Talk:RS-232

Vulnerability to Electromagnetic Interference (EMI)

Based on what I've read in the book "USB Complete" by Jan Axelson, if anything, USB should be less vulnerable to EMI than RS-232. Beyond "low speed" operation, USB is required to have high quality shielding. RS-232 does not require RS-232 cables to be shielded at all (though many cables are shielded slightly, anyway). In all cases, USB devices must use differential signaling. That fact alone should make USB less prone to EMI, and it is in practice. RS-232 almost never uses differential signaling, and if it operated at a higher speed than it does, it almost certainly would corrupt data easily.

Does anyone else have something to say about this?

74.178.57.187 (talk) 23:12, 24 February 2018 (UTC)

It isn't really straightforward to compare the two except as complete system, checking feature one against the other doesn't really give any sense of how vulnerable the link is. USB was designed with economic factors very firmly in mind - it had to be cheap to implement - whereas this was much less of a concern for RS232 when it was designed. Therefore when features such as shielding are added which add appreciably to the cost ask why they are needed as opposed to simply counting them as a good thing.

Sure, USB works over a twisted pair and has screening, those are both things that help with noise immunity, although the screening is arguably less useful than it was now laptops (lacking a ground connection to anchor the screen to) are so prevalent. RS232 has neither baked into the spec. However compared to USB RS232 has much higher working voltages (needing a bigger disturbance to alter the interpretation), much greater currents (the more current the less interference can pull the voltage levels from where they should be), has much lower slew rates (less mutual interference between connections) and the lower speeds allow brief glitches to be discarded, both through active techniques such as majority voting and simple passive properties such as the capacitance of the receiver. USB needs the features you cite to get an acceptably reliable transmission at the levels, currents and speeds that it operates: it doesn't make it "better" than RS232 in this regard, that is simply needed for USB to work at all: the higher speeds tend to make the error margins much tighter.

Yes, you do notice the occasional error on an RS232 link, mostly on longer links that have more scope to pick up interference. You can't really hold that against it compared to USB when USB would be unable to span such distances in the first place. Even with the twisted pair and the shielding in place the standards acknowledge that errors are still going to be detected - checksumming and retransmission are an integral part of the standard and have been since day one: if the immunity was that good this complexity wouldn't be needed. 3142 (talk) 11:51, 19 February 2019 (UTC)

Common ground

Since my edit yesterday was reverted maybe we need to be a little more explicit. From the article:

Use of a common ground limits RS-232 to applications with relatively short cables. If the two devices are far enough apart or on separate power systems, the local ground connections at either end of the cable will have differing voltages; this difference will reduce the noise margin of the signals. Balanced, differential serial connections such as RS-422, RS-485, and USB can tolerate larger ground voltage differences because of the differential signaling.

Using USB as an example here is completely misleading. Sure the signal is transmitted on a differential pair, but the reference is to the bus as a whole otherwise the comparison is meaningless. USB (prior to version 3 for simplicity, although the same issues hold there too) consists of four wires - the differential pair, +5V, and, erm... a common ground. It is also limited in terms of the length to a quite hard 5m (~16ft) by propagation delays. RS232 has no specified length limit - the preamble to the standard mentions 50ft as a target but 1000ft+ is easy enough in practice.

So, we are comparing the two and saying USB is superior because it lacks something that it has, and this missing thing that is present makes it better because it is worse at the very metric we are using as justification.

