Painless remote control options

[Ajohn]

The companies that maintain Linux distro's have a way of ensuring people upgrade - eventually people try to install something and find they can't because things have moved on.  I had that happen last week also found I can't update my favourite photo editor -not that I need to do that.

Back on topic.

There is something about that will link usb over wifi according to one source 

http://www.raspberrypi.org/forums/viewtopic.php?f=31&t=97179

The person who wrote virtualhere mentions that it's stand alone code rather than using the Linux facilities. That might explain the speed. Not sure. Like most I pull the card out of the camera and plug it into a reader. I haven't used the camera download for a long time but vaguely remember it's slower. It's free to try on one device. Makes me wonder about wifi cards.

Interesting demo of remote control here. There are some others on youtube. The INDI telescope end has been ported to a 'pi but there is a 'pi look alike around. Odroid. 1.5khz 4 core and some reports that it's IO is faster. It also has a gigabit lan connection. There is an option to install a different type of flash memory to it as well.

Treasure Island Britain makes fibre optics way too expensive as the prices leave a bad taste in the mouth and that's just the cable.

John

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[JamesF]

The person who wrote virtualhere mentions that it's stand alone code rather than using the Linux facilities. That might explain the speed. Not sure. Like most I pull the card out of the camera and plug it into a reader. I haven't used the camera download for a long time but vaguely remember it's slower. It's free to try on one device. Makes me wonder about wifi cards.

From the documentation it appears to use the USB/IP drivers in the Linux kernel.  If you're using Linux-based machines at both ends it might be sufficient just to use that.  Effectively you get a virtual USB device on the client with traffic being transmitted over the network to the real USB device at the other end.

I'm struggling to convince myself it would work for high bandwidth data transfers such as are common with planetary imaging and I'd guess there will be problems with client-side code that expects a response from the USB subsystem within a specific time interval because the USB specification guarantees that to be the case, but it might well be ok for plenty of other stuff.

James

[Ajohn]

It reads to me that the server side must be Linux based, 'pi etc, but the client side can be windows, linux, mac or android. I became a little curious and found there is a cross platform usb library about that can be run in and used in user space. It's likely to be very compatible with all kernels it can be compiled for. What it appears to do is standardise the usb interfaces across various platforms so the kernels are still being used. It looks like the latest versions will also handle usb 3.

The wifi dongles come in 3 flavours, 150,300 and 600 odd Mbits/sec. The later is only available on USB 3 and run at 5ghz. I believe some step back under certain circumstances and that data rates are upped by using more than one channel. If that can't be done due to use or interference the speed drops back. There are some interesting numbers on here,  bluetooth might work out faster than some. the latest is supposed to reach 24Mb/sec

https://en.wikipedia.org/wiki/Comparison_of_wireless_data_standards

I should add that it looks like INDI can be run via wifi or wire but I haven't really ;looked at the detail.  Maybe a sophisticated cable reel and solid cored cable is the fastest option.

John

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[r3i]

Ethernet can also be sent down a mains cable but I've not looked at bandwidth.

HI John,

Powerline adapters are worth looking into.  In my experience, bandwidth doesn't have to be great.  I run a pair of fairly old 85mb adapters: one in the observatory, the other in the house wired into the router.  Test software is saying I typically get 30mb between them, which I've found is more than sufficient for me to monitor imaging sessions wirelessly from the lounge on an Android tablet using TightVNC.

[Ajohn]

I've just come to the same conclusion Mike. 500mb/sec ones can be bought. It should even be possible to use a remote usb hub with these plus the ethernet to usb adapters. There does seem to be some USB2 ones about but I'd guess that would mean actually buying them to find out.

I'm after something portable rather than fixed otherwise I would probably just run an ethernet cable.  Having seen the UK price of reinforced cable I would probably bury it in toolstation hose pipe if I did that. 

In real terms I will be using an extension lead plus the powerline units and a dslr which is why the bandwidth concerns me. On that score the Odroid looks more favourable than a 'pi.

It's an interesting area to look at. The manufacturers are producing chips to actively boost usb 3 because the max lead length is so short. There are also some fibre optic usb 3 leads about as well but the prices are a bit high at the moment. Even a fast usb wireless standard but no one seems to be doing anything with it. At 10ghz it will probably only work over short distances anyway.

Only remaining problem is Altar Astro. I ordered a light weight mount off them specifically for this and expected them to get in touch if it wasn't in stock. Not a peep out of them even after a message on their answer phone.

