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If the host device is a Modem like device and has a RS232 DCE port, then use a "straight cable". That is, connect the signals with the same name from one side to the other. DCE TXD IR100S-L TXD RXD RXD RTS RTS CTS CTS DSR DSR DTR DTR CD CD RI RI GND GND - 15 - Version 1.20 May 27, 2002 ACT-IR100SL and IR100SL-M User’s Manual If the host device is a PC like device and has a RS232 DTE port too, then you need to use a "Null Modem" connection between the two. That is, in/out signal pairs need to be