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AM7910 modem configuration questions

neazoi

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Hi I have configured an AM7910 modem IC to use only bell103 and bell202 modes without back channel, no handshaking. I ignore all the other settings/modes of the chip. Here is the rough schematic attached.
My questions are in the page 7 and 11 of the datasheet.

1. If set for Bell103 answer mode and no TX data is sent across, does a default mark tone of 2225Hz continuously present on the TC pin if RTS is grounded?

2. If set for Bell202 and no TX data is sent across, does a default tone of 2025Hz present on the TC pin if RTS is grounded?

3. When I change the state of RTS/ will these default tones deactivate? (TX off)

4. If I want to change modes from bell103 originate, to bell103 answer and vice versa, I could follow the 4-steps process on page 7, DTR/pin description. However, can I switch off the modem, configure MC0/MC1 and then switch it back on, or do I need to also manipulate the DTR pin accordingly?

Thanks a lot
 

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neazoi

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Any news on this please?
I am trying to find out the right configuration so I can make an acoustically coupled modem with it, that will be able to drive transceivers through their VOX function. This is why I am asking.
 

betwixt

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I haven't used one of those devices in the last 30 years so I would have to go back through my archives for details on usage but if my memory serves me well, all you do is change the mode pins. The device has no intelligence, it is just a tone encoder/decoder and a control interface.

I would caution you that acoustic modems were seldom used above 300 bauds in the past as higher speeds were found to be unreliable. A transceiver generally has a narrower audio passband which would make matters worse. You would have to use AM or FM for transmission, SSB would require very accurate tuning and a wider than normal filter.

Brian.
 

    neazoi

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neazoi

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Thanks Brian,
I am going to use the bell103 just for it's shift (200Hz) but on HF, I am going to use lower baud rates. The shift does not change. The 200Hz shift is pretty close to the 170Hz that hams use and if you adjust your TRX frequency accordingly, you can have a 15Hz error in both mark and space. I bet most HAM radio software can deal with this nicely. This will make it compatible with HAM RTTY.
Acoustic coupling will be used as a universal connection (it will work also through GSM) where each device/mobile has it's own interface, as an easy way to transfer data. I actually need the bell202 only to be compatible with my CB2 micro (http://cb2.qrp.gr) so as to transfer data, as this project does not support lower speeds than 1200 baud.

However, I was wondering about these questions mentioned in post 1. How about these tones when no TX data is sent (but TX channel is active), have you got any idea?
 

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The device isn't intelligent, it just switches tones according to the logic level at the input so if it is enabled, the tone will always be present and selected by whatever level is there.

Don't forget that RTTY does not use standard ASCII codes. In fact it is a two level code so you need to decode it to switch character sets.

Brian.
 

    neazoi

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neazoi

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The device isn't intelligent, it just switches tones according to the logic level at the input so if it is enabled, the tone will always be present and selected by whatever level is there.

Don't forget that RTTY does not use standard ASCII codes. In fact it is a two level code so you need to decode it to switch character sets.

Brian.
I was not aware of that! Bell103 also has other incompatibilities with RTTY, like the diddles when no data is sent on rtty, in contrast to the mark tones for bell103 and 202 (the ones I asked for. I will use these tones as a VOX enable. But I thought that RTTY code was compatible with bell103.
Thanks
 

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RTTY is a 5-bit code designed optimized for mechanical teletype machines. As the early machines all used impact printing (like an old fashioned typewriter) and there wasn't space for the full character set on the print head, they used all the combinations of 5 bits twice with one bit being used to toggle between the two halves of the printable font. On the early machines the code raised and lowered the print mechanism so the other characters were lined up with the paper. You have to remember that the first TTY machines were entirely mechanical except the motor and solenoids. You also needed ear defenders to be in the same room as them!

RTTY uses Baudot code (see https://en.wikipedia.org/wiki/Baudot_code ) and the 'diddle' was a later addition to keep the sending and receiving machines in the same shift mode. If the transmission was to break there would be no way to tell which character set the receiving end should start with.

Brian.
 

    neazoi

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