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In addition to the commands defined by Meade Corporation, Gemini supports additional,

LX200 like commands following the syntactic rules described above. Both are described

in Appendix 8.4, Serial Line Protocol.

6.2.2  Gemini Native Commands

Access to the Setup data, modeling parameters, tracking rates and much more is possible

by means of Gemini' s Native Command set. These commands have a format that allows

for easier parsing and execution. The use of a checksum allows for safe transmission.

Command

Return Value

Description

<:#

Get Value (from L2 up)

value>#

>:

Set Value (from L2 up)

value>#

Native Serial Command Syntax

Each command has a numeric identification,   id   that is interpreted as an integer; leading

zeros are ignored.  To request a value from Gemini, the command begins with the   less

than   character   <  , and to set a value, the command begins with   >  . Some commands

are grouped (e.g. the tracking rates are group 130). If any one of the commands within

this group is used in a   Get Value   command, the actual setting (e.g. 131 for Sidereal

Tracking) is returned.

The checksum for the native commands is calculated by a bytewise XOR operation of the

transmitted characters, including the Get/Set command sign (  <  ,   >  ). The highest

significant bit of the result is cleared (modulo 128 operation) and 64 is added. For

sending, the checksum is separated by a colon. On receiving, there is no colon, but the

checksum can easily be recognized after the parameter value because it is never a number

(in the ASCII alphabet, number characters range from 48 (30H) to 57 (39H), so the

checksum will always be greater.)

Commands received with a wrong checksum are not executed. If Gemini is in Debug

mode, both the received and the expected internally calculated checksums are displayed

as hexadecimal characters instead.

6.3  Connecting a GPS Receiver

The serial ports of many GPS receiver units have a special, non standardized design. To

connect these GPS units to a computer, you will have to obtain a cable from your GPS

manufacturer for that purpose. On the PC side, these cables have a DB9 connector like

the one shown in section 6.1.  There are two options for connecting this cable to the

Gemini.  The first is to buy a commercially available null modem adapter (male DB9

connectors on both sides). A simple   gender changer   will not work, since it would

Gemini Users Manual

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