
Gemini Users Manual 80
6 Serial Communication
Gemini supports two serial ports. The first one is available to the user via the RJ22 RS-
232 socket, and is described in section 6.1. This port allows you to connect devices such
as GPS receivers, computers, and handheld devices to extend Gemini’s functionality.
The second serial port is normally used to drive the LED displays in the hand controller
(or in the electronics box in older Gemini units). By connecting a separately available
adapter to the DB15 hand controller port, Gemini level 4 allows you to use this second
serial port to connect another device to emulate the hand controller using a serial line
protocol. This is described in section 6.2. The new serial port supports the complete
command set and works independently from the first. It is up to the user to assure that
the commands sent do not conflict.
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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