Issue
Can certain Analog Input values be read with up to 3 decimal places in WorkPlace Tech?
Environment
- WorkPlace Tech
- MNL series
- MNB series
- MicroNet 2000
- Network 8000
Cause
The MNL, MNB, MicroNet 2000 and Network 8000 controllers have the capability of working with values from -163.83 to 16,383 and never use more than 2 digits after the decimal place. This is something that all of the controllers have used for many years. This is documented on page 25 of WorkPlace Tech Tool 4.0 Engineering Guide (F-27254).
Resolution
In certain applications where it is necessary to read very small values beyond 2 decimals, a work around has been used. The technique should only be used by those who fully understand the concept and its implementation. The technique to overcome this is to set the output scaling factors of the Analog Input object (in WorkPlace Tech) to values at a factor of 10 or 100 (or other suitable number) thus giving the resolution and therefore (no loss of) accuracy that is required. This requires scaling the value down by the same factor at the head-end (or wherever) to get the proper and accurate reading. Multiply the scale properties by a factor then divide by the same factor for display or logic calculations.
Example: A pressure transmitter that reads a pressure range of -.25 to .25 in. wc at 4 to 20 mA, the Analog Input properties needed for such a transmitter would be as in this table:
Input Low Value: 4 mA
Scaled Low Value: -.25 inwc
Input high Value: 20 mA
Scaled High Value: .25 inwc
There is an issue with a device that has such small sampled values. The controllers can only register at most 2 digits after the decimal. Therefore the controller can be fooled by setting the properties such that the controller thinks that it has a device transmitter attached that reads pressures from -25 to 25 In. WC. This is a factor of 100 and all values would be 100 times larger than the true reading. This of course needs to be accounted for in any calculations and GUIs. The properties for this would be as in this table.
Input Low Value: 4 mA
Scaled Low Value: -25 inwc
Input high Value: 20 mA
Scaled High Value: 25 inwc
In using this technique, remember to factor the number when displaying it or using it with other logic. Since the controller can use negative numbers no smaller than -163.83, no properties can be entered smaller than that value. In the table below the same transmitter is used but a factor of 1000 is used for the properties. This can not work due to the scaled low value being less than -163.83.
Input Low Value: 4mA
Scaled Low Value: -250 inwc
Input high Value: 20 mA
Scaled High Value: 250 inwc
The workaround is well proven and sufficient to satisfy needs.
