Talk:RS-232/Archive 3

Connector naming
(This seems to be a separate discusion so I have taken the liberty of providing a new section for it - I B Wright (talk))

"RJ45" is possibly the second most abused term after "DB 9" - I've seen thousands of 8-pin modular connectors in my life, but I don't think I've ever seen a real honest-to-Bell RJ 45. Is it just the computer biz that mushes language to pulp, or do other industries and fields have wide-spread abuse of what should be perfectly clear unambiguous terms? --Wtshymanski (talk) 20:21, 5 April 2013 (UTC)
 * We build a WAN fibre communication network using thousands of RJ-45 connectors, as labeled on the packages by the connector manufacturers. Also appearing in the same style was the RJ-31 but the sockets had a set of auto circuit bypass contacts built for shared voice circuits downstream. 174.118.142.187 (talk) 03:19, 6 April 2013 (UTC)
 * Well, see, there you go. A connector that is not wired to anything cannot correctly be described as "RJ-45". RJ-45 is a usage and wiring standard (single circuit POTS with a programming resistor) that happens to use 8P8C connectors. Jeh (talk) 04:13, 6 April 2013 (UTC)

Ignorance is not bliss, and certainly not a good foundation for editing WP
A lot of useless but well-intentioned edits and reverts and arguments on this article could have been avoided if people actually had a copy of the standard the article is describing. I realize that it costs $160 for all of about 40 pages; this is not what most volunteer editors are going to be doing with their spare cash. But, really. If you don't have a copy you can refer to it'd be better if you asked here first rather than just assuming that what you read in "Baby's First Big Book of PC Interconnections" was correct and plopping it into the article. Jeh (talk) 19:45, 5 April 2013 (UTC)
 * Ignorance of the subject material is one of the Five Pillars of the Wikipedia! This is one of the few articles about a technical standard that actually says something useful about the standard without being a copy of the table of contents. Few technical standards are notable in the Wikipedia context. I used to have a handbook of all the different 9-pin connectors that were used with serial ports; seems Allen Bradley and IBM never agreed on this and you could never go to Staples and buy a cable that would go from your AT-compatible to an AB PLC or Data Highway interface. --Wtshymanski (talk) 20:12, 5 April 2013 (UTC)

Voltage levels
Could you add to this section the RXD and TXD voltage levels, when the line is idle? Apologies if it is stated somewhere and I missed it.


 * In async serial (which is what is used on e.g. the common PC serial port) the idle condition is a continuous "marking" (as opposed to "spacing") level, or bit value 1. And the start of a character frame is defined by the appearance of a "start" bit, which is "spacing", bit value 0. RS-232 doesn't actually define any of the previous (it leaves the usage of the signals to other standards), which is why it isn't mentioned here. But RS-232 does say that on RxD and TxD, bit value 1, or "marking", is the negative voltage range. So (connecting the dots) the idle condition corresponds to the negative voltage range. Jeh (talk) 01:34, 11 July 2013 (UTC)

rx / tx pins wrong?
It seems to me from a DTE point of view, that TX should be pin 3 and RX pin 2. The table shows it the other way round. — Preceding unsigned comment added by Mattvenn (talk • contribs) 17:04, 5 August 2013 (UTC)
 * No, the table is correct. However, please note that the pin numbers are for the DB-25 connector, not the DE-9 connector, which is more common today, but not part of the RS-232 standard (therefore not listed in the article). They have pins 2 and 3 swapped. --Matthiaspaul (talk) 02:38, 6 August 2013 (UTC)

Incomprehensible caption
The oscilloscope capture image was captioned with the following "...Same signal was input into an RS-232 to UART converter..." I have a degree in computer engineering and deal with UARTS and RS 232 every day and have never heard of a "UART converter". By definition UARTs are universal, therefore there can be no such thing as a UART converter. CombatWombat42 (talk) 23:05, 24 June 2013 (UTC)


