1489, 75189 RS232 to TTL level shifter. A inputs are RS232-level inputs, C inputs are response control, a TTL signal which could be used to adjust threshhold and hysteresis but is generally unnecessary and unused. It is often tied through a 300 - 470 pF capacitor to ground. +----------+ 1A |1 +--+ 14| VCC 1C |2 13| 4A 1Y |3 12| 4C 2A |4 1489 11| 4Y 2C |5 10| 3A 2Y |6 9| 3C GND |7 8| 3Y +----------+ [This information is part of the GIICM]