I got another toy to play with: A digital multimeter with RS232 interface and True RMS power measurement. Sadly, it comes with Windows-only software, which I interpreted as a challenge!

Since this is electrical measuring equipment, the serial communication should be electrically insulated from the device. The VC-940 uses a photodiode. Since the cable connecting the multimeter to the computer only contains an optical receiver, it is impossible for the computer to communicate with the meter. This significally reduces the complexity of a protocol.

This optical part showed to be the first problem. When I insert my RS-232 tester when running the Windows application, I could see data passing; under MacOSX no data would pass.  Besides showing no data, the tester also indicated that the control lines were asserted differently. By asserting the control lines the same way under MacOSX, the problem was resolved. Apparently, the voltages on the control lines are used to power the optical receiver electronics.

The baudrate and other settings were found by simply trying different values. When receiving at 2400bps, 8 databits, no parity and 1 stopbit (2400 8N1 for short), measurements were recorded in the Windows application.

This perl-script opens the specified serial port, asserts the control lines as required (RTS logical 0, positive voltage; DTS logical 1, negative voltage) and outputs the data to stdout.

The message format

When observing the data flow, it was clear that the device sends out a byte-string of 11 bytes roughly every second:


Every message seems to be terminated by 0x0D8A. I used this to synchronize the input. The following table shows what I discovered for the other bits in the message. The nibbles (4 bit group) are numbered from left to right starting from 0.

Nibble Description
13 The measurement mode (see below)
1 First digit
3 Second digit
5 Third digit
7 Forth digit
9 Fifth digit (not in all modes)
11 Decimal point position and unit selection (depends on mode)
17 bit 0 Negative (1) or positive (0)
17 bit 1 Auto (1) or manual (0) range
15 AC, DC or AC+DC (depends on mode)

Note that I didn’t figure out what every bit means…

The modes are listed below:

Mode Description
1 V DC
2 V AC
3 mV
4 Ohm
5 Farad
6 ºC
7 uA
8 mA
9 A
a Ohm with beep
b Diode junction voltage
c Frequency and duty cycle
d ºF
e W
f % of 4-20mA loop

This perl-script parses the binary input (from stdin) to readable output:

2009-05-01-16-48-52 : 313834b5b3b0b3b0b0 :  184.53 mV
2009-05-01-16-48-53 : 3138343434b0b3b0b0 :  184.44 mV
2009-05-01-16-48-54 : 313834b534b0b3b0b0 :  184.54 mV
2009-05-01-16-48-55 : 313834b3b0b0b3b0b0 :  184.30 mV
2009-05-01-16-48-56 : 313834b037b0b3b0b0 :  184.07 mV
2009-05-01-16-48-57 : 3138b338b5b0b3b0b0 :  183.85 mV


  1. HyP3r says:

    Nice Work Thank you!

  2. Florian Bach says:

    The source code for read out data from the VC 9X0 can be downloaded at http://hardtest.florian-strunk.de/showthread.php?tid=57

  3. Ballet says:

    Without your page, I can’t use my Voltcraft in Dephi.
    Really good work.
    Best regards from France

  4. quetzalcoatl says:

    You might find this one interesting:
    the function codes match exactly to those you’ve posted above.

    There’s a great dumpbin for lots of datasheets and other information at http://www2.produktinfo.conrad.com . Try searching for your VC940, and maybe you’ll get something more interesting. Unfortunatelly, the languages are mostly random and the search is done by simple file-name comparison. For example, I’ve found for myself a *very* important document http://www2.produktinfo.conrad.com/datenblaetter/125000-149999/125335-da-01-en-Datenprotokoll_SD_card_file_Formatv1_2.pdf that explains the binary formats for Voltcraft Energy Loggers 3500/4000. Not returned by seaches for EL4000.

  5. icke says:

    This rather old post helped me finally getting my VC850 to talk to my Linux box. Asserting the RTS line to 0 did the trick. Thanks!

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