The analog input card is where the twisted pair ultimately ends up (this is the entry point into the PLC), depending on whether you have a separate marshalling rack, or if you wire direct to the card. The twisted pair goes to a specific channel on the AI card.
The 4-20 mA is then available to the PLC sytem.
A controller card then reads the particular channel for your 4-20 input, and converts the mA signal into an equivalent engineering unit (e.g 4-20 mA to 0-200 psig). This is also available to the PLC.
To control your VFD, you probably need a PID control function block (I am not familiar with Auomation Direct's products and offering). The input is fed to the PID block, and the output of the PID block is fed to an analog out channel, which corresponds to a set of twisted pair wires in an anlogy output card.
The wires then ultimated leads to a motor control center for your VFD (via a marshalling rack if there is one).
Is this what you mean?
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?