John

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[gkec]

I am not entirely sure what you are trying to achieve but there are some points that I think need clarifying. First of all the USB/Ethernet adapter on Amazon. This is just a device to add USB and Ethernet ports to your PC or RPI. It does not convert USB to Ethernet.

Whilst it is probably possible to connect USB to USB as both a PC and an RPI USB ports act as a host port to devices this is not the normal mode of operation.

However wired Ethernet will do this and will operate over about 100m.

I can't see how hijacking Ethernet wires can work. Surely the obvious solution is a simple UDP protocol between the PC and the RPI would facilitate what you are trying to achieve.

Linux development essentially consists of a compiler,GCC,and an editor. For compilation you can either compile on the RPI or cross compile on the Pc.

By mounting the RPI sdcard on your PC so it looks like a local disk you can use whatever editor you like.

There are IDEs that can pull all this to get here but I find they are either expensive and/or more complicated than the original problem.

Eclipse I reserve a particular contempt for after trying it on four separate occasions the last wasting 3 months of development effort.

[Ajohn]

There are usb to ethernet extenders about. I thought that the link I posted was to one but that turned out to be incorrect. These do convert or maybe just drive ethernet cable usb style - there was a post somewhere that reckoned they had extended one by simply splicing cat 5e cable into an ordinary usb cable that had been cut in half. Other things could be going on though. Most of the extenders state usb 1 some say usb 2 compatible, some usb 2. Personally I'm not sure if usb 2 devices will drop to usb 1 in much the same way as ethernet drops speed if it can't be maintained so I see these things as being a bit dubious. Any built in chip could force USB 1 anyway. If you search ebay for usb extender several are listed. The length of cable they will drive varies. There are also active usb cables that use a chip to receive and boost the signal taking power from the connector, some have a separate power supply. Lengths vary again.

While watching a pretty awful spaghetti western 1/2 hr ago I realised that while the powerline ethernet adapters look great they probably don't have gigabit lan drivers in them so data rates may be 100mb/sec in practice. That seems to be the limit for a 'pi anyway. The Odroid does seem to be gigabit and will run the software that interests me personally. Same price more or less but people seem to be having problems with INDI and wifi at the moment. However as powerline could be a private network, nothing else on it, it might turn out to be the fastest option even at 100mb/sec.

It should be possible to treat the 4 USB ports on the new 'pi effectively as a hub so there isn't any reason why something shouldn't go in one and be split off into others but 100m USB cables appear to have their problems. Some suggest that the active ones can be daisy chained but if so why do some have separate power supplies.

As some one pointed out earlier cross compiling does seem to be the only "dev kit" available in practical terms. Having seen what INDI and Kstars/Ekos can do I don't think I need to write any software at all -. other than I will wonder why they haven't put their remote focus in it as well but that may be down to the number of timers in the processor. If some one uses windows and wants to work in Linux it should be fine working in a virtual machine. I have done that in the past the other way round but not for a long time, My biggest concern  in this area is if Kstars/Ekos will run on a decent netbook. I've always intended to run a Linux on it but since iPad it's just been lying around. In this case I don't need a long cable - I only need that to run it all from my desktop.  Maybe perusing that via vnc on something else at times.

According to the wiki the ethernet cable length limit is 100m which can include some flexible. Like most things I'd guess it could be longer but 100m with decent cable should work 100% and not drop packets.

I'd guess that the camera usb ports place another limit on max speed. While they may be usb 2 this doesn't mean that they have to send 480mb/sec continuously, just pulse at a rate they can manage. Bit like what ISP's do to our connections.  I haven't downloaded from a camera this way for ages as direct from the card was much quicker.  Guess i had better try that and find out.

This might seem all a little strange but I want the option to use it all close to the scope and away from home - ie portable. This rules out using watty parts close to the scope for me anyway.

John

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[Gina]

I am hoping to  migrate to using the Linux platform for my AP in time - need to get the equipment working properly first, and this thread looks very interesting and shows what can be achieved.  Thank you to all those who have contributed

[Ajohn]

 I've never tried to do this so much of it is new to me but from habit - work - I have done what I would do there. Want to do this, ok what are the options. I'm reluctant to write any software but I found a couple of sites that may help

http://free-electrons.com/docs/

Seems I am now largely what is now called a bare metal person, mostly in assembler in my case so that one is of great interest to me as I feel the need to know what lies underneath

Mixed c and assembler on a 'pi and maybe some help with make and configuration files. not sure. My eyes glaze over when ever I look at what is in one of those.  I suspect the worst problem is producing the first one in the same fashion as writing a first very early windows application.

http://www.cl.cam.ac.uk/projects/raspberrypi/tutorials/os/

And the USB library I mentioned. I'd be a bit surprised if any thing else is used.

http://libusb.info/

The other dev tool that can often be of use is an emulator. As I suspected a google for arm emulator came up with some results. It would take a while to see which offers what and what else is needed.

