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?
Electronic – usb daq device to rs232
rs232serialusb
Related Solutions
Here is how I would suggest going about debugging this problem:-
Check the connections.. if you've mixed up signal ground and chassis ground (shield) you could get funny behavior just like you describe. Pin 7 on the DB-25 is the one you want to connect to DE-9 pin 5. I'll assume you're using a commercial adapter/cable or have one wired like this.
Maybe check the transmit out line to ensure it's idling at -12 ( < -3 anyway).
Make sure the instrument is set to RS-232, and not current loop (Jumper 7,8 and 1,2).
Try the most common serial port setting (9600/8-N-1), 9600 baud, NO parity, 1 stop bit switch 1, 2, 3 OFF switch 4, 5 ON switch 6 ON switch 7, 8 ON
Make SURE you understand which is 'OFF' and which is 'ON'. Don't laugh, I've seen this happen more than once, and some types of DIP switches are kind of ambiguously marked. If in doubt, an ohmmeter across the switch.. in fact you could check all of the switches to ensure they're working.
I like Realterm as a terminal program. It's free, and easy to change the parameters. Try it on something known working first of course, and look for an online tutorial. Really worth it.
If still gibberish after that, get a scope screen cap or photo and post it.
If you have a continuing need for a logic analyzer, this one is very cheap and relatively useful. Not as good as my Tektronix boat anchor, but this one will fit in a coat pocket. But I'd get a scope first if you don't have one. Even the worst scope is better than nothing.
My advice is to use a UART and RS485.
If you try to use any non-differential signalling protocol on a lead that long, you will get a terrible BER at useful data rates. Differential half-duplex async is the way to go.
Your concerns about sharing the bus with USB are valid, but RS485 drivers can (generally) be put in a high-impedance mode that might be willing to co-exist with USB.
Best Answer
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.