We have a piece of data acquisition equipment (voltage data logger) that connects to a PC via a USB cable for power and communication. Because of some other equipment we already own, it would be convenient to have this connected to a serial port using RS232 protocol. Is there any easy way to do this (e.g. an off the shelf product)? I have found numerous USB to RS232 converters, but they seem to go in the opposite direction. Can I simply use one of these cables and splice in to provide external power to the unit?

  • \$\begingroup\$ What does "daq" mean? \$\endgroup\$ – JYelton Aug 22 '13 at 21:08
  • \$\begingroup\$ @JYelton data acquisition \$\endgroup\$ – Passerby Aug 22 '13 at 21:41

USB to RS232 or TTL Serial are USB Slave/Peripheral Devices. They need a usb host to work. It does not work backwards by themselves.

Your three options are 1) if the existing device already uses a serial to usb device inside, you can modify it by removing that part and wiring to the existing serial connection. If the serial inside is TTL, you would need a TTL to RS232 level adaptor.

2) If you don't want to modify the device, and it is a simple CDC/virtual serial port device, a small linux computer with a usb and a serial port can be set up as a pass through device. Like 10~20 dollars and an hour to set it up if you have linux experience.

3) If the device is not a simple serial port device, requiring special drivers and software to work, then you are looking at a problem that will require a high number of man hours to reverse engineer and implement, if not money.

A fourth option is replacing the DAQ with one that suits your need.

  • \$\begingroup\$ I like the linux computer idea, but is $10-$20 realistic? I'd buy a handful if I can find them at that price, but the best I can do is the Pi or Beaglebone Black, so we're in the $40 range. \$\endgroup\$ – Scott Seidman Aug 22 '13 at 22:25
  • \$\begingroup\$ @ScottSeidman all it needs is usb and a serial port. Doesn't need more than like 16mb ram or 4mb flash, all it is doing is booting up, and echoing one virtual serial to the physical serial port. As for linux computer prices, 10 dollar routers, 35 dollar android tv sticks, 30 dollar android cell phones, pogoplugs, 10 dollar digital picture frames, hell, 30 dollar Wifi-enabled sd cards running linux. Embedded computers are cheap, and the less options it needs the cheaper. \$\endgroup\$ – Passerby Aug 22 '13 at 22:36
  • \$\begingroup\$ I figured this might be the case. Thanks for the answer. I checked the device and it does not have a serial to usb device inside. Since we will have to have a computer involved anyway, I can just use the computer (e.g. Pi) to read the data over USB and record it. \$\endgroup\$ – SKempf Aug 23 '13 at 13:29

No, you likely cannot marry the USB to RS232 converters. In the USB world, you need a device, and you need a host for the device (ignoring USB OTG for the moment). The converters are devices, and you'll have no host to control them with.

I suspect it will be easier to build your own voltage data logger w/ serial communication from scratch than it would be to buy or build a USB-to-serial converter. The biggest obstacle I see is that RS232 does not have power on the connector, so you'd have to power your device.

Alternatively, native serial voltage loggers are probably a dime a dozen! http://www.g4jnt.com/SerialLogger/HF-10.htm, for example.

  • \$\begingroup\$ I looked for some alternatives to replace the unit we have, but haven't found anything equivalent. Building one would probably be pretty easy, but this is a bit out of my skill set! Thanks for the answer. \$\endgroup\$ – SKempf Aug 23 '13 at 13:56

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