Posts: 2
Joined: Sat Feb 02, 2013 8:41 pm

±12v RS232 Serial adapter

Tue Oct 13, 2015 9:31 am

Ok, I've been getting lost in this one for a while, and I'm sure it shouldn't be this difficult to find. Basically, I'm trying to attach my Pi to a device that demands a ±12v RS232 connection. I've already tried a cheap USB to serial adapter, and that turned out to be TTL levels. No go there. I know that I could put something together to run off the GPIO pins with appropriate parts (perhaps the Sparkfun RS232 Shifter kit), but I'll admit that my electronics knowledge is somewhat weak, and my soldering is worse! Additionally, this approach would require a new case for my Pi. However, if this is the best way to do it, I'm willing to try.
For me, the ideal solution would be a simple USB to RS232 ±12v adapter with the converter board in the USB end of the cable. Why the USB end of the cable? Because the device has an RJ14 (6P4C) socket, and if it's that way round, I can replace the DB9 with the right plug to remove the need for an extra inline adapter cable. If anyone knows where I can find such a cable, I'd love to hear of it. Failing that, any recommendations for USB to DB9 RS232 adapter that is confirmed to work with a Pi and provides a ±12v signal would be a great help.
For those interested, I'm trying to connect to a Celestron Nexstar SLT handset. I've already got the Pi Camera and eyepiece working nicely, with camera control running over ad-hoc wifi to tablet/smartphone. So far, everything's working nicely, but the next step in the project is controlling the telescope mount from the same software. Custom software, work in progress. The setup is often assembled in the dark, hence I'm trying to minimize adapter cables.
Any suggestions and or/advice is welcome!
Thanks in advance, P.

edit: After having submitted this, I spotted the link from RoddyJoff's post below, Add a 9-pin Serial Port to your Raspberry Pi in 10 Minutes. It looks promising, but it's not entirely clear whether it's providing 5v or 12v output. Anyone more knowledgeable than me able to comment?
Last edited by ichneumon on Tue Oct 13, 2015 10:15 am, edited 1 time in total.

Posts: 3172
Joined: Sat Aug 18, 2012 1:21 am
Location: Korea

Re: ±12v RS232 Serial adapter

Tue Oct 13, 2015 10:15 am

Hi there,

I'm pretty sure you want one of these:
http://www.dx.com/p/usb-to-rs232-adapte ... ngth-24799

And then, to avoid soldering, you want one of these:
http://www.dx.com/p/db9-female-to-rj45- ... pter-48128

You can actually get RJ11 6p4c versions, or you can cut two small pieces of plastic and glue them in place to make the RJ45 socket narrower.

Then you need a telephone cable, with RJ11 6p4c plugs on each end and straight through wiring on all 4 pins. You can probably get this at a dollar store.

You'll end up with a bunch of adapters, but no soldering. You configure the RJ45->DB9 adapter by pushing the pins you want into the backshell of the DB9. If you snip the unused wires off then they won't get in the way. You'll have to trace out which order you want the 4 pins from the telescope to appear in, but that should be easy.

Pi -> USB extension -> USB serial -> DB9 to RJ45 -> RJ11 telephone cable -> telescope.

* Yes, I know it's DE9. Bite me.

Posts: 2
Joined: Sat Feb 02, 2013 8:41 pm

Re: ±12v RS232 Serial adapter

Thu Oct 15, 2015 1:30 pm

Yep, I know it's DE-9 as well, but I figured more people know them as DB9s. I've ordered the RS232 adapter, and I'll make the extra conversion cable myself - I can do that bit, as I've got 6P4C plugs and crimpers kicking around. I'm not 100% certain it'll go, as one of the comments mentions that it provides a 0-12v signal, rather than ±12v. I'll let you know if it works.

Posts: 5
Joined: Sun Feb 18, 2018 11:49 am

Re: ±12v RS232 Serial adapter

Sun Feb 18, 2018 11:54 am

Hi. I am planning to interface a gps module to my raspberry pi 3.. gps module has a RS232 12v DB9 port.. so I bought a usb- RS232 DB9 adapter.
Can I straight away connect it to the raspberry pi's usb port.? Or will I need some voltage level shifter? Because power supply to the gps module is 12V.

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