Serial buses Maximum speed - Page 1
Forum section: electronics / beginners

author: JustAnotherGuy999
author_rank: Contributor
timestamp: February 29, 2016, 11:04:13 am
content: Is there a maximum speed that serial buses can't go beyond, like the case with parallel buses

author: Rerouter
author_rank: Super Contributor
timestamp: February 29, 2016, 11:15:43 am
content: It looks to be based upon the interface chips at either end, FTDI claim some of there chips can handle up to 3Mb/s, and have found many claims of even the arduino microcontroller pulling off 1Mb/s however this limit seems more or less linked to the limitation of what settings a program called hyperterminal make available,

author: JustAnotherGuy999
author_rank: Contributor
timestamp: February 29, 2016, 11:23:28 am
content: Actually im not talking about SPI, im talking about serial communication in general, thanks anyway

author: Rerouter
author_rank: Super Contributor
timestamp: February 29, 2016, 11:24:40 am
content: I was never referring to SPI, but RS232 Serial,

author: amyk
author_rank: Super Contributor
timestamp: February 29, 2016, 11:27:21 am
content: https://en.wikipedia.org/wiki/Serial_ATAhttps://en.wikipedia.org/wiki/HDMIhttps://en.wikipedia.org/wiki/List_of_device_bit_ratesA lot of these are serial...

author: JustAnotherGuy999
author_rank: Contributor
timestamp: February 29, 2016, 11:30:22 am
content: Is there a limited speed that serial communication in general can reach

author: daqq
author_rank: Super Contributor
timestamp: February 29, 2016, 11:35:41 am
content: QuoteTheoretically only the limits of the medium, which can be pretty high considering fiber optics are serial. At some point you have to involve physics that involve wavelengths, individual particle energies and stuff like Planck time. There is probably some upper limit when you divide the available energy of the Universe with some weird number, but it'll be very high. So don't worry about crossing it

author: JustAnotherGuy999
author_rank: Contributor
timestamp: February 29, 2016, 11:46:18 am
content: Yes but in practice some problems may arise like in the case of parallel communication (synchronization, crosstalk ...)serial communication might have its issues too, that would limit the speed to a certain value, isn't?

author: coppice
author_rank: Super Contributor
timestamp: February 29, 2016, 11:51:04 am
content: The channel capacity theorem tells you how much data you can push through a single serial stream, whether its over a radio channel or over some other medium - like an optical fibre, or a pair of wires. The more bandwidth you can find the more bits you can send. The more SNR you can find, the more bits you can reliably send. Radio could conceptually be from kHz to gamma rays in a single channel, offering quite a lot of bandwidth, but it seems unlikely that such a channel would ever be technologically feasible. In more practical terms, an optical fibre has far more capacity than current commercial systems are able to extract from it, and systems already send at 1.6Tbps.

author: daqq
author_rank: Super Contributor
timestamp: February 29, 2016, 11:52:24 am
content: Serial communication = concept of sending data serially. As such there is no upper limit defined by said concept. However, at one point you will arrive at physical limitations of your data channel and your hardware.You are asking a very broad and unclear questions.

author: Rerouter
author_rank: Super Contributor
timestamp: February 29, 2016, 07:22:25 pm
content: for RS232 the maximum speed selectable in a number of windows applications is 921600, while 115200 appears to be the fastest to be universally supported by almost all computers,

author: bson
author_rank: Unknown
timestamp: February 29, 2016, 07:48:31 pm
content: Quote from: JustAnotherGuy999 on February 29, 2016, 11:04:13 amYes, the maximum data rate is less than the effective bandwidth / 2.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: February 29, 2016, 08:01:55 pm
content: Quote from: JustAnotherGuy999 on February 29, 2016, 11:46:18 amThose aren't problems inherent with parallel communication, those are problems with the transmission medium (lack of shielding, etc) and the specific protocol being used. You haven't specified either of those in your "serial alternative", so it's impossible to answer. If you want a specific answer, you need to ask a specific question.In general, a serial bus would have a bandwidth one Nth that of an equivalent N-bit parallel bus.

author: langwadt
author_rank: Super Contributor
timestamp: February 29, 2016, 09:15:16 pm
content: Quote from: suicidaleggroll on February 29, 2016, 08:01:55 pmthe advantage of a serial bus, or a parallel bus of serial streams like PCI-Express is that the clock and data can bea single stream so you don't have to rely on delays in different line being exact

