Knowledge Base

There is no hardware exclusively devoted to the OBQC function. Basically, the OBQC comes “free” in the design of the SPLat board. On some custom boards, however, the OBQC may be associated with special purpose input conditioning circuitry. An example of that would be inputs designed to work off TTL levels rather than NPN sensors.

That said, the OBQC operates in conjunction with two hardware inputs. Usually this is simply a pair of regular inputs that can be used for counting via the OBQC or as a normal input. The ideal maximum count rate is 4kHz (input state changes per second), as determined by the Firmware‘s sampling rate of the inputs. The actual maximum frequency will be limited by:

  • Asymmetries in the input waveforms (outside our control)
  • Time jitter in the sample time (within our control but not perfect)
  • Asymmetric turn-on and turn-off delays at the inputs caused by input EMC filters. As an example, see MMi201 input response times

We have specified a maximum of 2.5kHz for most controller models. This is the number of input state changes per second (equal to the count rate). That will be 4 times the pulse frequency on either input channel. This can only be achieved if the input transitions are absolutely equally spaced. Unless you have an oscilloscope and can check your input waveforms, it can be very difficult to know how symmetrical your inputs are. For example, proximity switches or a photo cells on an encoder disk or cog wheel could produce a quite asymmetrical waveforms, depending on mechanical and sensitivity adjustments.

Warning: The OBQC is liable to miss counts while the processor is writing to any of the non-volatile memories (non-volatile memory writes block the processor for up to several milliseconds). If your program writes to non-volatile memory you must make sure it does not happen at a time when it has to track a quadrature encoder.

Please see the documentation for the SPLat controller you are using for information on which inputs are used.