The CTD, also known as the Count Down, instruction is used to decrement the value of the counter associated with it. This instruction will detect a FALSE to TRUE transition of the logic leading to it and decrease the “Accum” value of the counter by 1. The effect of using this instruction is the exact opposite of what the CTU Instruction does to the counter. Just like its counterpart, this instruction will be found in a rung which will count a certain number of events. Due to the fact that it decrements the value, the use cases need to be very specific in order to be clear to the next programmer.
This is a companion discussion topic for the original entry at https://www.solisplc.com/tutorials/plc-programming-intermediate-instructions-ctd-count-down