The following picture shows a cable to connect the Gemini to a PC equipped with a DB9 

serial COM port (The locking tab of the RJ22 and the four pin row of the DB9 face away 

from the viewer): 

Serial cable to connect Gemini to a PC 

The maximum usable cable length depends on several factors including the serial 

interface of the PC connected, the cable used, electromagnetic fields at the location and 

even the power supply' s ground resistance. 

The longer the distance to be bridged, the higher must be the quality of the cable used. 

The heavier the gauge of the wires, the less is their resistance and resulting voltage drop. 

Straight (not coiled) cables are recommended. Twisted pair cables can be used, but be 

careful not to twist the RxD and TxD wires. Twist each of these signal wires with a wire 

connected to GND instead. For long distances, using multiple wires for GND helps 

reduce the disadvantages of the asymmetric (common GND wire) circuitry.  

A shielded cable with the shield connected to the metal of the Sub D connector on the 

computer side is also recommended. 

With good cabling, communication over distances of about 50 ft. should be possible. 

Longer distances may be bridged by using relay modems. These units change the 

asymmetric circuitry to symmetric circuitry (no common GND wire) that is relatively 

insensitive to electromagnetic fields and ground potential differences, and can therefore 

communicate up to several miles.  

The serial ports provide an electrical connection between the Gemini and the computer. If 

there are differences between the ground potentials of the devices, ground loop currents 

may occur, potentially damaging both pieces of equipment.  To avoid such currents, you 

should ensure that both devices have the same ground potential. Whenever possible, the 

Gemini Users Manual 

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