Cable Crisis: How to make your own DB9-RJ12 cable for Synscan Handcontrollers

After finishing the observatory and starting to shift my scope and gear into it, I very quickly found that all my essential cables were not where I thought they were... USB cables for my cameras, rj12 to db9 and usb to serial cables for connecting the handcontroller to my laptop, even the RA and Dec motor cables essential to control the mount - all missing. After turning the house upside down searching for them without result, I've had no choice but to replace them. The usb cables are no trouble in this regard, they are standard types which can be bought wherever, but the motor cables and the db9-rj12 cable are proprietary and much more difficult to replace.

The RA motor cables for my mount don't appear to be available for purchase anywhere online, not even from Orion themselves. The official Orion website would have you believe you can only get them by ordering a whole new motor upgrade kit for the Skyview Pro. At the cost of several hundred dollars though, it's obviously not a good option just to get some cables.

Contacting Orion directly they seemed to indicate that they could supply replacements but refused to entertain me further until I could provide proof of purchase for my mount, which I no longer have. This is usually how all my interactions go with Orion unfortunately. Instead I got in contact with my local Orion supplier who confirmed that there is, in fact, a spare parts catalog from which these cables could be ordered. It cost me $40 NZD in the end, which I think is a bit steep for cables, but ultimately I didn't have much choice. It's this seething resentment of overpriced cables though that would lead me down the DIY path for my next cable.

The normal way to connect a pc to a mount is via the handcontroller. If the controller is new enough it will have a usb port which makes things easy. If it's a bit older it will have a rj12 port and an rj12-to-serial (RS232) cable to go with it. Of course not many laptops and PC's have serial ports these days but you can just use a serial-to-usb adapter to complete the connection instead. This latter method is the one I have to use but there is a third option, albeit an unofficial one.

This third option is a usb cable that can connect direct from a usb port on the pc to the rj45 port on the mount that the handcontroller normally connects to. This method, known as EQDirect, allows you to bypass the handcontroller completely, meaning less cables and adapters to get in the way and less potential comms issues.

As someone missing all his cables, I'm left with 2 options. A) Replace the rj12 to serial cable and get a usb to serial adapter, or B) Just go straight for an EQDirect cable. The latter option is a $70NZD purchase and the former is, well, a bit of a lottery to even find a cable that will work. The problem is that, much like the motor cables, there's no rj12 to serial replacement cable readily available for my mount. Even though you can find cables with the same plugs/connectors out there (such cables are used in point of sale devices and what not), there's no guarantee it will be compatible with my mount as the pinout to the serial port could be completely different.

Being a cheapskate, the only thing that makes sense at this point is to make my own cable - but which one? EQDirect or RJ12 to serial? EQDirect is a superior connection method and isn't anymore costly to put together, in fact, it's probably cheaper as you don't need to get a usb to serial adapter to use it, but there is one caveat: it's a little bit dangerous. You can potentially zap your mount if you make a mistake and end up routing some volts to the wrong pin or there's a short. It's a minor threat but a threat nonetheless. I believe using such a cable may also have warranty implications as well. In the end, I decided to make up an rj12 to serial because of this. I'd like to get some actual astro done before I risk frying my gear. I probably will put together an EQDirect at some point in the future though. I may make a guide for it but there's already a bit of material out there on this (get started here if that's what you're looking for).

To get this done, all you need is an RJ12 cable and a DB9 RS232 adapter that will let you terminate your own pinout. A suitable cable stripper would make the job easier and a screwdriver will be required if your adapter has screwdown terminals.

In my case I used the exact style of adapter pictured above but I've seen other types such as ones with gator clips instead of screwdown terminals. Any style will work as long as it's RS232 and you are clearly able to identify which pins its terminals map to. This $1 adapter on ebay will do the job just fine. Also make sure that the plug gender for the adapter will pair with whatever gender your usb to serial adapter is so they can actually connect. I overlooked this detail so had to use a gender changer. It's not a big deal to do this but it's better to avoid the problem if you can. I've heard that gender changers sometimes don't have all the pins connected which can only lead to a lot of frustration...

As for the RJ12 cable, you may think you've got a bunch of this stuff laying around spare somewhere from old phones or networking gear but chances are you don't have the right stuff. That's because you need a 6p6c cable which means it has 6 wires inside it all crimped into the 6 positions and contacts on the plug. Most of the stuff you have laying around will only have 2 wires which is not suitable. A 4 wire 4p4c cable would probably work but that's not what I used. Fortunately this stuff is still pretty cheap so just buy some if you have to.

To get started just cut one end of the RJ12 cable and strip a couple inches back to the wires. As can be seen in the diagram above, you only need to use 3 of the wires, the rest can be cut back so they're not in the way and can't short on each other. Why does Orion use rj12 when they only need 3 wires? No idea. Maybe just a consequence of whatever I/O happened to be present on their controller board? Anyway, strip the wires were actually going to use back to the copper and screw them down into the corresponding terminals according to the diagram. The colour of the wires will vary depending on the cable but just look at the bottom of the connector to see which colour is in which position. As long as the wire in position 2 terminates to pin 2, position 3 terminates to pin 5 & position 5 terminates to pin 3, then your cable should be good to go.

Also, when getting a serial to usb adapter, there appear to be 2 main chipsets that you will encounter: one from Prolific and the other from FTDI. I've used both types successfully but I've read in a lot of sources that the FTDI is superior and less prone to comms issues so I'd say make the effort to acquire one with their chip if you can. They seem to cost the same amount regardless.

If you want to keep it tidy you can get clip together casing to contain and protect the db9 adapter and cable. For me though, I've just taped the whole thing to my pier so that the cable isn't hanging down and pulling on the wire terminals.

I've been using this cable with no issues for a few weeks now so I think it's safe to say the pinout above is good. I've been able to get some observing done but I'm still dealing with some longstanding autoguiding problems on my mount.  I believe the issues all come down to stiction (and to some degree, backlash) internal to my mount that maybe I can't do much about but I will still document the issue and seek help. This will likely be my next entry but until then... clear skies.