 * To repeat the comment you've already ignored twice:
 * Look up what a UART does and it might become clearer. It's not just voltage shifted, it's not even just cleaned by a Schmitt trigger (which would remove the ringing), the timing has also been cleaned up. This is no longer an analogue signal that has been cleaned up, it's now a regenerated signal: the analogue signal is converted to a digital word (not just serial digital levels, a data word) and this word is then used to regenerate a serial analogue signal. Andy Dingley (talk) 22:57, 24 June 2013 (UTC)
 * Andy, there is no UART involved here. The upper picture shows the signal as it goes over the line, and the lower picture shows it after it has been received and fed through a RS-232 level shifter like a MAX232 (just to name one). The output of the level shifter has a TTL or CMOS compatible voltage level and typically (but that depends on the specific chip) reversed logic. This is the signal that is fed into the UART (a separate chip or a block in a microcontroller). The receiver in the UART samples the incoming signal (and thereby also debounces and dejitters it) in its process to convert it into some parallel presentation in a number of registers. The level shifter will do no such things, it's just a line driver and level converter.
 * On a different note, I don't think this picture is particularly helpful at all. Instead of offering various arbitrary magnified views in difficult to view colors, it should just display four plain B&W views without much clutter: On transmitter side: 1) Signal TOUT coming from sending UART and going into RS-232 line driver. 2) Signal TxD coming from RS-232 line driver/level shifter. On receiver side (with long cable): 3) Signal RxD before level shifter (same as 2) but possibly distorted). 4) Signal RIN after level shifter and before receiving UART.
 * --Matthiaspaul (talk) 09:57, 25 June 2013 (UTC)
 * It's hard to tell what the image is adding to the article, certainly. As I look at it though, the timing of the clean trace appears to have been cleaned up though, not just the voltages. That requires a UART(-like) behaviour, not just a MAX-232. Andy Dingley (talk) 10:25, 25 June 2013 (UTC)
 * The image is misleading without careful scrutiny, the two traces appear to have been taken from different screens and a zoom has been used. In the top trace, the trigger is the leading edge of the first bit but in the lower it is several character lengths earlier. The zoom is offset and as a result the 'cleaned' version appears to arrive before the analogue. The digital version doesn't seem to serve any purpose since it is not showing the voltages in the standard, the analogue does show typical edge glitches and voltage drift so may be useful. What would improve it would be annotation of where the start, data and stop bits. The apparent improvement of timing (reduced jitter) may be only the time resolution of the scope which is shown as 4us. George Dishman (talk) 09:30, 28 January 2014 (UTC)


 * Andy, Where have I ignore'd it twice? I put a complaint on the image talk page, but that was a separate complaint about the image itself. Discussions about this page should be had here on the talk page for this article. I also don't see the second time you've posted that comment. I would have accepted a change to "signal conditioner" but just reverting my change was both lazy and wrong, instead of reverting to factually incorrect statements next time, improve it, don't just revert.. CombatWombat42 (talk) 14:16, 25 June 2013 (UTC)

Citation needed
There's been a minor revert skirmish going on regarding the citation tag for why RS-232 has died out on the desktop. While I agree with User:Andy Dingley that it is indeed self-evident that USB has replaced RS-232, I also think it's fair for somebody to ask for a citation. Citing reliable, authoritative sources is one of the WP:FIVEPILLARS we work under. To blow off a citation request with it is self-evident gets us perilously close to WP:OR: I don't have to prove this because everybody knows it's true.

In addition, there are a number of things being asserted in that paragraph. Not just that USB has pushed RS-232 off the desktop, but a specific list of reasons why. Those are worth validating. Moving somewhat further away from the original topic, I also think a discussion of the RS-232 -> USB evolution should also include mention of other technologies that were tried along the way, such as Apple Desktop Bus.

-- RoySmith (talk) 13:47, 4 February 2014 (UTC)


 * Hi everyone, I'm the editor who first added the citation tag yesterday. User:RoySmith echoed my thoughts exactly. While I think that User:RoySmith's justification is sufficient on its own, I should also divulge that I added the citation tag because after reading why RS-232 has disappeared, I simply wanted another link to follow. In essence, I want us to find a reference that substantiates the claims we make on the wiki page, and also provides an avenue for the interested in order to continue researching.


