6  Serial Communication 

Gemini' s RS 232 port allows you to connect devices such as GPS receivers, computers, 

and handheld devices to extend Gemini' s functionality.  This chapter describes the 

hardware and software interfaces used by the RS 232 port. 

6.1  RS 232 Interface 

6.1.1  Pinout and Wiring 

Gemini connects to external devices via a serial RS232C port. To avoid any possibility of 

accidentally plugging in a wrong cable, a smaller RJ22 modular socket, (4P4C   4 pin, 4 

connectors) is used for this port. The pinout of Gemini' s serial port is shown in the 

following table: 

Gemini RJ22 socket  Function 

Pin 1 

Not Connected 

Pin 2 

GND Signal Ground 

Pin 3 

RxD Received Data (Input) 

Pin 4 

TxD Transmitted Data (Output) 

The serial port does not use any modem control signals. Gemini can be connected to a 

computer using a null modem cable. While both GND pins are connected directly, the 

receive (RxD) input of one connector has to be connected to the transmit (TxD) output of 

the other (and vice versa). 

Personal computers usually have DB9 or DB25 connectors for their serial (COM) ports. 

If your PC does not have a serial port but a USB connector, there are several units on the 

market to convert RS 232 signals to USB.  

To build a null modem cable, connect the pins as follows: 

Gemini RJ22 

DB9 

DB25 

Pin 1 

Not Connected 

Not Connected 

Pin 2 (GND) 

Pin 5 (GND) 

Pin 7 (GND) 

Pin 3 (RxD) 

Pin 3 (TxD) 

Pin 2 (TxD) 

Pin 4 (TxD) 

Pin 2 (RxD) 

Pin 3 (RxD) 

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