Linux is distinctly different to windows in the way it's organised. Linux is the basic software system that runs the machine, often called the kernel. Windows has one too  really but it's not so apparent. Above that sits library files which in loose terms are similar to DLL's and services such as X windows and others that look after specific things. The desktop or windows system sits on top of that with currently another graphics add on. I believe most use Qt these days. There are several desktop packages. The most popular of late is probably Gnome, it has been another called KDE in the past. There are also light weight ones. As I understand it packages mostly just rely on them using some version of Qt so that they can run on all of them - best check as I generally have only used one.

Next comes distributions. These people package software so that it will run on their Linux. This means a package from one distribution may not run on another and the method of easily installing them can be different. Even the act of installing it all in the first place varies enormously.  I would say the 2 that avoid that sort of thing most are Ubuntu/Kubuntu and OpenSuse. The K in front of Ubuntu means it runs the KDE desktop rather than Gnome and that fact plus problems introduced with a complete rewrite of KDE is why Gnome is probably more widely used these days. I've stuck with KDE on OpenSuse all of the time but these days the Ubuntu stable probably has a lot more activity and help around. I some times have to compile code and chase down dependencies - the web will explain what that means. Compiling is generally dead simple  and usually stated in the readme file that comes with source code. Chasing dependencies isn't.  eg

./configure

make

su

<password>

make install

make clean  ---------- removes some junk left

su john

On Ubuntu make install would be replaced with sudo make install rather than su which actually means switch user. If no name it assumes the root user. Some think su stands for super user.  It doesn't. A root account - same sort of thing as a windows administer account isn't available on Ubuntu - wasn't when I last tried it anyway. The name root user comes from it being the only account that can do anything with the root directory of a disc eg make a directory or write access a number of system files etc.  I'd guess that on Ubuntu if some one wanted to run a desktop application such as a file browser it's possible to create a desktop icon that runs sudo what ever the applications is called. I just create one that runs the application as root so I can browse and edit system files as root from the desktop - not much need for that these days. Source files are provided compressed so it's just a case of down loading, right clicking extract here and it will be extracted under a version number directory. Then right clicking on the directory formed and selecting open console here.

The console has this thing called BASH, borne again shell. It's like using DOS in an MsDos shell window but has way way more bells and whistles to such and extent that getting fully to grips with it is a real task. I don't bother unless I really need to. It's probably possible to burn dvd's, write and send emails and even browse the web in it. Pass. Where it can be useful is that it's likely to offer more facilities than a desktop application will offer. The commands are documented by a thing called man. As the output disappears when that is exited it's often better to google man command name. If the command name isn't known google linux - what ever you want to do.

Both opensuse and ubuntu offer graphical software installers that will contain what people are likely to want Opensuse offers a huge check list when it's installed. These are dead easy to use. They also offer console based ones aptget or something like that on Ubuntu and zypper on opensuse.   I might have the spelling wrong on that one. Apt get is a Debian distribution thing, Ubuntu is based on Debian. I have looked at installing that but I use a complicated set up with specific disk partitioning. Looked a lot more difficult on Debian. Most distributions offer various releases. I stick to the stable one. It will have the least number of bugs especially after it's been out for a few months. The latest greatest isn't a good idea on Linux other than for people who want that and can live with the problems they MAY have.  I have seen ex windows people get into trouble by assuming all will be ok. The safe option is to let the automated update system do it's job as it will only update from it's official repository - where the software is kept. It's also safer to install software from there. Next best bet is to compile and add any missing library files from the repo. Last is anything you can get your hands on.  I have had problems from doing just that. An unofficial repo and later using the official one. The installer took code from both and they wouldn't work together, result no desktop. My answer to that is a quick visit to the distro's forum for help if I can't sort it out myself. I use an iPad for that or some one else's windows laptop.

As far as imbedded work goes Linux is all over the place, even in wifi SD cards it seems. The list is endless. At the pro level they are probably using things like Linux From Scratch - build a kernel with just what is needed in it, BusyBox - a light weight shell and similar things plus a little code. Dlink for instance make what they use available but just provide the run code for what they have added. It looks like OS-X developed by IBM and run on Mac's is basically a Linux rip off. Many software packages and idea originate from open source, often stolen for free as I see it. Pity really as people who maintain it often called dev's don't make a decent living which is why they come and go. Many just write or add something for the simple reason that they want it or for shear fun or interest.