No, that is not a valid example. 3142 (talk) 20:22, 18 February 2019 (UTC)

But the USB bits aren't "read" by reference to the common ground. As in any good differential scheme, they are compared against each other, so ground differences at the end don't matter. RS-422 and -485 have a "ground" pin in their connectors too, you know. Jeh (talk) 12:48, 19 February 2019 (UTC)

Once the link is up and running and transferring real data as opposed to bus enumeration metadata. The bus as a whole is dependent on a common ground, as is the initial connection even on the differential pair. Since I discussed USB2.0 in the first instance we'll stick with that, since it's still current and keeps things compared to USB3.0. Read through section 7.1 of the specification and take a good look at figure 7.1, there are a couple of other places this is covered but will stick with there. You see a pair of pull down resistors to ground terminating the pair, fine, that's standard differential stuff. However we also have a switchable pull-up resistor on one of the lines used for signalling the device presence and ultimately triggering the enumeration process. We also have the single-ended receivers necessary to detect those unbalanced conditions. The text also makes this explicit:

This state is achieved by using the low-/full-speed driver to assert a single ended zero...

transceiver activates an internal current source which is derived from its positive supply voltage and directs this current into one of the two data lines...

The link isn't balanced in those conditions, the common ground is necessary. Without it the link can't be established and no data can transfer. It is absolutely essential to the protocol even without the requirement to supply power.

I would suggest referring to the RS-422 and RS-485 standards. Neither specifies a common ground. One is often included for a few possible reasons - I've seen shielding, power supply and unbalanced handshaking signals all done on the same connector as both of them, but the standards make no mention of one. Both are fairly barebone standards (e.g. lack of any pinouts or even connectors) so a practical implementation has to build on them by necessity.

Ultimately though this is getting far more technical than it needs. Remember, the article makes two assertions by citing USB as an inappropriate example:

• USB does not use a common ground.
• USB can span greater distances than RS232.

Neither assertion is true. How can it be a valid example? Yes, it is a shit one and there is nothing wrong with calling it as such. Refer to WP:CIVIL, that is not a personal attack but simply calling a spade a spade. 3142 (talk) 15:06, 19 February 2019 (UTC)

Extra DB-25 Pins

The standard defines a whole heap of extra connections on DB-25. These obviously cannot be used on a DE-9 connector.

Perhaps a description of when these extra DB-25 connections are used and what sort of systems use them?

See: pinouts.ru/SerialPorts/RS232_pinout.shtml for a description of the extra connections.

Also, if somebody knows why the DE-9 connector has never been added to the recent revisions (revision F as of writing this), please add it to the text.

Chris Fletcher (talk) 14:24, 10 March 2019 (UTC)

Add the 9-pin connector here?

(This next comment was originally a comment to the preceding section. Since this is a different discussion - "details of 9-pin connector should be added to this article" rather than "why aren't the DE-9 connector ever added to the TIA-232 standard", I have split the next comment and all that follow into a new section. Jeh (talk) 21:11, 14 March 2019 (UTC) )

I don't know why, but this article should list 25 and 9 pin in the same table, because 9 is the most common in this world. Quote from this article - "TIA-574 (standardizes the 9-pin D-subminiature connector pinout for use with EIA-232 electrical signalling, as originated on the IBM PC/AT)". • Sbmeirow • Talk • 14:50, 10 March 2019 (UTC)

Re the DE-9 connector: this has been proposed before, and the arguments for exclusion still seem valid. Articles should be about their subjects. Do you see the article title? Does it say TIA-574? No. The 9-pin connector is described in the Serial port article, to which this article links. Jeh (talk) 17:39, 10 March 2019 (UTC)

It doesn't matter how many times it's been proposed, because any topic can be rediscussed on Wikipedia. There is nothing wrong about putting a comparison column (DE-9 (TIA-574)) against another standard, unless you can quote a specific rule against it. Knock off the snarky comments about the title. • Sbmeirow • Talk • 02:15, 12 March 2019 (UTC)

Yes, any topic can be rediscussed, but it seems pointless if someone doesn't have a new argument to bring up. It's not snark to point out that articles should be about their subject. The subject here, as dictated by the article title, is very narrowly specified. Discussion of topics not covered in the RS-232 standard are off topic. I believe the term "subject creep" or "topic creep" came up the last time. It is fine to mention that other, related standards and topics exist - particularly in the "See also" section - but adding the comparison column you suggest is a bridge too far. It's too much detail about a not-topic.

