 Case Study". It's now on the Transcat website. It covers a field test that I did when these devices first became available.
what am I missing.
