Getting Started with High Speed Counters in Siemens Step 7....

In this post, i will explain the use of high speed counters in Step-7 PLCs. Typically, a high-speed counter is used as the drive for a shaft timer, where a shaft rotating is fitted with an incremental shaft encoder. The encoder provides a specified
number of counts per revolution and a reset pulse that occurs once per revolution. The clock and the reset pulse from the shaft encoder provide the inputs to the high-speed counter.

The high-speed counter is loaded with the first of several presets, and the desired outputs are activated for the time period where the current count is less than the current preset. The counter is set up to provide an interrupt when the current count is equal to preset and also when reset occurs. Compare function is also used for this purpose to provide an interrupt when the current count is equal to the preset counts. We can easily use memory bits for the interrupt at different count positions i.e at different angels.

You can use the HSC Instruction Wizard to configure the counter. The wizard uses the following information:

Type and mode of counter, counter preset value, counter current value, and initial counting direction. To start the HSC Instruction Wizard, select the Tools, Instruction Wizard menu command and then select HSC from the Instruction Wizard window. To program a high-speed counter, you must perform the following basic tasks:

- Define the counter and mode.
- Set the control byte.
- Set the current value (starting value).
- Set the preset value (target value).
- Assign and enable the interrupt routine.
- Activate the high-speed counter.

to be continued....