Whenever the PLC scans the decoder input, it must see stable data.  In order to ensure this, the PLC gives a data transfer command and a predetermined time later the PLC synch circuit stabilizes the data for the PLC to read.  This time is adjustable on some AVG Automation units (2 µs to 30 ms), where as it is fixed on others (50 µs, 100 µs, etc.).  The variable time feature, when available, can be used to provide the most fresh data to the PLC.

For example, the time interval between the data transfer and read commands might be 12 ms and say the time delay is set at 5 ms.  After 5 ms of the data transfer command the stable data is available to the input modules of the PLC and when the PLC commands the data to be read 12 ms later, it is stable and valid.

The Microfreeze can also be called as transparent PC-Handshake.  This feature is particularly useful when interfacing data directly to a microcomputer where speed of operation is much higher.  In this case the decoder position data is continuously updated at full speed.  The data are frozen for 100 µs ±10% after a delay of 10 µs from either transition edge of data transfer command.  The microcomputer can read stable data during these 100 µs and it automatically unlatches.

The problem of synchronizing BCD data to a PLC can also be addressed by software filtering.  Software filtering is usually done in one of the following two ways:

1. A window is created around the last correct reading based on the known operating speed of the decoder.  If new position is outside of this window, the data is rejected.
2. Three samples of position data are taken, of which two must agree before data is accepted.

Either of these approaches will increase the scan time of the PLC.  Since scan time is an important factor in system speed and resolution, the software approach is usually not a viable approach.

Notes:

1. The synchronization problem does not exist when using Gray Code absolute decoders because only one bit changes state at a time.
2. The synchronization process described above does not result in faster machine operation.  The system resolution and permissible decoder speed will still be limited by the PLC scan time.  As a rule of thumb, a PLC with 16.67 ms (One AC cycle) scan time will permit 1" resolution at 10 RPM (The rule 10:1:1).