Omega OMEGAETTE HHT=1501 - Manual (Page 14)

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3.6. External Signal(TTL) Input (RM1501)
Instead of receiving signal from light source, users can input external
TTL signal (High: > 4.5V, Low: 0V) through pin 8 of RS232C
connector with pin 5 as signal ground.
NOTE: It is strongly recommended that the emiting/receiving unit be
covered to avoid undesired light signal coming in.
3.7. Digital Pulse Signal Output (RM1501)
Users can output digital pulse signal through pin 8 of RS232C
connector with pin 5 as signal ground to oscilloscope.