Alcorn Mcbride 1756-Show I/O, Руководство пользователя, Страница: 28 / 33

22 Using the Module as a SMPTE Reader
Monitoring the Status of the Reader
The 1756-Show I/O reader/regenerator communicates its status to the PLC
through the Input File. See the Module I/O Space chapter for a diagram of the
Input File.
Current Time
The current incoming SMPTE time code is always available through four words
in the PLC Input File.
Word 0, hours, will always be between 0 and 23, inclusive.
Word 1, minutes, will always be between 0 and 59, inclusive.
Word 2, seconds, will always be between 0 and 59, inclusive.
Word 3, frames, will always be between 0 and 29, inclusive.
Note: Word 3, frames, can change 30 times per second. This could impact your
programming technique if your program’s scan time exceeds 33 milliseconds.
Current Frame Rate
The low byte of word 1 in the Input File always contains the number 24, 25, or
30 to indicate the frame sequence of the incoming time code. No distinction is
made between 23.976 and 24 frame rates, or between 29.97 and 30 frame rates.
Locked
When the received time code is valid, this bit in the Input File is set (one). If the
received time code drops out or becomes corrupt, this bit is cleared.
Self-Generating
Whenever the internal SMPTE generator is running, this bit in the Input File is
set (one). If the internal generator switches into idle mode, this bit will be
cleared.
Drop Frame
If the incoming time code has its drop frame flag set, then this bit will be set. If
the incoming code is not a drop frame code, then this bit will be clear.
Error Codes
The bits in the error code word indicate invalid entries in the Output File time
fields. The SMPTE reader does not produce any additional error codes. See the
Generator chapter for a complete description of the error codes.