Here is some info I've compiled on the NexStar GPS Telescope operation.
Disclaimer:  Use this information at your own risk!  It is my opinion only.

Feel free to mail me at: andre@paquettefamily.ca for any comments, corrections, or suggestions.

NexStar AUX Command Set
Hand Controller
Motor Controller
GPS Module
Serial Problem with HC
Autoguiding with Joy(stick)
PC Control Cables

• use the PC port to control the telescope
• access information like GPS time/site via HC serial port or PC port
• control all aspects of MC operation, like leveling, movement and positioning, backlash settings, etc.
• perform firmware upgrades of the MC
• design other devices that plug into the AUX port that can be accessed from the HC serial or PC port

• Polls the versions of the MC ALT/AZM, GPS, and M/IB boards.  Can be used from either the HC serial or PC port interface.
• Usage: getver <comport> <mode> <count> [debug], where:
• <comport> is the com # (ie., 1 for com1)
• <mode> is 0 for PC port and 1 for HC serial
• <count> is the number of times to poll each device
• [debug] is an optional debug level.  Use a value of 2 to see packet contents.
  

• Monitors traffic and displays raw bytes on the PC port interface.
• Usage: monitor <comport>, where:
• <comport> is the com # (ie., 1 for com1)
  

The NexStar GPS hand controller employs a PIC17C756A processor.

The HC firmware can be found in two socketed  chips.  These chips are Atmel AT49F040 512K flash devices . 

With the HC display on the top, the leftmost flash device is the "even" part -- the chips have 2 dots on the top. 
The rightmost device is the "odd" part and has 1 dot on the top.

Firmware can be extracted from the flash devices and units upgraded via a programmer.
Note that the sockets are quite fragile and it takes a fair bit of force to remove the chip, so be sure to use proper chip removers if you intend to play -- and be careful.
Finally, here is the manual for HC 1.6 features.

Debug mode:
There is a hidden mode in the hand controller that gives you access to some low-level information.  To enter this mode, hit 6, then 7 from the "NexStar GPS" level.
Here's what this mode gives you:

For the detailed GPS info, the displayed format is:
"A B H:M:S"
"CD E F G"
Where:
A is the number of tracked satellites,
B is the MSB of the RCVR Status
H:M:S is the time in GMT
C is the month
D is the day
E, F, and G are the raw values for the latitude.

Not all PC serial ports are made equal.  When not in use, most serial ports idle with a stop bit.  However, some serial ports send a continuous start bit (break signal) when not in use.  If the HC serial port receives a continuous start bit, it gets confused or bogged down and has difficulties sending characters the next time you connect.  For instance, if you try connecting to the HC with "TheSky" you'll get errors 9/10 times.
Often, BIOS upgrades are available that will change the serial port behaviour and work around the problem.  Also, you could get a serial-to-USB device and hope that it idles with stop bits.
Either way, this is a bug with the serial handling on the HC and hopefully Celestron will correct it on the next spin.

Autoguiding with Joy(stick):

It it is really easy to make a cable that allows the autoguider port to interface with the old-style Commodore/Atari joysticks. These old joysticks will fit into a male DB9 connector and are great for doing manual guiding. Wico command/control meets NexStar GPS...

AG pin 1 -> N/C
AG pin 2 -> DB9 pin 8 (GND)
AG pin 3 -> DB9 pin 4 (+RA)
AG pin 4 -> DB9 pin 1 (+DEC)
AG pin 5 -> DB9 pin 2 (-DEC)
AG pin 6 -> DB9 pin 3 (-RA)

 PC Control Cables:

Cables for sale
Parts used
9-pin serial pinout
HC serial pinout
PC port pinout

Cables for sale:

I'm selling cables (at just over cost) for interfacing your computer to the NexStar telescopes.  
There are two kinds of cables:  one to connect to the hand controller, and one to connect to the PC port.  The table below lists the prices.  Shipping is extra.

Cables are made from 4 or 6 conductor flat cable with an RJ45 connector at one end and either an RJ45 or RJ11 handset plug at the other end.  The RJ45 connector plugs into an adapter that converts the RJ45 to a 9-pin female end for use with a computer.

If you wanted both a PC port and a HC serial cable but weren't going to use both at the same time, you could buy one "DB9F to RJ45 Adapter" and then the "Cable Only" parts for each type of cable.

If you're interested in purchasing a cable, send me an email with the type, length and number of cables you would like and the address to use for shipping.  I'll then send you an email when the cable is ready and the expected cost including shipping.  You can then send me a cheque and I'll send you the goods.

Here are the parts used to make the cables:

DB9F to RJ45 modular connector
RJ45 plug
RJ11 handset plug
"Silver Satin" 6-conductor cable
"Silver Satin" 4-conductor cable

Here is the pinout for the DB9F connector:

Pin 2 -> data to computer from telescope
Pin 3 -> data from computer to telescope
Pin 5 -> ground
Pin 7 -> RTS
Pin 8 -> CTS

Here is the pinout for the HC Serial interface:

Pin 1 -> data to computer from telescope
Pin 2 -> ground
Pin 4 -> data from computer to telescope

Additional information on how to make a HC serial cable can be found on Mike Swanson's site.
Important: Pin1 is the pin on the lower right side of the diagram on Mike's page.

Here is the pinout for the PC Port interface:

Pin 1 -> ground
Pin 2 -> data from computer to telescope
Pin 3 -> ground
Pin 4 -> data to computer from telescope
Pin 5 -> RTS
Pin 6 -> ground
Pin 7 -> CTS
Pin 8 -> ground

Additional information on how to make a PC port cable can be found on Mike Swanson's site.
Important: Pin1 is the pin on the lower right side of the diagram on Mike's page.