+ Post New Thread
Results 1 to 6 of 6
  1. #1
    Advanced Member level 5
    Points: 16,646, Level: 31
    Achievements:
    7 years registered
    neazoi's Avatar
    Join Date
    Jan 2008
    Location
    Greece
    Posts
    3,365
    Helped
    13 / 13
    Points
    16,646
    Level
    31

    TCM3105 what happens if data is sent at a higher rate?

    Hi I design a modem using the TCM3105 (datasheet available on the net).
    The computer device that will be connected to it has an RS-232 with only 1200 baud RX and 1200 baud TX half duplex.

    1. What will happen if I configure the tcm3105 for 600RX/600TX whereas I am trying to send and receive data to it from the computer at 1200?
    On sending, will the modem throw away sent characters because it can't cope?
    On receiving, will the modem work ok, since it receives only 600 baud but it is configured for 1200. Or it could not cope with it at all?

    2. Will the tcm3105 matter if it is connected to the RS232 port (+/- voltages available) or does it need 0v/5v for logic?

    Thanks
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!

    •   AltAdvertisement

        
       

  2. #2
    Super Moderator
    Points: 260,228, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    45,442
    Helped
    13826 / 13826
    Points
    260,228
    Level
    100

    Re: TCM3105 what happens if data is sent at a higher rate?

    The modem chip processes bits and doesn't know about "characters". Respectively it's not able to perform any kind of baud rate conversion. Not easy to predict if using foreign bit rates will give meaningful results. Why would you do this?

    It's a 5V chip and uses TTL logic level. Connecting RS232 input signals may damage it.


    1 members found this post helpful.

    •   AltAdvertisement

        
       

  3. #3
    Advanced Member level 5
    Points: 16,646, Level: 31
    Achievements:
    7 years registered
    neazoi's Avatar
    Join Date
    Jan 2008
    Location
    Greece
    Posts
    3,365
    Helped
    13 / 13
    Points
    16,646
    Level
    31

    Re: TCM3105 what happens if data is sent at a higher rate?

    Quote Originally Posted by FvM View Post
    The modem chip processes bits and doesn't know about "characters". Respectively it's not able to perform any kind of baud rate conversion. Not easy to predict if using foreign bit rates will give meaningful results. Why would you do this?
    The system that controls the tcm3105 has fixed 1200TX/1200RX speeds. So this should work ok if the chip is configured for 1200/1200.
    However I want to use the chip for lower data rates (comms on HF radio for example) which require much lower speed (even 75 baud). So I wonder what is happening inside the chip, does it throw the data or it won't be able to communicate at all? This is a kind of long time question for me
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



    •   AltAdvertisement

        
       

  4. #4
    Super Moderator
    Points: 81,430, Level: 69
    Achievements:
    7 years registered
    Awards:
    2nd Helpful Member
    betwixt's Avatar
    Join Date
    Jul 2009
    Location
    Aberdyfi, West Wales, UK
    Posts
    13,348
    Helped
    4456 / 4456
    Points
    81,430
    Level
    69

    Re: TCM3105 what happens if data is sent at a higher rate?

    Based on the TI data sheet, it should work at almost any speed as it doesn't time the frequency shifts, it just responds to them as they happen. It is only an FSK modulator and demodulator, it doesn't care what speed you feed to it but obviously there is a limit determined by the number of cycles per bit in the audio waveform. The data sheet shows configuration examples for 600/600 and for 150/150 Bauds. You should keep to the same crystal frequency though as it determines the filtering characteristics and tones.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.


    1 members found this post helpful.

  5. #5
    Advanced Member level 5
    Points: 16,646, Level: 31
    Achievements:
    7 years registered
    neazoi's Avatar
    Join Date
    Jan 2008
    Location
    Greece
    Posts
    3,365
    Helped
    13 / 13
    Points
    16,646
    Level
    31

    Re: TCM3105 what happens if data is sent at a higher rate?

    Quote Originally Posted by betwixt View Post
    Based on the TI data sheet, it should work at almost any speed as it doesn't time the frequency shifts, it just responds to them as they happen. It is only an FSK modulator and demodulator, it doesn't care what speed you feed to it but obviously there is a limit determined by the number of cycles per bit in the audio waveform. The data sheet shows configuration examples for 600/600 and for 150/150 Bauds. You should keep to the same crystal frequency though as it determines the filtering characteristics and tones.

    Brian.
    So can I leave the settings of the tcm3105 at 1200/1200 and just send ttl data at lower speeds?
    But not the other way round, right?
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



    •   AltAdvertisement

        
       

  6. #6
    Super Moderator
    Points: 81,430, Level: 69
    Achievements:
    7 years registered
    Awards:
    2nd Helpful Member
    betwixt's Avatar
    Join Date
    Jul 2009
    Location
    Aberdyfi, West Wales, UK
    Posts
    13,348
    Helped
    4456 / 4456
    Points
    81,430
    Level
    69

    Re: TCM3105 what happens if data is sent at a higher rate?

    Correct - the different mode settings change the frequency pairs and maybe filtering but not the bit rate.

    It's worth considering how much audio the modem has to decode, at 1200 Bauds it only gets ~1 cycle of audio to work out the frequency so you can see why pushing the speed higher will lead to failure.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



--[[ ]]--