 * -- Mmpozulp (talk) 16:38, 4 February 2014 (UTC)
 * Drs. Horowitz and Hill discuss this even in the 2nd edition and I imagine the lnog awaited 3rd ed. of "Art of Electronics" will describe it in detail.  But by all means let's leave the useless tag in there because someone is bound to document it some day. --Wtshymanski (talk) 22:06, 4 February 2014 (UTC)
 * A (completely cited) discussion of the evoloution of computer serial ports is off-topic for this article, whcih is about one standard. --Wtshymanski (talk) 22:07, 4 February 2014 (UTC)

The main problem here is that there seems to be a disparrity between what is being challenged and what some users think is being challenged and possible even reality! That the RS-232 is not found on modern PCs but that USB is is not in doubt and such a statement would not require a citation (See WP:BLUE). Even stated in his edit summary that: the disappearance of the RS-232 is self evident - I don't think any of us can argue with that. But that is not what the section of the article says. It states that the slow speed, (relatively) high voltages and size of connector of RS-232 motivated the development of USB. As a claim, that requires a citation. But one will not be found because it is not true. It is true that those characteristics motivated the development of faster interfaces which eventually resulted in the USB, but the developmental route was not a direct one.

The motivation behind the development of USB was to develop a much lower cost version of the (then) up and coming Firewire (which became IEEE-1394) serial interface. Firewire had (and still has) a few problems. It is limited to just 64 devices on any bus (including any host port); only one of the (then) 3 available speeds can be used on any bus system; the communicating speeds were considered excessive for most real applications of the time and (the biggest motivator) the licensing cost of producing port chips was considered excessive (at well over 50 USD per chip set) - this last part is no longer as true as it used to be. USB was developed as a low cost version of Firewire that addressed all of the above points to a greater or lesser extent.

USB was never developed as a replacement for RS-232. Indeed, for a surprisingly long time, the two co-existed on computer motherboards side by side. Few owners of one of the original Pentium series based PCs would have been aware that there was a fully functional USB port on the motherboard along with the RS-232 ports. It was by now a standard port on the Northbridge chips, but at this stage, the header on the motherboard was seldom (if ever) connected to a physical port on the outside of the box. This was not an issue because the version of Windows that shipped with these machines (usually 3.1 or 3.11) did not have any support for USB. It was the Pentium II motherboards that first had an externally accesible USB port, and most now had a root hub to make 2 accesible ports (and they still had a pair of RS-232 ports). Windows 95 was the first version of Windows that provided support for USB - for one device only and then only if you could find a W95 driver.

The Pentium III motherboards were the first to feature fully useable USB ports as they were fully supported by Windows 98. Those RS-232 ports were still there though but mainly because there were still hardly any peripheral devices equipped with USB ports. There was a surge of peripheral devices once operating system support was available and Pentium 4 based motherboards finally dropped RS-232 ports (though they still could be added as expansion cards or even using RS-232 to USB converters - the later still an option today). These last motherboards started to implement the high speed USB standard.

Having said all of that: is correct that a history of evolution of serial communication is off topic for this article, but a brief discussion of the demise of the RS-232 port and the reasons behind it is quite acceptable. DieSwartzPunkt (talk) 12:48, 5 February 2014 (UTC)
 * Just to make it quite plain here, I removed your tag solely because it was a tag that Wtshymanski had first removed and I saw your action (for an article where you had no recent edits) as being more about Wtshymanski and less about RS-232.
 * Now, as is very well known, my idea of a fun day out with the kids is dancing around the flayed corpse of Wtshymanski as some sort of grisly wiki-maypole, but let's reserve that sort of fun for when he actually deserves it. Neither RS-232 no longer being around on the desktop, nor cos phi>1, are justifiable reasons for kicking off yet another edit war with the same parties. Andy Dingley (talk) 13:12, 5 February 2014 (UTC)


 * Nope. I restored the tag because the statement that it was requesting a citation for was patently false given how long the USB and RS-232 ports survived along side each other.  And in case you missed it: I actually agreed with  in my last paragraph above.  It does happen from time to time.  DieSwartzPunkt (talk) 13:32, 5 February 2014 (UTC)

Nominated for WP:LAME.  CombatWombat42 (talk) 15:29, 5 February 2014 (UTC)


 * I just checked again and no you have not. What is so lame about challenging a statement that just is not true?  You may also want to find out what edit warring is before throwing mallicious allegations around.  Since I only made 1 (one) edit reverting the tag, it doesn't remotely qualify as edit warring.  (My second edit where I merely added the reason to an already existing {cn} tag is not edit warring either).  DieSwartzPunkt (talk) 17:03, 5 February 2014 (UTC)


