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What I used was:

• An old 25' coiled handset cord I had lying around (you can buy these in 12' or 25' lengths from Radio Shack for $5 or $6).  I used its full length.
• Radio Shack part number 276-1538 9 position female D-Subminiture Connector ($1.29)
• Radio Shack part number 276-1539 D-Subminiture Connector Hood ($0.99)

The tricky part is interpreting the diagram on page 50 of the N5 manual:

That says "jac" but it looks more like a plug!  I can't figure out which way the telephone handset plug would go into it.  What I found worked was if you hold the end of the handset cord like this:

so that the cord itself is down, the long surface of the gold pins are facing you, the short surface of the gold pins are up, and the latch is away from you, the pins in the plug are numbered as above.   These pins then map to the DB9 like this:

Note that pin 2 or 3 of the handset plug can be wired to DB-9 pin 5.  I had a cable wired with 3 to 5 and it worked fine.   I made this mistake because I originally thought that the Celestron drawing was oriented so that the handset plug was inserted from the bottom of the "jac" with the gold contacts facing away from you - hence I also had transmit and receive reversed and the thing didn't work.  Then I reversed DB-9 2 and 3 and it worked - I never bothered to swap handset plug 3 with handset plug 2 on DB-9 pin 5.  I have since rewired my cable though.

If your DB9 isn't numbered, or not numbered like mine, this is how mine was numbered (viewed from the front, where the back is where the wires connect):

To test it, you can use Windows Hyperterminal (HYPERTRM.EXE) with the following settings (Mac users should get the same results using a similar "dumb" terminal program):

• Connect Using: Direct to ComX (where "X" is 1 or 2 or whatever)
• Bits Per Second: 9600
• Data Bits: 8
• Parity: None
• Stop Bits: 1
• Flow Control: None (this is what tripped me up for several hours - I had it set to "Hardware")

To talk to the N5, all you have to do is a fake auto align:

• Turn it on
• Choose auto-align
• Enter Date, Time, Location, etc.
• When it slews to its first star, just press enter then align
• Press enter to go to the second star and press enter and then align
• The N5 will congratulate you on a successful alignment
• When you get control back, select MENU
• Scroll to RS-232 (press DOWN 6 times)
• Press ENTER

Now you are ready:

• Go to Hyperterminal and type the character "?".  You should almost immediately see the character "#" appear.  
• Now type "A" followed by four other keys (any will do, I used four spaces) the N5 will take off headed for that Alt/Az address.   The N5 handset will say "SLEWING".
• When it gets there, "SLEWING" will be replaced with the RS-232 mode message and an "@" should pop up on your terminal program.  Press "?" again and see if you get "#".  If you do, the N5 is ready for another command.
• Type "A" followed by four other keys different from step 2 (I used "aaaa") and the N5 will go to that address.
• When it is ready again you should get the "@".  If you retype the sequence "?A    " (that's four spaces) it will go back to where it was.   By alternating this with "?Aaaaa" you can zip it back and forth between the two positions.

For those of you who are wondering how to translate the characters you type to celestial coordinates, you have to convert the character sequence to its binary 16 bit representation, and then correct that for the encoder accuracy.  To convert to binary, here are some instructions I posted to the NexStar5 eGroup:

Take Alt and Az - the N5 wants Alt and Az each expressed as a 16 bit number communicated by two bytes (8 bit characters). Since a 16 bit number represents a maximum of 65535 in decimal, it is like dividing the 360 degrees of Alt or Az into 65535 "steps" to which you can direct the N5. 0 would be Alt or Az 0, 65535 would be Alt or Az 360.

The 16 bit number for 0 is in binary: 0000000000000000, and as two bytes it would be 00000000 00000000. The ASCII character corresponding to 00000000 is NUL so this 16 bit number is represented to the N5 as two NULs. So if you send the N5 the character "A" followed by 4 NULs, it will take off slewing to Alt = 0, Az = 0 (assuming it is not already there). Consider now what would happen if you sent the N5 the "A" command followed by "AAaa". The ASCII character "A" is 65 in decimal or 01000001 in binary, so "AA" is 0100000101000001 in binary. To convert this to decimal, multiply the decimal value of the "high" byte or left most byte by 256 and add it to the decimal value of the right most byte: 65*256 + 65 = 16705 (see below on how to convert 8 bit binary to decimal). To convert 16705 to Az in degrees: (16705*360)/65535 = 91.76 degrees or 91:45:36 This is just the ratio of the parameter over the maximum 16 bit number (65536) times 360 degrees. The Alt value of "aa" is 24929 or 136.94 degrees. So, the N5 would slew to Az 91.76 and Alt 136.94.

However, what I missed in this explanation was that the N5 only has a 20 arc-second accuracy which means it can only point to 64800 unique positions in a circle (20 arc second accuracy means 1/3 of a minute accuracy or 3 steps per minute, so, 360 degrees * 60 minutes per degree * 3 N5 steps per minute = 64800), not 65535, so the desired Alt or Az has to be rounded to the nearest 1/3 of a minute step before conversion to 16 bit binary.

John Carlyle has written a program NEXSTAR.EXE (for Windows/DOS computers only) which allows you to control the N5 from a DOS command prompt.   The program and its instructions are available in the eGroup at message 3440: http://www.egroups.com/group/nexstar/3440.html?

Below are the complete details on the command interface, lifted (and improved) from the N5 manual (if you want to know the mapping of the ASCII code, go to http://members.tripod.com/~plangford/index.html):

NexStar5 communicates at 9600 bits/sec, No parity and 1 stop bit.. All angles are communicated with 16 bit numbers.

Before all commands, the following INITIALIZATION is necessary:

• PC sends one byte (63=ASCII “?”) to check that NexStar is ready.
• NexStar responds with one byte (35=ASCII "#") when NexStar is ready to respond. After NexStar sends a 35, the buttons to the hand control do not respond until the command from the PC has been received, then the direction, rate, and undo buttons are active.

Goto RA-Dec positions

• INITIALIZATION
• PC sends (82=ASCII “R”)
• PC sends the RA high byte, RA low byte, Dec high byte, Dec low byte.
• When the scope is finished slewing, it will send back a “@”.

Goto Alt-Az positions

• INITIALIZATION
• PC sends (65=ASCII “A”)
• PC sends the Azm high byte, Azm low byte, Alt high byte, Alt low byte.
• When the scope is finished slewing, it will send back a “@”.

Get RA-Dec positions:

• INITIALIZATION
• PC sends (69=ASCII “E”)
• NexStar sends the RA high byte, RA low byte, Dec high byte, Dec low byte.

Get Alt-Az positions:

• INITIALIZATION
• PC sends (90=ASCII “Z”)
• NexStar sends the Azm high byte, Azm low byte, Alt high byte, Alt low byte.

February 16, 2002