DTR stands for Data Terminal Ready.

Which is a reasonably weird name until to remember that the names of all the RS-232 connections are based on connecting a Mainframe Terminal (Data Terminal Equipment, DTE) to a modem (Data Communications Equipment, DCE).

This then makes it reasonably obvious that DTR indicates if the receiver (The Terminal in this case) is ready. If the receiver is not ready then the modem will not send data to the receiver! In fact DTR is often used to hang up modems, so that when the Terminal would say that it was not ready the modem would hang up.

Needless to say for RTK we need to have DTR on, or ignored.

The easiest way to do this is to use the init string AT &D0. This turns off the need for DTR. Technically all modems should support this feature. A number of cell phones don't seem to.

For modems that don't allow the overriding of DTR you need to create a cable that asserts DTR , either the whole time or when there is a call.

I recommend connecting the DSR (Pin 6) and the DTR (Pin 4) connection from the the modem.

Name	Pin	Receiver Side	Phone Side	Notes
CD	1	Not connected	Not Connected
Rx	2	Straight though	Straight though	If using a normal download cable cross over with 3
Tx	3	Straight though	Straight though	If using a normal download cable cross over with 2
DTR	4	Not connected	6 on phone side
GND	5	Straight though	Straight though
DSR	6	Not Connected	4 on phone side
RTS	7	Straight though	Straight though	Connect to pin 8 if not using a 37779 Rev B
CTS	8	Straight though	Straight though	Connect to pin 7 if not using a 37779 Rev B
RI	9	Not connected	Not connected	Not Needed, Not recommend to be connected

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