Alcorn Mcbride 1756-Show I/O, Руководство пользователя, Страница: 26 / 33

20 Using the Module as a SMPTE Reader
The regenerated time code is always available to the PLC through the Input File.
The PLC can use this information to trigger execution of rungs.
The SMPTE reader performs the following tasks each frame:
1. Receive and decode incoming SMPTE bits.
2. Verify that the received code matches the previously received code or is one
frame greater than the previously received code.
3. Automatically detect the frame rate of the incoming code.
4. Lock the internal SMPTE generator to the received code’s frame edge.
5. If the incoming code has dropped out, advance the internal generator by one
frame.
6. If the signal loss has lasted longer than the dropout tolerance, stop the
internal generator.
7. Make the current internal time and frame rate available to the PLC through
the Input File.
8. Indicate external lock and internal generate status to the PLC through the
Input File.
Configuring the Reader
The 1756-Show I/O reader/regenerator is entirely controlled by words in the
PLC Output File. See the Module I/O Space chapter for a diagram of the Output
File.
Operating Mode
When the MODE bit is set (one), the module will generate SMPTE time code
internally. When this bit is clear (zero) the module will read and regenerate
external SMPTE time code.
Dropout Tolerance
If incoming SMPTE time code drops out or becomes corrupt, the 1756-Show
I/O will flywheel, or self-generate, time code to cover the gap. The DROPOUT
TOLERANCE word sets how many frames of self-generation are allowed
before the unit stops self-generating and goes IDLE. A setting of zero frames
selects infinite tolerance; the unit will never stop self-generation.