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!
Contents
- TIA (RS) Complete Standards
- TIA (RS) Electrical Standards
- 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
RS-423-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
RS-423-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. |
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
Signal |
Max
Signal |
Out
Imp |
In
Imp |
Comments |
| 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. |
Notes:
- RS-485 is frequently used as a simple three wire system in many half-duplex, multi-dropped environments with TX and RX being alternately switched onto the same two pins - it does not matter whether they are the TX or RX pair (as long as both ends are the same, obviously), the third pin carries a GND.
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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