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The thing I’ve gotten most confused about historically is whether RS232 is voltage levels or a protocol
by klysm 2y ago
The thing I’ve gotten most confused about historically is whether RS232 is voltage levels or a protocol
- dgfitz 2y agohttps://www.optcore.net/difference-between-rs-232-rs-422-and-rs-485/#:~:text=What%20is%20the%20difference%20between,the%20single%2Dended%20transmission%20mode https://www.optcore.net/difference-between-rs-232-rs-422-and.... Serial is usually ascii, not always but usually. The different types of serial depend on things like duplexity and line noise, or lack thereof. 232 is -15v to 15v, 422 is -6 to 6, and 485 is -7 to 12
- YZF 2y agoRS422 and RS485 use the same differential signalling IIRC. The difference between two lines (>some mV <some mV) is what encodes zero or one. There's some allowed voltage range defined from ground but it's not that level that encodes the signal, it's the difference. Running a differential signal over a twisted pair reduces the noise. In RS232, to contrast, the voltage defines the values, +/- 12V is also common and I think you can go pretty low while still meeting the standard. ASCII is orthogonal, you can send anything over the serial link. EDIT: apparently 3V-15V are the defined voltage levels (-3V - -15V) and drivers should tolerate up to +/-25V.
- Gibbon1 2y agoBig difference between RS422 and RS485 is the drivers for RS485 are required to survive a minus 7 volt difference between the two grounds. Old R422 drivers would smoke if you got more than 7 tenths of a volt difference. Which would happen if you had a open ground. Or lighting hit. Or some big motor turned on. We had an RS422 based system with 50 units on one pair. One of them was holding the line. Coworker unplugged each one till he found it. And half of them didn't work after that.
- epcoa 2y agoFor years the most common use for “serial” would be dialup internet and computer mice (among other peripherals), neither of which would have been primarily ASCII. Not sure what you have in mind by “usually” ASCII, but it’s not the layer that serial links operate at, they’re just a dumb pipe.
- terinjokes 2y agoRS-232 defines the levels, the different signals, and the physical connectors. However, baud rate, parity, stop bits, and the like are at a "level" above RS-232, and so outside its scope.
- jcrawfordor 2y agoWe also have to bear in mind that the computer industry often uses "RS-232" in reference to something that uses neither the connector nor the signaling levels defined by RS-232, something we have IBM to thank for.
- metaphor 2y ago> ...something we have IBM to thank for. Egregious rent seeking grifters at TIA demanding $165 per half-assed scanned copy[1] of a near 3-decade-old document that they couldn't even be bothered to properly OCR, and somehow IBM gets to shoulder the blame for the interoperability clusterfuck that ensued as everyone and their mother was profiting off their innovation? Oh, come off it. [1] https://store.accuristech.com/tia/standards/tia-tia-232-f?product_id=2594289 https://store.accuristech.com/tia/standards/tia-tia-232-f?pr...
- mschuster91 2y agoYet another fine example showing why standards should be at least readable to everyone for free, eh? The general public at large already pays huge amounts of money to various standardization organizations, be it ISO, IEC, DIN, UL, whatever. The resulting works should be public domain, similar to patents.
- cwillu 2y agoAlso number of bits per byte.
- wrs 2y agoDoes that framing even have a real official name? It’s usually handwaved around as “async serial” or “UART”. And remember, with whatever it’s called, in addition to sending characters of various bit lengths, you can send a “break”.
- YZF 2y agoThe standard defines the electrical characteristics (including voltage levels) and things like connectors and pinouts.