Jimbo Harris was good enough to tell me about his USB modification of my digital setting circles project. Jim found the FTDI TTL232R and was able to adapt his DSC interface to communicate via (and draw power from) the USB port.

Jim wrote to me:

Hi Dave,

The TTL232 modification was a success; I connected to the board in place of the MAX232 at pins:
+5v – pin 16
GND – pin 15
Tx – pin 11
Rx – pin 12
(all corresponding pins of the MAX232), and I left out the MAX232, 7805, and all the corresponding caps, etc. The interface booted right up. I connected via both your program and also via the ASCOM driver (in ASCOM Validator and in DSLR Focus). I got the same results both ways.

Here’s the marked-up schematic that Jim sent me:

schematic for USB conversion

Jim also sent me a photo of the TTL232R kit:


And here’s a picture of his connection of the TTL232R to the interface board:


10 thoughts on “USB For Your Digital Setting Circles

  1. Hello David,

    Do you know if for this modification Jimbo used the TTL-232R-3V3 or the TTL-232R?

    They have different specifications in the Vcc.

    Thank you.

  2. Hi George,

    I wrestled with this same question before purchasing.

    I decided to go with the 5v version of the unit instead of the 3.3v (3V3) version, since the original circuit had a 7805 5V regulator in it (U1).

    There are also several styles of the 5V version. I got the “base” model, not the “AJ” (audio jack) or “PCB” version.

    I happened to order from Mouser (it’s available elsewhere, too). The (complete) Mouser part number is: 626-DLP-TTL232R

    Keep looking up,

  3. i am curious and new to electronics in general but i am following and starting a project using a palm device for my DSC using this board. there seems to be some concern about the board eating 9v batteries does this adaptation cure this problem and if so how to i get it. i have not ordered anything yet to begin. just got the palms and some of the info to build.

  4. James,

    Nope, this won’t improve battery life. It must means that you can connect it to a PC’s USB port instead of a serial port. Since you’re talking about a Palm device, this modification won’t work for you. For improved battery life, I recommend you use a sealed lead-acid battery (sometimes called a gell cel. You can often get them surplus for not a lot of money.

  5. Jim, thanks for this. Up till now I’ve been using a Skyspan serial to USB adapter. Works great and I haven’t had any problems with it.

    It’s just that I wish I’d known about your mod before I paid the $70 for the Skypan!!

    I have a couple of friends who’re making the Ek box and I’ll tell ’em about this before they get too far. Is there a new circuit board with this mod available? –geoff

  6. Geoff,

    Nope–no new circuit board for this mod.

    Wow–it seems like you could find a USB-to-Serial converter for a lot less than $70 these days–but then again, I haven’t been looking for one…


  7. Dave, yes you can but I was told that there were good serial converters & bad serial converters, some of them don’t allow good passage of all the signals. Now I don’t know if this is so or not.

    The Skyspan I ended up buying was from an Apple store so that explains why the cost is so high. But it works a treat. –geoff

  8. Hi Dave,
    I’m just jumping on the DSC bandwagon, so to speak. I have a palm w/ USB cable connector for hot syncing. My palm also has bluetooth. I was wondering if I could use one of the adapters above (Mouser lists several), use a female-female connector, and then use a USB bluetooth signal generator (dongle?) on the end? I would like to build your system, but the inevitable march of technology has kind of passed the serial port by, and I’m looking for a solution that doesn’t add another $80 to the cost. Thanks for your help and suggestions,
    Mike D.

    • Mike, I don’t think that would work. A better solution would be to purchase a serial-to-bluetooth adapter like what is sold at aircable.net, plus a null modem adapter. There’s quite a bit of info on using bluetooth in the palmastro Yahoo group.


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