Read Once Setup of S7 node

Hi, Guys,
I am new to node-red need help on the usage of node-red-contrib-s7. What I am trying to do is with a trigger or event, the node will read a variable from a PLC endpoint(setup via S7_Control node). Whatever I setup it with Trigger Read or Cycle, what I got from the Debug node connected to the S7_In node works periodically...

How I can just read once when triggered?