Terminology wrt UART Active Low (Magnova) vs Idle Low (Siglent) - Page 1
Forum section: electronics / general

author: Sorama
author_rank: Frequent Contributor
timestamp: December 14, 2024, 02:23:46 pm
content: hello,As there is a difference between the logic levels (0/1) and their electrical equivalent when talking about UART (TTL) and RS232, what do you guys think is correct or faulty in terms of considering the signal as Active Low/High (Magnova) or Idle Low /High (Siglent)?2 screenshots are attached; one when measuring an RS232 signal with the Magnova that only decodes correctly (I'm sending O LF) when the decoding is set to active Low although the signal when idle is (electrically) Low and one screenshot of the Siglent on the same signal, same settings except the setting is Idle Level =Low (which to me is the opposite of the Magnova; Active Low).I find this setting/description, Active Low, not the right terminology when a signal electrically goes from low (-7V) in Idle to a positive voltage of +7V when active.The Siglent, when hooked up the same signal, decodes correctly when decoding set to Idle Level =Low, which I do find a correct description of the signal itself.I'm not talking about the logical levels as indeed for RS232 signals, the Low (negative) voltage is actually a digital logic 1 where for UART data is generated per TTL logic where logic 1 means 2,5 to 5V.I'm I the only one that find the terminology of the Magnova contradictionary/plain wrong?

author: Sorama
author_rank: Frequent Contributor
timestamp: December 16, 2024, 07:53:09 am
content: I know this is not the most interesting topic on the forum, but if some of you guys could share your thoughts of it, I would highly appreciate it.Tnx.

author: Andy Chee
author_rank: Super Contributor
timestamp: December 16, 2024, 08:10:59 am
content: Commonly accepted standards are useful for expediting development of interfaces, in both the hardware and software world.But standards do not need to be upheld as if they were “the law”. As long as the documentation is good, there shouldn’t be any problem.

author: Sorama
author_rank: Frequent Contributor
timestamp: December 16, 2024, 09:09:31 am
content: it is a setting that has a high impact on the decoding results, not a commonly accepted standard.When a serial communication like RS 232 which has its idle level high, it is not correct that you have to set the decoding of the scope on Active Low.It is inversed, so a bug IMHO.

author: 2N3055
author_rank: Super Contributor
timestamp: December 17, 2024, 06:52:30 pm
content: Ok, I see...Yes, in industry it is customary to specify "Idle" state (Idle low or Idle high) for UART type interfaces.I don't like changing naming for something that worked well for many decades.So I think using logically inverted "active something" naming for this was not necessary. If it ain't broken, don't fix it.BUT, while weird, it is not so hard to understand.And definitely NOT a bug. Bug would be if they say "Active LOW" but scope actually performs "Idle LOW" or something.I think it is just not so fortunate choice of naming in my opinion.So maybe a polite note to manufacturer that customers would preferer "old school" naming that is standard for decades.That is my opinion.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 17, 2024, 07:41:35 pm
content: Yeah, sure, "active" is kind of opposite of "idle" so clearly it's wrong, or "bug" if you so like. Although, "active" is such weird choice of words that users expect anything and try whichever setting works. Which is what people do anyway, we are so used to things specified weirdly and having to iterate over possible options to find what practically works, so that's why I think you are not getting too many replies. People are just not interested, companies getting names and interfaces wrong is similar to water being wet. Like, remember how every other television set had "S-VHS" input when the correct term was s-video?

author: m k
author_rank: Super Contributor
timestamp: December 17, 2024, 07:45:24 pm
content: Quote from: Sorama on December 14, 2024, 02:23:46 pmCheck MC1488/89 datasheets.

author: nctnico
author_rank: Super Contributor
timestamp: December 17, 2024, 08:48:25 pm
content: Where it comes to dealing with UART / RS232 I'd prefer 'idle low' or 'idle high'. If only the RS232 levels wheren't inverted from what is coming from a UART...Similar where it comes to SPI interface decoding. You want to specify rising edge or falling edge of the clock. Not 'active high' or 'active low'. The decoding configuration on R&S scopes like the RTM3004 is really good as it shows how it interprets the waveform based on the settings. There is no guessing needed.