• ​Basic operation achieved. Modulation recorded and analyzed.

    Ricardo • 01/28/2018 at 15:43 • 0 comments

    The unit is powered with a 5v wall adapter and alligator clips instead of four 1.5V coin cells. The mode switch (at the lower right) is set to "OPER" and a letter 'P' is shown in the screen. Enter "0406091001" and press "ent"; a letter 'H' is now shown. Enter "001290" and "ent", a '0' is displayed: that was the end user ID. Now, different product codes can be stored.

    Begin with 'H' or 'P' and type digits, then press "ent". Information is briefly displayed like:

    0004      0451

    with the first number increasing and the second decreasing. Probably means the number of occupied and free memory positions. After a few seconds, '0' can be seen again. The arrow keys let you check previously stored data but I don't know how to edit or delete a position.

    After storing a few codes as "H12345678" or "H96385274" I place the mode switch in "SEND" and press button "SND/=". The screen displays "SEND BUSY" and this audio sequence is emitted:

    For 4.4s a 1.200 Hz tone is heard. Then, for 6s, it is 1 kHz. For 2.8s there is a modulation. For the last 3s, the 1 kHz tone is sent again. A careful look at the spectrum shows the frequencies are not very accurate. The tones actually seem 980 and 1180 Hz.

    I installed this program: http://www.whence.com/minimodem/minimodem.1.html

    and ran:

    minimodem -r -f msi55_bell103_3.wav -M 980 -S 1180 300

    which returned:

    ### CARRIER 300 @ 1000.0 Hz ###
    �H00��90+�H00��90+�H00��90+�H��3�56��+�Ʊ�3�56��+��9��+�ƴ56+�H963�5���+�
    ### NOCARRIER ndata=74 confidence=2.026 ampl=0.147 bps=294.55 (1.8% slow) ###

    This looks like a Bell 103 modulation: https://en.wikipedia.org/wiki/Bell_103_modem

    Even though Bell 103 has 1070 and 1270 Hz tones instead of 1000 and 1200 as I found.

    Maybe the components have aged and the tone and symbol frequency have drifted. I edited the audio clip for speeding it up 1.8%. The audio tones were now close to 1000 and 1200 Hz and the symbol rate close to the theoretical 300 bps:

    minimodem -r -f msi55_bell103_4.wav -M 1000 -S 1200 300 -R 8000 -8 --startbits 1 --stopbits 1

    ### CARRIER 300 @ 1000.0 Hz ###
    �H00��90+�H00��90+�H00��90+�H��3�56��+�Ʊ�3�56��+��9��+�ƴ56+�H963�5���+�
    ### NOCARRIER ndata=74 confidence=2.090 ampl=0.148 bps=299.50 (0.2% slow) ###

    In both cases the demodulated data makes sense in spite of some errors. The code "H12345678" is now "H��3�56��". It could be due to some 50 Hz mains noise on top of the modulation, caused by the cheap wall adapter. minimodem program reports a low confidence value (2), consisten with a distorted signal. All in all, the esential features and technical specs have been understood.

  • Able to store data and send it by the modem.

    Ricardo • 01/14/2018 at 22:35 • 0 comments

    I got some advice from a former user and I was able to make it work, at least partially. An initial code must be entered and then product codes can be stored. The terminal also sent the data through the speaker. The sound seemed a 300 baud modulation. I will provide further instructions and audio samples.