Can you explain why the coverage in the Serial port article not sufficient?

Alternatively - Heck, why don't we add the corresponding pinouts for RS-422, RS-423, EIA-530, etc.? They're all related. Jeh (talk) 04:12, 12 March 2019 (UTC)

RS232 via "DB9" connector is far more popular than all of those others, including the "DB25" too. • Sbmeirow • Talk • 14:36, 13 March 2019 (UTC)

Popularity is not the point. Precision in article topics is the point. WP:COMMONNAME pertains to article titles (it's why this article is still titled "RS-232" instead of "TIA-232-F"), but not to content. The Serial port article covers the information about the DE-9 connector. Why do you think it should be duplicated here? (Note: A "DB-9" would be the size of a DB-25 but populated with only 9 pins.) Jeh (talk) 14:58, 13 March 2019 (UTC)

1) Popularity is the point, per my comments above. 2) Numerous articles have been merged on Wikipedia, making the smaller article a section within a related larger article, thus making this point moot. 3) TIA-232-F redirects to this article, making your point moot. 4) I didn't say duplicate Serial port article here, I said "add a column". Many topics are compared on Wikipedia, making this point moot. Just because something is stated in one article doesn't automatically mean it must be excluded from another article. 5) I put quotes around the common name "DB9", though you purposely overlooked the quotes. (Note: I already knew about DE-9). • Sbmeirow • Talk • 05:16, 14 March 2019 (UTC)

5. "Purposely overlooked"? No, I inadvertently ignored your quote marks. I thought your use of "DB-9" was a mistake rather than deliberate. But it's an error either way. If you were making a point, it was too subtle for me to see.

4. Adding a column describing the pinout of the 9-pin connector would be significant coverage. Yes, many things are compared on Wikipedia, in articles with titles appropriate for such coverage. For example, the Serial port article includes a table that compares the pinouts of many connectors that have been used to carry RS-232 and similar signals for serial ports. That is appropriate for an article called "Serial port". It is not appropriate for an article called "RS-232" unless the standard is modified to include significant coverage of that connector.

"Just because something is stated in one article doesn't automatically mean it must be excluded from another article." No, it doesn't. But it does mean that the material in question already has an appropriate place on Wikipedia. And if the material is not within the second article's topic area, then copying the material to the second article is not appropriate. The topic here is the material in the standard originally known as RS-232.

3. I don't think you know what "moot" means. A "moot point" is one that is valid, but inapplicable to the argument at hand. For example, if you said "2 plus 2 equals 4" and I replied with "yes, but 2 plus 1 equals 3", as if that was a counter to your claim, then I would be making a moot point: 2 plus 1 does equal 3, but that fact doesn't refute your claim. As to this case, if anything is moot it's your observation that the TIA-232-F redirect exists. The existence of the redirect does not mean that WP:COMMONNAME is not being followed. WP:COMMONNAME is about titles of articles, not titles of redirects. WP:COMMONNAME even suggests that redirects be created for alternative names.

2. Yes, article merges happen. If the result is an article with a broader subject, then the article title is almost always changed to reflect that shift. Are you proposing a merge? From what article? - noting that there is no article for the TIA-574 standard. But there is no need, because the broader-topic article you're looking for already exists. It's called Serial port.

1. Popularity of the 9-pin connector is apparently your point, but it is irrelevant. The article topic here is not "serial ports that are pretty much compliant with RS-232, or at least similar in some ways". The topic here is the RS-232 standard. That standard is described in a document. That document does not mention the 9-pin connector. Therefore a serial port with a 9-pin connector is not compliant with RS-232, and this article should not give significant coverage to such ports. But! Nobody is disputing that the 9-pin connector is popular. Accordingly, it is covered in the Serial port article. Which is pointed to by a hatnote here: "For RS-232 variants, including the common 9-pin connector, see serial port." You still have not described why you consider that coverage insufficient. Jeh (talk) 19:12, 14 March 2019 (UTC)