Building a
Data Cable for the
Losmandy
Gemini GoTo System
Introduction
The Gemini user
manual provides all the necessary information for making a null modem cable to
connect between the Gemini RS232 port and a standard DB-9 or DB-25 PC serial
port. However, no details are provided regarding the pin connections for a
DIN-8 plug required to connect to a standard Apple Macintosh serial port.
Although the DIN-8 pin schematics are readily available elsewhere, I thought it
would be useful to post this page as a “one stop shop” for building a null
modem cable for the Gemini, regardless of your computer platform.
It’s worth
noting that it has been several years since serial ports were last included on
new Apple computers. However, there are readily available USB to serial
converters on the market that will fill the roll. I use the Keyspan Twin
USB to Serial converter which works well under both the MacOs Classic
environment and native MacOs X (and indeed, will also be recognised as a PC Com
Port in Virtual PC 5.0 or greater)
Required Connectors and Pin
Identification
The Gemini RS232
port requires a RJ22 Modular plug identical to those used for telephone handsets.
The connector required for the other end of your cable will depend on the type
of serial port on your computer or serial port converter (either a DB9 or DB25
connector for a typical PC, or a circular DIN-8 connector for Macintosh)
Use the following pictures to identify the pins required to make the necessary link between the two connectors.
Constructing the Cable
To make a null
modem cable, each of the ground pins are connected directly, while the input
(RxD, receive data) pin of one connector is coupled to the output (TxD,
transmit data) pin of the other (and vice versa).
The following
chart details the connections required for each type of connector:
The longest
cable I have used successfully is a modified “wind-up telephone cord-caddie”
which is 20 metres in length. Surprisingly, despite its length, the cable does
not have to be fully unwound to function correctly.
Disclaimer
The above
material has been checked carefully, and I have personally used this
information to make several working data cables for the Gemini. However, I
cannot be held responsible should there be any damage to equipment or loss of
data as a result of using the information contained within this page.
6 June 2003
Stephen Voss
E-mail: stevoss@mac.com