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ttl <-> ttl: what can go wrong?

Discussion in 'General Electronics Discussion' started by muffinman, Jul 19, 2013.

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  1. muffinman

    muffinman

    17
    0
    Dec 10, 2012
    Hello all,

    I've got a device with a ttl output which I need to connect to rs232. They sell a 400 euro box to do that, but I figured I can do that cheaper. So, I've got an rs232 to ttl converter and all works fine. I've hooked it up to an oscilloscope (just leaned how to use one). All voltage levels are well within the limits (I wanted to verify the specs and learn how to use an oscilloscope).

    Now, I've got a minimum background in (and knowledge of) electronics. Therefore my question: is there something that I have overlooked and could void my warranty (or hurt my device), e.g. sucking too much ampere's, or ... also read a scary story how a x1 probe can hurt your system ... well, I don't know.

    Thanks in advance, Maarten
     
    Last edited: Jul 19, 2013
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,271
    2,718
    Jan 21, 2010
    Hi muffinman, welcome to Electronics Point. (edit: 5 posts, not 1 -- well have another welcome anyway!)

    Presumably you mean that the TTL output uses RS232 serial protocol, but with TTL voltages?

    I don't know who "they" are, but if the device needs only convert voltage levels to get correct RS232 electrical specifications, then that seems like quite a bit.

    What do you mean by "all work fine"?

    Does it communicate correctly with some other RS232 device or does it jut look OK to you on the scope?

    Perhaps you can tell us what voltages you're seeing for mark and space. RS232 has minimum as well as maximum voltage specs. Are you "within" as in between these limits, or "within" as in below the maximum?

    The electrical specs are (in brief) logic "1" between -5 and -15V, Logic 0 between 5 and 15V, and the ability to drive a reasonable current (and I can't remember what the spec for this is). It should also not be damaged by shorting any pin to any other pin.

    The TTL inputs and outputs are not easily damaged if they have limited current capability, have common grounds, and are not exposed to voltages out of spec. Connecting an oscilloscope probe should not do any damage.

    Show me this story. I suspect it is where the grounds of the two systems are at different potentials. This can cause damage with any probe, it's not limited to x1 probes.

    I tend to use my x10 probes by default anyway. I see no reason to use x1 probes for logic.
     
  3. gorgon

    gorgon

    603
    24
    Jun 6, 2011
    The standard say -3V to -15V, and +3V to +15V, for '1' an '0'. (+3V to -3V) is not valid RS232 levels. Most drivers I've seen has a 300ohm resistor in series, to protect from short circuit. So from this the max current should be in 50mA range for +/-15V drivers.
    It is this resistor that limits the max lenght/speed of the RS232 Interface, due to the slew rate.
     
    Last edited: Jul 21, 2013
  4. muffinman

    muffinman

    17
    0
    Dec 10, 2012
    Hello all,

    Thanks for your replies. Reading back my initial post I realize I wasn't too clear. I'll try to correct that below.

    Kind regards, Maarten

    True, actually inverted TTL voltages. Idle is at 0v and 5v is logic zero (I think [edit] I think it is logic zero, that is [/edit]). Anyways, it is sortof compatible with rs232. Computers with an rs232 interface can communicate with the TTL device (stereo amplifier). However, to reduce the chance of damaging my precious amp, I've put an rs232/serial to inverted TTL converter in between my amp (ttl) and computer (rs232). This is set at idle 0v and 3.3v logic zero. I could have gone to 5.5v logic zero, but the manufacturer of my amp said it should not go past 5v.... Though 5.5v probably wouldn't matter much, but since 3.3v works, why go above what the manufacturer specifies.

    It has a bit more functionality, but none I'm interested in.

    Communication between these devices is good.

    I think above answers this one. Mark/iddle is at 0v and space is at 3.3v for the computer side and 5v at my amp. I only care about my amp though the 5v should be fine for the rs232-ttl converter, as it can run on both 5 and 3 volts...

    The voltages are within the specs, the devices are hooked up to the same power socket (so I suspect common ground?). As for current capability, I have no clue.

    at 2:20 and 3:20
     
    Last edited: Jul 22, 2013
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,271
    2,718
    Jan 21, 2010
    Good

    In that case I'd say you're good to go.

    OK, that tells you about effects the probe can have on the signal being measured.

    It doesn't say a 1x probe is going to hurt your system. (OK, it *might*, but then so might anything).

    What is more likely to happen is that a high impedance, high frequency part of your circuit will be loaded down and fail to oscillate, or amplify, etc.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,271
    2,718
    Jan 21, 2010
    Not completely sure, but I think you'll find that RS232 receivers should be capable of operating from +/- 3V signals, but the transmitter should provide a higher minimum voltage swing -- it's the same with digital logic.

    If you can point me to the actual RS232 technical specs I'd appreciate it. A casual googling didn't retrieve them. This is all from memory from 30 years ago...
     
  7. gorgon

    gorgon

    603
    24
    Jun 6, 2011
    You are right, you stated the voltage values for the transmitter, and I the voltages at the receiver end. I found this document referring the RS232-F standard. http://www.ti.com/lit/an/slla037a/slla037a.pdf
     
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