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TTL and True RS-232 Serial – What’s the Difference?

Posted January 21, 2016

CELE038The differences between a TTL RS232 and a True RS232 interface purely have to do with data being sent, not with powering the barcode scanner. The data is being sent as an electrical block-signal representing a sequence of logical zero’s and ones.

  • For a TTL device a zero would ideally be 0 volt, and a 1 one would ideally be 5 volt. The receiving device therefore has to decide whether a the signal at a given time is meant to be 0 or 5 volt, in order to tell if it is looking at a binary zero or a binary one.
  • For a True RS232 device the zero is represented by ideally -12Volt and a one is represented by ideally + 12 Volt. Here the receiving device has it a bit easier because the difference between -12 and + 12 = 24 Volt. (specification allows -5 to -15V and +5 to +15V)

In practice we see that a signal gets weaker the longer it has to travel and can sometimes drop 1 volt a meter going via cable from device to host. With a True RS232 device both positive and negative signal can drop 10 volts and still have 4 volts difference in polarity left to tell zero’s from ones. The TTL interface has already less polarity difference if the signal drops just 2 volts.

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