Siemens SIMATIC S7 System Manual page 104

Programming instructions
6.1 Basic instructions
CTU: CTU counts up by 1 when the value of parameter CU changes from 0 to 1. If the value
of parameter CV (Current count value) is greater than or equal to the value of parameter PV
(Preset count value), then the counter output parameter Q = 1. If the value of the reset
parameter R changes from 0 to 1, then the current count value is reset to 0. The following
figure shows a CTU timing diagram with an unsigned integer count value (where PV = 3).
CTD: CTD counts down by 1 when the value of parameter CD changes from 0 to 1. If the
value of parameter CV (Current count value) is equal to or less than 0, the counter output
parameter Q = 1. If the value of parameter LOAD changes from 0 to 1, the value at
parameter PV (Preset value) is loaded to the counter as the new CV (Current count value).
The following figure shows a CTD timing diagram with an unsigned integer count value
(where PV = 3).
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S7-1200 Programmable controller
System Manual, 11/2009, A5E02486680-02