May 12th, 2002, 05:04 AM
I am using Moore APACS/Quadlog PLC. The analog cards are Moore SAM (Standard Analog Module). The SAM accepts 4-20 mA DC two-wire transmitter signals and provides the 24 V DC loop power. The inputs need to be sinking type.
I have one input from an analyser which is 4-20 mA DC signal which does not need loop power because it derives its power from the analyser and is non-isolated. I want to connect this to the SAM module and achieve signal isolation for input to the SAM at the same time. Does anyone know of the solution, a suitable interface card (for example, from the range of Pepperl & Fuchs?).
May 13th, 2002, 09:03 AM
There are several manufacturers of opto-couplers
for 4-20 signals. choose one!
May 13th, 2002, 09:49 AM
You may not even need an isolator, depending on the internal configuration of the I/O card. I'm not familiar with Moore's unit, but we often mix loop powered and self powered transmitters on the same non-isolated I/O card.
We have gotten a lot of questions about 4-20 mA loops lately that could be answered by observation of the loop diagram and application of fundamental principles to them. It looks to me like a lot of the confusion arises from expecting to know the answer, rather than knowing how to find the answer!
The first step is to sketch out a diagram of the loop, showing all of the connections and enough of the internal electronics to understand the current path. For example, if the commons of all channels are tied together and to the power supply "-" show this. If the transmitter is self powered and limited to 750 Ohms total load, show this. If an intermediate indicator is loop powered, show the diode and identify the max equivalent load. Then just follow the current around the loop, and make sure you have a complete circuit and total the loads or voltage drops as you go. (Just like you did in EE-101 when you learned Ohm's Law.) If the load is too high, or you have multiple paths to common, it should become obvious. If you apply the proper methodolgy, and go through it one step at a time, you can solve any problem.
Just like the football coaches say, "It all comes down to the fundamentals of the game!"