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Tech Stuff - RS 232 & ITU Alphabet Soup

This section describes the various RS-xxx (more properly TIA-xxx) standards and some of the V.xx standards from our good friends at the ITU (did we really say that - they are the guys who still charge for specs!).

A partial apology: The ITU, bless their hearts, now have a policy that you can get two docs for free when you sign up. It's a start but why don't they do like the IEEE 802 guys. Free but only after 6 months. If you are desperate you pay, if not you wait.

Note: Until February 11th, 2011 these specs referenced a EIA/TIA number (sometimes even a TIA/EIA number). EIA is no more. TIA has taken over its functions (for the Telecomms sector). Thus all specs should reference TIA, for example, TIA-232 etc.. Since this is the third change in recent memory (RS -> EIA/TIA -> TIA) it is probably bettor to stick with the RS terminology. Technically it is incorrect but it is stable!


  1. TIA (RS) Complete Standards
  2. TIA (RS) Electrical Standards
  3. ITI V.XX Standards

TIA Standards (a.k.a. RS)

EIA/TIA defines Complete and Electrical standards.

Complete Standards

Complete standards may define the electrical characteristics or they may call up one or more Electrical standards. They always define a connector since they are supposed to be complete.

Name Electrical Rev Connector Description
RS 232 RS-232 E DB25 Unbalanced point-to-point serial interface on a DB25 connector. Max. official data rate 20Kbps (really).
RS 449 RS-422-B
- DB37 Balanced multi-drop serial interface on a DB37 connector. Max data rate 2Mbps. Not quite the biggest connector in the world - that goes to the old ITU V.35 connector but... Sensible people use 530-A instead.
RS 530 RS-422-B
A DB25 Covers Balanced (RS-422 and RS-485), multi-drop (RS-485) and unbalanced (RS-423) serial interface on a DB25 connector. Max data rate 2.1Mbps.
RS 561 RS-562 - RJ45 (MJ8) Unbalanced point-to-point serial interface on a RJ45 (Modular Connector 8 positions). Max. data rate 38.4Kbps.
RS 574 RS-232 - DB9 Unbalanced point-to-point serial interface on a DB9 connector. Max. data rate 20Kbps (really).
RS 613 RS-612 - ? Balanced point-to-multipoint serial interface on a ? connector. Max data rate 52Mbps.

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EIA/TIA Electrical standards

These are Electrical standards and are not Complete that is, they do not define signal names. They are typically referenced in a complete standard. Table below highlight the major characteristics.

Name Min
RS-232 +- 3V +- 25V 300 3K - 7K Unbalanced point-to-point.
RS-562 +- 3V +- 25V 300 3K - 7K Unbalanced point-to-point.
RS-422-B +- 7V +- 12V 100 4K Multi-drop balanced driver and receiver. Max. of 1200 meters all terminals, max of 500 meters between any two terminals.
RS-423-B +- 7V - - - Multi-drop unbalanced driver and balanced receiver
RS-485 +- 7V +- 12V - - Multi-point balanced driver and receiver. Max of 1200 meters all terminals, max of 500 meters between any two terminals. Up to 32 stations/terminals assuming 12K load. Lower load allows more terminals.
RS-612 -0.5V +2V - - Balanced driver and receiver. Max differential <= 1.5V.


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ITU V.XX Standards

The ITU, the 500 pound gorilla of the standards world (and the guys who still charge for specs).

Name Electrical Description
V.24 V.28 Defines the signal names used in serial interfaces. Connector defined in ISO 2110 and ISO 4902. Practically the same as RS-232
V.28 - unbalanced point-to-point serial interface. Practically the same as RS-232.
V.35 - Term still widely used but standard withdrawn years ago and replaced with V.10 and V.11. Still holds the record for the biggest serial connector known to mankind - the infamous CCITT 37 pin connector. Mostly uses a DB25 connector today but not every manufacturer uses the same wiring standard.
V.10 - unbalanced driver and balanced receiver point-to-point serial interface (a.k.a RS-423-B).
V.11 - Balanced driver and receiver point-to-point serial interface (a.k.a. RS-422-B).

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