Meade LX200 RS232 Port Connections

To connect an LX200 to a computer you need a special cable. You can sometims buy one from the company that makes the software you're using but it's much cheaper to make one for yourself. The parts only cost a few dollars and it will take only a few minutes. All you need is a soldering iron, the appropriate connectors and the information below. It's really a lot simpler that it looks.

Pin-outs

For those of you who know what you're doing all you need to know is the pinouts:

Description LX200 DIN-8 DB-9 DB-25 Name
+12v DC 1        
Misc Serial Out 2        
PC Transmit Data 3 3 3 2 TxD
Ground 4 4 5 7 Gnd
PC Receive Data 5 5 2 3 RxD
Misc Serial In 6        
In the diagrams below the colors indicate which pins connect to each other.

LX200 RS232 Port

At right is a diagram of the LX200's RS-232 port as you see it looking into the panel. Note the odd pin numbering: 1-4-2-5-3-6, read left to right looking into the port (This numbering is taken from the LX200 User's Manual, don't expect to see it elsewhere.)

At left is the corresponding 6-pin modular plug (RJ11) viewed from the flat side (top as it is inserted into the LX200; not the end); the connecting pins are on top, the tab on the bottom of the diagram represents the wire.


Mac Ports

The Macintosh's modem and printer ports use a DIN-8 jack as shown at right looking into the Mac. The diagram at left shows the pins from the end of the male plug. Of course, it's a mirror image of the diagram of the port. The pins are usually labeled in tiny raised letters.



Making a Simple Cable

Making a cable is just a matter of wiring up the connectors with matching colors as shown below. Connect pin 3 to pin 3, pin 4 to pin 4, and pin 5 to pin 5. (The exact correspondence of numbers is purely accidental.)
(Crossing lines do not connect)

Making a Dual Cable

Though they are labeled as "Unused" in the LX200 manual pins 2 and 6 are actually an extra RS232 port (pin 4 is a common ground). Thus you can make two simultaneous RS232 connections to your scope. This is useful if, for example, you want to connect to a computer and your CCD camera at the same time.

Connect one connector as above. For the other one, connect pin 6 to pin 3, pin 2 to pin 5, and the common ground pin 4 to pin 4.


(Crossing lines connect only where a dot is shown)

Other Computers

Other computers use a variety of different connectors. Most common are DB-25 (right, top) and DB-9 (bottom); they are shown here looking at the pins from the end of the male connector. Connect as for the Mac substituting per the pin-out table above.

Pin Nomenclature

The various RS232 pins go by a confusing variety of names. Fortunately for this purpose, we need only be concerned with three of them: Ground, RxD (receive data), and TxD (transmit data).

The diagram at right shows how to connect a DIN-8 (top) to a DB-25 (below).

Richard Mathews says, "On the DIN-8, pins 5 and 8 are both Receive Data (actually RxD- and RxD+). The voltage difference between them is what matters. ... To reduce noise, you should connect pin 8 to ground by tying it to pin 4. Note that your diagram for connecting a DIN-8 to a DB-25 already shows this connection, but you don't show it in the info on connecting the RJ-11 to the DIN-8." Richard is probably right but my cable works fine anyway.

USB

Modern computers have USB instead of serial ports. To use an LX200 with a USB computer you need a cable such as above plus a serial-USB adapter. I use a Keyspan USB Twin Serial Adapter; there are several other similar products available.

Disclaimer

Check your cable carefully before using it. A miswired cable could fry the RS232 ports on either end.

I have made a cable using this information. I have tried to make it correct. But I'm only human. If you burn up your LX200 it's not my fault.

Notes



Credits

This page was made possible by the generous contributions of Don Dillinger, Michael Hart and others on the MAPUG mailing list (plus a little help from the LX200 manual, appendix F). I am grateful for their help but any mistakes are mine.



Bill Arnett; last updated: 2002 Feb 28