The device shown on KDPV was designed to automatically send orders from a branch to a central store. To do this, it was necessary, after entering the articles of the ordered goods into it, to call the number of the central store and send the data according to the principle of an acoustically interfaced modem. The rate at which the terminal sends data is assumed to be 300 baud. It is powered by four mercury-zinc cells (then it was possible), the voltage of such an element is 1,35 V, and the entire battery is 5,4 V, so everything worked from a 5 V power supply. Three modes can be selected with the switch: CALC - a regular calculator, OPER - you can enter numbers and other characters, and SEND - sending, but at first it was not possible to squeeze out a sound. It is clear that you can somehow save the articles and then send them, but how? If you can find out, the author will try to analyze the sounds , or even somehow adapt the terminal for digital modes of amateur communication.
The device is on the reverse side, the dynamic head and the battery compartment are visible:
The most important thing - how to squeeze sound out of the terminal - the author learned from a person who once had the same terminal. You need to enter the initialization code, and then you can enter articles. We move the switch to the OPER position, the letter P will appear. Enter 0406091001 (the author does not explain what it is, probably the username) and press ENT. The letter H appears. Enter 001290 (and this is probably the password) and press ENT again. The number 0 appears. Article numbers can be entered.
You need to start the article with the letter H or P (here the author made a mistake, there is no letter P on the keyboard, there is F), then there are numbers. After pressing the ENT key, a line like 0004 0451 appears, where with each subsequent article the first number increases and the second decreases, from which it follows that this is the number of occupied and free cells, respectively. Using the arrow buttons, you can scroll through the entered articles, but the author does not know how to delete them (which means that the CLR key did not help). How to indicate the quantity for each of the articles is not said.
After entering the articles, you must then turn the switch to the SEND position and press the SND/= key. The indicator will display the message SEND BUSY, and the transfer will begin:
For 4,4 seconds, a tone with a frequency of 1200 Hz sounds. Then for another 6 s - 1000 Hz. The next 2,8 seconds are spent on transmitting a modulated signal, and after them another 3 seconds are again on transmitting a tone at 1000 Hz.
If you look closely at the spectrum, in fact, instead of 1000 Hz, you get 980, and instead of 1200 - 1180. The author recorded a WAV file, installed the program mentioned above ("man" to it ) and run like this:
minimodem -r -f msi55_bell103_3.wav -M 980 -S 1180 300
Happened:
### 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) ###
It looks like . Although there are generally 1070 and 1270 Hz.
Didn't the frequencies "float away" at the terminal? The author edited the WAV file so that the speed increased by 1,8%. It turned out almost exactly 1000 and 1200. Restarting the program:
minimodem -r -f msi55_bell103_4.wav -M 1000 -S 1200 300 -R 8000 -8 --startbits 1 --stopbits 1
And she answered:
### 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 result carries a semantic load, despite the errors. The article H12345678 was “pulled out” from the signal as H��3�56�� — the numbers that we managed to make out are in their places. There may be poor filtering in the PSU, which is why a 50-Hz background is superimposed on the signal. The program reports a low confidence value (confidence=2.090), which indicates a distorted signal. But now it is at least clear how the terminal sent data to the computer of the central store when it still existed.
Source: habr.com