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 01, 2016, 06:25:39 am
content: 1. Bitrate is twice the bandwidth.2. Bandwidth is how much EM frequency you can shove down the structure, and receive at the other end.3. If you have parallel wires, then the bandwidth includes DC, and extends to a modest upper limit, where the waves start to break up because their wavelengths are less than the wire-to-wire spacing or separation. (This limits coax and CAT5 to a few GHz.)4. If you shrink the structure, you can fit smaller waves between them. But losses go up.5. If you replace the wires with dielectrics, then you can't carry DC anymore (there will be a LF cutoff), but losses go down. (Waveguides are an example of this: consider a coax cable with the center conductor removed.)6. If you replace everything with dielectrics of varying material (instead of a shield and conductor), you get fiber optic cable.Fiber optic cable has a fairly strange frequency response, where bands (of some 10s of GHz width) propagate smoothly (meaning, without dispersion: some frequencies showing up sooner or later than others within the band), separated by anti-bands (as it were) where propagation is poor (highly dispersive, the velocity is changing with frequency: so pulses of information won't stay together as pulses). The number of bands (modes) is quite reasonable (hundreds, for near IR I think..?), so the total bandwidth can be quite high (Tbps).Whether this counts as serial anymore is up to you, I guess. Typically, each band is treated as a serial channel, with a clock recovery system, and modest bandwidth (minding that "modest" here is comparable to the CPU-memory buses of last decade's state-of-the-art PCs). Data is incoherent between channels, because of the differing delays (the velocity of each band is different), so a single bitstream would have to be reconstructed with the help of buffers. In a sense, it's neither parallel nor serial: the channels certainly aren't synchronized (as in a parallel bus), and there isn't a single serial stream.I don't think there can be a technology that offers higher bandwidth than fiber optics, not more than arithmetically so.First of all, electrical bandwidth of sufficient magnitude is "light" (yellow is around 500THz). Frequencies that high can't be confined by metallic conductors, so we must use dielectric structures: dielectric waveguides, fiber optics.I don't know that a waveguide can be designed in such a way as to eliminate modes; perhaps a gradient index and exponential cross section could confine smaller wavelengths to a smaller section of the waveguide, keeping the geometry, and therefore velocity, constant, thus allowing nearly full, continuous bandwidth without breaking it into bands (i.e., using a solid, say, 50THz or more, around a 500THz carrier).Significantly higher carrier frequencies aren't practical, because many materials absorb UV. There are still ways to deal with it, like vacuum and glancing-angle optics (which is how the Chandra X-ray Telescope is constructed), but I think I would be surprised if the efficiency of such a system could ever be practical.Other media can transport information: acoustic waves for example, but that's much too slow (and lossy above mere ~MHz). Gravity waves propagate much the same as light, but it seems unlikely that there could ever be a means of manipulating that for more than very low bandwidths (perhaps uHz or less, since we're talking about altering the orbits of, say, neutron stars..). The propagation also isn't any faster than light, though it does penetrate through, well, everything in the universe.So we're a fraction of the way there already: some Tbps in a single fiber. Perhaps 100Tbps will be achievable some day. Beyond that, I think it will come down to printed (or grown) structures: optimizing for lower energy (perhaps middle to near IR, which costs less voltage to emit) and greater numbers of channels (perhaps a multi-core fiber that packs a thousand, or a million channels, into one strand). Any more than that would seem to strain any kind of information-theoretic requirement; it will be much more expensive to distribute such quantities of information over such distances where this would be required, versus piling a bit more computronium* in one place to achieve a similar task. Even if it takes actually quite a lot more, because of network prediction requirements as well as just whatever's being computed along the way.*Computronium: a supposed substance, of macroscopically amorphous nature, which (somehow or another) receives a higher grade of power (be it electrical, thermal or what) and information signals (electrical, optical, etc.), and outputs low grade power (usually waste heat) and processed information signals. A block of silicon chips is a step in this direction, but the information-theoretic limit (in terms of computation/watt and computation/cm^3) is something like >6 orders of magnitude more dense than where we're at now. (The far-out fantasies being something like: suppose you have a computronium shell, of a sort which is powered by a thermal gradient. Construct the shell around a star. Now construct a shell around the shell, and so on, each layer operating on a lower average temperature, and whatever thermal gradient it gets. Now you have a Dyson sphere with truly astronomical thinking power!)Tim

author: hamster_nz
author_rank: Super Contributor
timestamp: March 01, 2016, 07:33:27 am
content: Around 40Gb/s is close to the current state of the art for a single link within a Data Center, with 10Gb/s over copper and 16Gb/s over Fiber being the more normal option for high bandwidth links.It is common to use port channels over multiple serial connections to boost effective bandwidth, so we have a few 128Gb/s links around the place, made up of 8 x 16Gb/s fibers.For copper, the cables have the standard SFP+ transceivers already connected, so you don't have losses and impedance bumps that connectors would give you.Using WDM over single mode optical fibers is common between data centers, but each wavelength is a separate serial channel.

author: bson
author_rank: Unknown
timestamp: March 01, 2016, 08:04:02 am
content: Quote from: T3sl4co1l on March 01, 2016, 06:25:39 amOops, you're right. The Nyquist limit is twice the bandwidth, not the other way around. My bad.

author: Rasz
author_rank: Super Contributor
timestamp: March 01, 2016, 04:44:48 pm
content: Quote from: bson on February 29, 2016, 07:48:31 pmbandwidth _is_ the maximum data rateor did you mean bandwidth in Hz? in that case YES, as evidenced by 56Kbit modems going over 3KHz audio link, or gigabit ethernet going over four 125MHz pairs, wait what?!?!?!11something something Shannon something

author: bktemp
author_rank: Super Contributor
timestamp: March 01, 2016, 04:55:55 pm
content: Quote from: Rasz on March 01, 2016, 04:44:48 pmIf you use only binary levels then bandwith/2 is correct. But as soon as you use more levels like QAM16/256 you can get more data over a channel.The 56kbit/s limit on modems has not much to do with the audio grade phone cables, DSL uses the same wires and achieves many MBit/s. The analogue phone signals get digitalized into 8bit/8kHz. This limits the theoretical datarate to 64kbit/s. Because the transmission and the AD/DA conversion is not ideal, the modem can achieve only 56kbit/s.