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What are the differences between TTL and a True RS232 serial interface?

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Update time : 2015-07-22 11:56:34
Question or Problem as reported by the customer
I do not know which device to select TTL or True RS232.
Can I use both TTL and True RS232 devices / what is the difference?
This knowledge article applies to the following scenario
  • RS232 TTL, True RS232

The answer / solution to the problem / question documented in this article.
RS232 is a standard protocol used for serial communication, it is used for connecting computer and its peripheral devices to allow serial data exchange between them.
RS232 more extreme voltages help make it less susceptible to noise, interference and degradation. The minimum and maximum voltages of a true RS232 signals are +/- 12/13V., while TTL signals are 0 to 3,3V/5V.
The differences between a TTL RS232 (also called RS232-C) and a True RS232 interface is only related to the signal level to GND (Ground) and not with the current to power the scanner.
  • Hardware working with a TTL level, zero would be ideally 0 volt, and a  logical 1 at 5 volts. The receiving device therefore has to decide whether 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 0 or a binary 1.
  • For a True RS232 device the 0 is represented by ideally -12 volts and a 1 is represented +12 volts. This provides higher data security as the data signal difference is from -12v to +12v is 24 volts (specification allows -5 to -15V and +5 to +15V).
The standard does not define a maximum cable length, but instead defines the maximum capacitance that a compliant drive circuit must tolerate. A widely used rule of thumb indicates that cables more than 15 m (50 ft) long will have too much capacitance, unless special cables are used. By using low-capacitance cables, communication can be maintained over larger distances up to about 300 m (1,000 ft).[12] For longer distances, other signal standards, such as RS-422, are better suited for higher speeds.
Honeywell does support only the RS232 TTL level by default for its Barcode Scanners.
Honeywell provides for its Barcode Scanner RS232 cables (CBL-020-500-C00) up to 5m as a straight or coiled cable. To support the True RS232-C interface a 'True RS232' (CBL-120-300-C00, 3m) cable has to be used. The data conversation is done in a hardware dongle implemented in the cable.

Although the RS232 standard defines RS232 to be a True RS232 interface, and TTL is actually not compliant with the RS232 standard, in practice the majority of the RS232 serial ports we connect scanners to are TTL ports. See more information on the differences between TTL and True RS232 in File 1.