Log two Multimeters with single serial port

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As promised in my previous post, here is the program to log the data coming from two multimeters. I opened the Rs232 box of one multimeter (we are talking about the DT4000-ZC model) and discovered that there is no GND reference ! Look at the picture :

rs232-ifAt left side, you can see the “circuit” that’s in the rs232 connector box ; at right, what I suppose it’s inside the multimeter. The GND reference isn’t needed ’cause the pin 2 of DB9 connector is normally polarized at -5V, that is the voltage level coming from the pin 3 (Txd) via 10K resistor. When the DTR signal (pin 4) goes high, the PNP transistor acts like a switch, commutating the pin 2 level from -5V to +5V depending on serial signal captured by the PNP phototransistor. In this case, the GND reference is inside the PC serial port and therefore there is no need to insulate the two multimeters one respect to the other.

But… I just want to use a single rs232 port on my PC to log both the multimeters, as channel #1 and #2. Ok, if the circuit inside the multimeter is like I supposed it to be, I can simply make an ‘OR’ of the outputs, powering via RTS one channel, and via DTR the other, once at a time. The schematic for this mixer is here:

That's not CAD, but MAD (Manually Aided Design)

That’s not CAD, but MAD (Manually Aided Design)

In the next picture you can see the prototype I built to test the program.

mixer-detailsNow, all is ready to test. I used a USB battery charger (very simple circuit, look on my previous articles for details) and two multimeters to measure the battery voltage and the charging current.

IMG_20130818_180318 Ok, the multimeters are running. Now I can launch my new application, having set the com port as in the previous program. Starting the log, I can see on the screen the samples, alternatively taken from the multimeter #1 and the #2. At the start of each line, I added a time stamp, for successive data elaboration, may be in graphical form.

dos2x2The application has been written in FreeBasic (look at the right column of this page for the link to the free, open source compiler) and doesn’t need for installation. Please, look at the previous post for more details on starting from batch. The new program acts exactly as the previous, only has enhanced data stream verification and, obviously, double channel capabilities :)

Note: this article was cited on “Hack-a-day” on Aug 26 2013.

Disclaimer: this application is provided with no explicit or implicit warranties of operation. I do not assume any responsibility for problems that may arise on the device where the application is installed. The program is a “demonstration” and no support of any kind is provided. By downloading and installing the program, you implicitly accept my terms of not taking responsibility. If you do not agree, do not download and / or install the file !

I have read and understood the note, accept the terms and want to download the file DT4000x2.zip (42.204 bytes)

Note: the zip file has a password. Check the hash of the zipped file, then extract it and you will have your executable ready to run. If you don’t have a tool for computing the hash, I suggest HashTab, free for personal use.
Note: DO NOT extract or use the downloaded file if MD5 doesn’t match with the following codeĀ : MD5 = 0EEA13D48A3E915B2C4E53CC54663679
The password is: eficara