Specifications

Microcomputer Synchronization
(Microfreeze):
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 continu-
ously 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.
Software Filtering:
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 posi-
tion 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 per-
missible 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 of 10:1:1).
A-21
800-TEC-ENGR
www.avg.net
Position Transducers/Sensors
SECTION
A
µ
µ
Decoder