High-Speed Counters (HSC), Shaft Encoders, Frequency Measurer

High-speed counter functions are built into the controller hardware.  This is why you do not ‘build’ a high-speed counter within your Ladder program.  Instead, you define it as part of the PLC’s hardware configuration by:

1. Selecting the counter type as shown below

2. Linking it to an MI that contains the counter value.

Note that the counter value is an integer with a range of -32768 to +32767.  After the counter reaches the maximum value of +3,2767 it will continue to count in the negative range.

The last on-board input on an M90 is the actual counter, and is capable of counting 5,000 pulses per second.  Note that the high-speed input is a pnp-type input, requiring a nominal voltage of 24V, a minimum of 15V.

The next-to-last input also serves a purpose in certain high-speed counter functions:

• Shaft encoder function: the next-to-last input serves to indicate the direction of the encoder.

• High-speed counter + reset function: the next-to-last input serves to reset the counter.

When the next-to-last input is used in a high-speed counter function, it is normally OFF.  It remains OFF until it receives a signal; the input then turns ON, stopping and resetting the high-speed counter.  The high-speed counter begins counting pulses only after the counter reset turns OFF.  Note that SB 10 High Speed Counter Reset Enable must be ON; it is ON by default.  

1. Open Hardware Configuration.

2. Click on the icon representing your controller model.  The appropriate hardware model window opens.  

3. Select a high-speed counter type by clicking the drop-down arrow to display the options, then clicking one.
   

4. The Select Operand Address box opens.  Select an MI to contain the counter value, and then click OK.

5. You can also select a Frequency Measurer option.
   

This MI contains the counter value which is current at the last program scan.  Use this MI in your program like any other MI.  You can reset the counter by placing a 0 value into this MI via the Store function.  Note that in order to reset the counter, SB 10 High Speed Counter Reset Enable must be turned ON; SB 10 is ON by default.   

The high-speed counter value is read once during every program scan. For this reason, it is probable that a counter value will not be read at the exact moment that a Compare function in your program is being carried out.  This can cause an Equal (=) function to miss the desired counter value; if the counter does not reach the value required by the Equal function at the moment the function is carried out, the Equal function cannot register that the value has been reached. To avoid this problem, use functions Greater Than Or Equal To (≥) and Lesser Than Or Equal To (≤).