 * As no actual nomination has been made, it is hard to understand why you regard this as lame. What has been posted above seems a reasonable argument (admittedly very long - but reasonable).  I have no idea if the USB port was developed to replace the RS-232, but now it has been mentioned, the two certainly coexisted for some time so it is a logical conclusion (in fact my older laptop has two of each).  Now we just need that all important reference to settle this either way.  212.183.128.111 (talk) 17:44, 5 February 2014 (UTC)
 * Here is one: . It makes no mention of a design goal to replace the RS-232 port, but says that, "[The USB] was originally developed in 1995, to minimize the number of ports in the back of the PC." and, "The major goal of USB was to define an external expansion bus which makes adding peripherals to a PC low cost and as easy as hooking up a telephone to a wall-jack" (my emphasis). It also says that seven industry leading companies were "Frustrated by Apples (sic) royalty fees on firewire devices" so at least that bit was right (I might have guessed it was Apple).  On this basis I propose deletion of the offending claim or at least a reword to bring it line with reality.


 * How about something along the lines of, "The difficulty of configuring an RS-232 interface motivated development of faster serial interfaces that were easy to use and were also able to interface with more than one peripheral device on the same port. This culminated in the development of the USB port which slowly displaced the RS-232 port from personal computers."  [Cite the above for the less obvious bits.  No one seems to be challenging that the RS-232 was displaced, but if anyone wants to be pedantic, I'm certain a citation exists.].  DieSwartzPunkt (talk) 18:29, 5 February 2014 (UTC)


 * I think you have established your argument. Since you are deleting a contentious claim (unsupported by cite and not mentioned in your cite) and substituting material with more citable provenance, I would say go for it.  It has to be an improvement.  Another editor has already conceded that the fact that the RS-232 port has been displaced is established enough not to require a cite, but you never know, there are pedants around.  212.183.128.111 (talk) 18:47, 5 February 2014 (UTC)


 * I was mostly making a sarcastic comment so people would take a step back and really look at what you are arguing about. Take a fresh look at it. Certanly as you say " What has been posted above seems a reasonable argument" and it the original statment does deserve citation. Is it really worth caring about whether USB was a direct descendant of RS-232 or more of a generic evolution of pheriferal busses? It certanly dosn't matter to me so I consider arguing about it in any way WP:LAME. CombatWombat42 (talk) 18:32, 5 February 2014 (UTC)

Real world RS-232 image.
I added back the RS-232 scope capture back into the image. I consider it important because it show the real world effect of thinks like noise, rise time and ringing. As far as I remember RS-232 does not define voltage or timing characteristics so this image could be considered misleading, but I consider it a valuble addition to the article. CombatWombat42 (talk) 15:12, 5 February 2014 (UTC)
 * OK fair comment, I only picked the one I left because it was simpler, but no matter. I have therefore deleted the other image, because they are dual redundant and it shows nothing of substance not contained in the other.  DieSwartzPunkt (talk) 16:50, 5 February 2014 (UTC)
 * I apologies for my dual revert, but I truly beleive providing two views of the waveform is important. Having the ability to side by side compare the optimal to the actual is what I was asking for, I wasn't particularly clear on that. If you feel strongly the other way please revert one of my reverts, but when I was learning electronics it was very important to have them side by side. CombatWombat42 (talk) 16:53, 5 February 2014 (UTC)
 * OK, I take your second point. I don't quite understand the point about a software engineer needing the first image.  A software engineer will just write the code for K to the appropriate register of the UART, and if anything else comes out, it is not his problem.  I do, however, still feel that the two illustrations are just showing the waveform (ideal or real) that emerges when the letter 'K' is sent over an RS-232 line.  Let's see if anyone else has any views.  DieSwartzPunkt (talk) 17:00, 5 February 2014 (UTC)


 * I don't think both views are necessary, and if we must have one of them, I would prefer to see the idealized one here. It's certainly true that when you're learning how to build or use interface chips you need to be aware that the signals on the real hardware won't be idealized, but that isn't the subject of this article; that's more in the signal processing realm, transmission lines, etc. For that matter, the only thing in either picture that's really covered by the TIA-232-F standard would be the voltage levels and (for the scope photo) the rise times. (The diagram is wrong when it comes to rise times; it shows them as very slow.) The standard does not and has never addressed start bits, stop bits, bit timing, etc. Maybe the diagram belongs better in the Asynchronous serial communication article? As for the scope photo, it's bordering on wp:OR. Jeh (talk) 20:28, 5 February 2014 (UTC)