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USB cable to control BS2 — Parallax Forums

USB cable to control BS2

Jason ShortJason Short Posts: 21
edited 2006-02-04 02:36 in BASIC Stamp
I'm working on some controllers that I'm driving with BS2s and I use a Keyspan USB-serial adapter to connect them. I thought it might be a nice trick to use a USB cable from the keyspan adapter to the stamp. The cable has 4 lines, Ground, +5, Data Out,Data In. I removed the DB9 connector from the adapter and soldered on a Type A USB connecter and ran the lines to it.

My power situation on the stamp is good. It powers on and works well. The issue is the RS232 connection. When I look at it with my little scope all I see is noise on the pins when they transmit. On my development board I see nice square waves.

My question is where is this noise coming from and how can I steal power and ground from this USB adapter? I figure I have some kind of ground issue but I'm totally ignorant on how to track it down.

http://www.keyspan.com/products/usb/USA19HS/

Thanks,
Jason
keyspan.jpg
400 x 300 - 50K
keyspan2.jpg
300 x 173 - 21K

Comments

  • allanlane5allanlane5 Posts: 3,815
    edited 2006-02-03 17:15
    Um, eeew.

    Why would you want to hack a perfectly good USB to serial adapter, to make a BS2 board which 'looks like' it has a USB connection, only it's some weird RS-232 over USB hardware hybrid.

    The RS232 hardware on the BS2 'Port 16' programming port uses the incoming RX signal to power its outbound TX signal, so that has to be good.

    And you can't 'reset' the BS2 over the serial link if you don't connect the DTR signal -- so a minimal BS2 cable MUST have TX, RX, GND, and DTR. And for programming to 'see' it, the RTS and CTS (pin 7 and 8) signals need to be looped back.

    So, to use it, you need TX,RX, and GND. To program it, you need those plus DTR, RTS, and CTS.

    Were I you, and in a hacking mood, I'd probably use an RJ45 connector and cable over the USB. You get more conductors (8), you get more distance, cable is cheaper, and you won't confuse people who WILL plug your BS2 module directly into a "REAL" USB port, with disasterous results.

    Oh, and powering the BS2 from the cable -- USB doesn't have enough pins for that, but you COULD use one conductor of the RJ45 cable for that purpose.

    For that matter, you could have 'lifted' one of the 'unused' DB-9 pins off the Keyspan (like pin 1, maybe), routed +5 to it from the Keyspan, and used THAT as your power source for the BS2.
    ·
  • Jason ShortJason Short Posts: 21
    edited 2006-02-03 17:26
    Well,
    I never intended to program the stamp from this. It's for a set of 4 embedded prototypes that need to be run off of a single keyspan adapter and swapped out many times. The USB is a nice cable for that and I happened to have the USB board mount connectors already. If I used an Ethernet I would run into a similar problem with pulling the power from the USB hub itself which is at the heart of my question. Of course I could add a real power supply and run separate power and ground to the stamp and I'm sure that would work, but I thought the USB already has 5v and ground so why not tap in?

    I'm just curious why the RS-232 isn't working, because the power is working fine. And yes I was in a hacking mood!
    Thanks
  • Bruce BatesBruce Bates Posts: 3,045
    edited 2006-02-03 18:19
    Jason -

    Please keep in mind that an un-enumerated USB connection can NEVER draw more than 100 mA under any conditions. To do so may cause the entire USB port to shut down.

    If you don't fully understand what I'm speaking about here, you probably shouldn't be trying to use parasitic power from a USB port. There is even more to it than what I've just said.

    Regards,

    Bruce Bates
  • Jason ShortJason Short Posts: 21
    edited 2006-02-03 23:13
    From what I understand it's possible to draw almost 500 mA without alerting the bus and I don't need anything near that much. I think my issue is a grounding issue, but I still think it will work. It's for a specific purpose so I won't be confused by the connector, and If I wire it right, it won't fry anyone's USB port. My final board will have a mii USB connector which is much smaller that the type B I have on the picture and much smaller than an ethernet connector.

    Jason
  • Bruce BatesBruce Bates Posts: 3,045
    edited 2006-02-03 23:30
    Jason -

    What you understand is NOT correct, per the USB 2.0 Specifications. Please download the complete USB Specifications from the link below and read through it, the same as I did, before you continue, since motherboards don't come cheap these days!
    http://www.usb.org/developers/docs/

    Regards,

    Bruce Bates
  • SteelSteel Posts: 313
    edited 2006-02-04 00:22
    Wouldn't it be easier, cheaper, and quicker just to throw this guy in?

    http://www.parallax.com/detail.asp?product_id=604-00043
  • Jason ShortJason Short Posts: 21
    edited 2006-02-04 02:25
    Cool, I've never used that chip before. I'll be sure to try it. How well does it work with OS X/Linux?

    jason
  • Jon WilliamsJon Williams Posts: 6,491
    edited 2006-02-04 02:28
    Yep: http://www.ftdichip.com/FTDrivers.htm

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Jon Williams
    Applications Engineer, Parallax
  • Jason ShortJason Short Posts: 21
    edited 2006-02-04 02:36
    I noticed the FTDI spec has a noise reduction circuit in the bus powered configuration. Might be the trick for solving my problem with noise.
    Jason
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