The trainer out / transmitter out PPM pulse train on th9x has following features (measured facts):

• Channel pulse is logical HIGH
• Channel minimum pulse length = 700uS (microseconds, thats 0.7 milliseconds)
• Channel minimum pulse length = 1700uS (microseconds, thats 1.7 milliseconds)
• 8 channels per frame
• Pause (logical low) between pulses is 300uS and has nothing to do with actual channel pulse width, pause has constant length (sum of pauses: 8*300uS = 2400uS = 2.4ms)
• Conclusion: one channel tries to be max. 2000uS which would make total max. 16ms)
• Total frame length = 20 milliseconds
• So: last “channel” is 20ms minus sum of all pulses in frame, about 10..12ms (based on this information the last channel length is minimum 4ms)