Logging two multimeters at (nearly) the same time

log-two-multimeters-at-one-time

It’s pretty common to have at least a couple of meters around to measure different values of a circuit at the same time. Where [Emilio P.G. Ficara] ran into a problem was logging the data from both at once. These Fluke meters have a serial-out, but his computer only has a single serial-in port. He cracked open one of the meters and figured out how to log data from both at the same time.

A lot of folks would look to a microcontroller to solve this problem. You use the chip to pull from each meter simultaneously and report back to a computer (or just dump the values onto an SD card). But this solution is a simple mechanical connector and a bit of creative programming. The way the serial output is set up on these meters they won’t interfere with each other as long as they’re read one at a time. [Emilio] wired them up as seen above, using his own software to manage the pins of the serial port. The example output he posted shows readings from the meters taken within about a tenth of second from each other. That should be good enough for most applications.

Comments

  1. Trui says:

    “A lot of folks would look to a microcontroller to solve this problem.”

    I would grab a serial -> USB converter cable, and create an extra serial port on the computer.

    • w says:

      “A lot of folks would look to a microcontroller to solve this problem.”

      Yes, there are a lot of muppets in the world.

    • meh says:

      Sure, using a 2nd adapter easily solves the problem, if you have one on hand or don’t mind waiting for it to ship from China. This hack lets you use 4 similar transmit-only devices on 2 ports.

      The real advantage to having 2 ports is not missing transmitted data while you’re reading from the other device. You can receive from both at once. And even then, that only matters if specific timing matters in your application.

      I like the idea. With a bit of clever hacking, like adding a 74HC154 (4 to 16 decoder — or a 138 for something smaller scale) on the DTR & RTS pins on 2 serial ports, you could easily address 16 different devices.

      He definitely loses points for using FreeBasic though (same story for his logger). There’s *tons* of far better options out there. Also, the schematics are very poor even for something hand drawn.

      • meh says:

        Oh, and for that it’s worth, I often (ab)use my serial terminal for most things RS-232 related instead of using a whole bunch of USB adapters. It was cheaper ($50 used on ebay), it’s more reliable and I can connect over tcp/ip.

        • The serial stream coming from the multimeters is not pure ascii, but bitmapped image of each segment and icon of LCD display, so the decoding job can’t be done with a simple serial terminal, but needs for an algorithm. Freebasic is an excellent way to do this. Don’t look at the “dos” output, it can be used for usual graphic style applications. I just programmed the minimal application that fits my needs (so lazy) :)

          • meh says:

            Sorry but FreeBasic is probably the best option if it’s the only language you know. That’s all I’m saying.

          • Oh, no. FreeBasic isn’t the only language I know. I also am able to speak Italian :)
            I don’t know why you consider so bad a free, open source tool that can compile executables for Windows and Linux, but I respect your opinion.
            By the way, I usually use the programming language that can give me a shorter time from idea to product. Look at my blog, there are applications for microcontrollers written in C, examples for web applications written in PHP (server side) or Javascript (client side), some accessory application written in VB, some small Android tool and if you dig in the deep, you will find some (old fashioned) motorola or zilog micro sources in assembler. Oh, there are some games for Windows Mobile 5/6 written in LUA. Nothing in Pascal or ADA, sorry.
            In the specific case, I just used a free tool ’cause I think the good thinghs of life are free :)

  2. Greg says:

    ^ That is what I was thinking. A second RS232 converter on another USB port.

    It may be that the photos on the site of his are shockingly small, and I can’t tell what computer he has (I doubt it has an RS232 port, it looks too modern for that) but I do have a spare old laptop that has a RS232 and parallel port for…just in case.

  3. Tiersten says:

    Just adding another USB serial dongle would be easier but this is still a neat hack overall. You may want to wire many multimeters into your uC/PC for some kind of test rig and doing it this way would mean just needing some GPIOs or a parallel port together with a single UART which may be 5V TTL.

  4. vonskippy says:

    You can get a brand spanking new Dual-Port PCI Serial card for under $10 US

    (of course that means using a Desktop computer instead of a lappy).

  5. NiN says:

    Nice hack. I guess it can be used for serial->dual_TTL interface if you supply two MAX232 chips from DTR and RTS lines separately.

  6. Finder says:

    I don’t think those are Fluke meters

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