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Comment by ryandrake | Hacker News Reader

Nice real world example of figuring out an unknown interface. There are plenty of different electrical interfaces for transmitting serial data, and they often use the same physical connector, so you definitely have to know what the device is outputting. For reverse-engineering an unknown device, you often will need a scope. If you're lucky, you'll be facing an interface that is documented, like RS-232, RS-422, or RS-485. If you're unlucky, you need to rely on experience, knowledge and familiarity like OP did. Only once you have the electrical signal figured out, you can move on to baud, word length, stop bits, parity, and so on, and then decoding the actual data.

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The thing I’ve gotten most confused about historically is whether RS232 is voltage levels or a protocol

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

RS422 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.

Big 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.

For 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.

RS-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.

We 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.

> ...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?pr...

Yet 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.

Also number of bits per byte.

Does 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”.

It's called UART, to the best of my knowledge.

There is no standard for this.

UARTs are called universal because they can be configured to work with "any" asynchronous serial protocol (within their bounds), which usually means

- 1 start bit

- 5-9 data bits

- even or odd parity

- 1-2 or sometimes 3 stop bits

per frame. Hardware UARTs also typically oversample 16x, which is why simple software UARTs are often somewhat glitchy.

The 5 bit data frame proved useful in the lab at uni where one of our tasks were to build a routable network over serial. :)

Split each byte in 4-bit nibbles and used the remaining data bit for not-quite-in-band-signalling.

See, you called it “asynchronous serial protocol” there. But one can certainly think of asynchronous serial protocols that are outside those bounds.

The Wikipedia article [0] calls this category “asynchronous start-stop signaling” but has no source.

I guess the best term is “UART protocol”, despite the limited size of the “universe”, since it’s really just a de facto convention established by decades of chips called “UARTs”.

[0] https://en.wikipedia.org/wiki/Asynchronous_serial_communicat...

UART refers to the chip that implements the framing. There are many different framings that can be implemented by a UART. Many of them have names.

The standard defines the electrical characteristics (including voltage levels) and things like connectors and pinouts.

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