@2N3055:
They do. It's called datasheet.
So we seem to be in agreement, that it is the datasheet that tells us what the instrument should be (at minimal) capable of.
Like I wrote before, it is stated in the datasheets that apply to the RTB-K1, RTB-K2, RTB-K3 software options (which are the specific software options for the RTB scope) that "Trigger and decode on four different interfaces at the same time from analog or digital channel signals."
And this data sheet does not list any reservation, any limitation or whatever.
If the datasheet unconditionally specifies it, then we cannot wave this away as an unreasonable expectation of the buyer of the instrument!
Again (and I am now repeating myself): I find it totally legitimate that R&S differentiates between their various offerings, like the RTB, RTM, RTA. But they do need, at minimum, to comply with the datasheets they publish for specific models.
I agree with you in general. But, as I said before, apart form datasheets, all other documents in big companies are being written (or at least edited) by lawyers nowadays, not engineers. Engineers contribute to it by giving base data, and then marketing and sales and legal departments make up phrases that are "technically correct" but sound better that they are.
I'm looking at datasheet now (Version 12.00, March 2020). I can't find sentence you're quoting. What I can see is: "number of bus signals 2" with comment " If a bidirectional bus is used (e.g. UART RX/TX or SPI MOSI/MISO), two bus decoders are occupied".
It is right there...
I dug a bit more, and found a brochure called "R&S®RTx-K1, -K2, -K3, -K5, -K6, -K7 SERIAL PROTOCOL TRIGGERING AND DECODING, For R&S®RTB2000, R&S®RTM3000 and R&S®RTA4000 oscilloscopes"
In that one they mention : "Decoding of up to four serial buses: Trigger and decode on four different interfaces at the same time from analog or digital channel signals"
But they say "Up to", for a family of products 2000/3000/4000. Expensive 4000 series does 4 decodes.... They didn't lie. That is a brochure for a product family.
In datasheet for RTB2000 they correctly state, that you practically have 1 decoder to use, unless you do 2 I2C buses or some other unidirectional serial bus..
But that's fine, really.. It is entry level scope after all. But 15000 USD retail RTM3000 also have only 1 bidir decode, and even more expensive RTA4000 series have 2 bidir decoders (yes those 4 are single direction, so only 2 UART decodes)
Keysight 3000T for instance has 2 decoders, full bidir... So that would be equivalent of 4 in R&S RTA4000...
But all that is clearly written in datasheets. Those are tech specs. Brochures and other marketing materials are not to be trusted and are marketing bullshit mostly, designed to confuse customer and painting a rosy picture..
And it is obvious that R&S doesn't need to make more decoders available (even if the could add them, maybe resources are limited), because it is obvious that current number of decoders is sufficient.
And we know for sure they are sufficient, because people keep buying it despite nominaly very limited decoding. Once they stop buying, and quote low number of decoding channels, they might make a change.
Until then, there will be no change.
Same is with Keysight 3000T (and 4000/6000 Infiniivision series). Everybody and their mother are bitching about how they have very short memory (2-4 MS compared with 200-500MS with even very cheap scopes today). And that is being a story now for more than 5 years.. And there is no new Infiniivision II with Megazoom 5 and 500 MS (or even 50MS) of memory in sight.
Why? Because they are best selling scopes on the market still in its current form, and obviously people don't need more memory... When sales start dropping and people say it is because of very short memory, then, you will see, how Keysight will very quickly introduce new series. like a rabbit from a hat. Of course they are working on next step, maybe even have developed product ready for production..
But why use an ace when you're winning the game anyways..