There are catches. Bugs of one sort or another do crop up. Often fixes can be found on the web. Things seem better of late in that area on stable releases in particular. Drivers for various bits and pieces can be a problem. eg I usually buy Samsung lasers for instance because they do come with Linux drivers. They also fixed one that didn't work in a couple of days. It will now support them itself. HP seem to have added some for their kit - written by HP people in their spare time. Not sure what state this sort of thing is on things like ink jets. On the other hand a bit of web searching and I could make my Samsung printer behave as an Apple printer via my machine for the Ipad's in the house over wifi. Linux supports Apple Talk. Not the most secure protocol, in fact I bug reported a serious flaw in that area and some one started working on it.  Distro's will often pick up serious bugs. It basically supports all sorts of things. There can also be odd problems in this area. Nvidia for instance maintain Linux drivers for their cards. At one point they just stopped working following an upgrade. People who used the open source driver didn't have any problems at all.

If some one wants to see what it's like - easy - just download one of the bootable DVD's  or CD's  from their web sites and boot it.  As Linux is very memory efficient it can run fairly well that way. These don't install anything. Many can now be booted from USB sticks as well. I keep one called Knoppix about for fixing things if needed. It's a distro intended to be booted this way rather than being installed. I used it recently to reset a windows laptop password.

I used Sun's VirtualBox last time I used one. I would probably use that one again if needed. Emulators are effectively virtual machines but some VM's may be more difficult to get up and running than others. Oracle now seem to  own Sun but wont currently emulate arm.

Dual booting is another option. I understand that MS finally offered a method for doing that some time ago. I'm pretty sure all Linux distro's will also install that way as well. I've never un dual booted a machine but assume it can be done. When Linux is dual booted I would be surprised if there aren't utilities about to allow the windows partition to be read from and written too. Dealing with windows partitions on some distro's has been omitted at times due to copy right worries but the info needed to do this has always been around some where. There is no 32 - 64 bit messing around on Linux but there may be limitations under a VM. Not that this matters all that much really.

Must add Linux isn't for everyone though. Forget running the likes of LightRoom and PhotoShop for instance. There are OS packages and they are capable in the right hands but just like PS they have a learning curve. There will be other areas that have similar complications. Linux in a VM is probably the best option then or 2 machines. Linux did run on slow older machines - since the C++ guys got at it particularly in the desktop area it's heavier than in was. Lots of the graphics bells and whistles can be turned off though - in fact KDE will turn some off automatically if it's struggling to cope. Not sure about Gnome. I'd suggest people have a look at Ubuntu or maybe OpenSuse. I've often stuck with OpenSuse as they have a tendency to offer more stable code but feel Ubuntu has caught up in that area now as there are lots of people working on it - at the expense of other distro's. One odd thing about Linux desktops is that 1 click rather than a double is traditionally used. Of late though sometimes 2 are needed are needed in places particularly in packages.

 Long post that and probably a bit terse in places. Might help some not sure. There is plenty of help out there though from linuxquestions.org and distro forums if it's needed. Probably a lot more help than there is on dealing with it on an arm.

John

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[gajjer]

The new RPi (announced yesterday?) looks like it might be interesting.  The older models are ok, but a bit sluggish at times.

James

Mines in the post. Woooo hoooo!

Must take care not to photograph it though. Apparently it has a light sensitive component. Oooops. More blu tak

cheers

gaj

[JamesF]

Must take care not to photograph it though. Apparently it has a light sensitive component. Oooops. More blu tak

Only an issue when it's powered on, apparently.  And even then only if the flash can deliver enough light.  I think I'd be tempted to use a sliver of insulting tape or a blob of paint over the component in question personally.

James

[jnb]

I'm having a fair bit of success using raspberry pi at the 'scope connected by about 30m of cat5. I can run a web cam from the rasperry pi recording data locally and providing a live web feed. The gpio pins on the pi can connect to my ra and dec handset so a simple web page is an easy way to drive it.

What doesn't work so far is knowing where I am as I'm using a simple EQ5 mount so for navigation I need some encoders or at the very least a second camera on a finder scope. I also need a good wayto predict and handle meridian flip.

The advantage of this approach is that it's all low powered and can run from batteries

. The only connection I need is one network cable. More details to follow when I.m not typing on a tablet

[Ajohn]

I'm having a fair bit of success using raspberry pi at the 'scope connected by about 30m of cat5. I can run a web cam from the rasperry pi recording data locally and providing a live web feed. The gpio pins on the pi can connect to my ra and dec handset so a simple web page is an easy way to drive it.

What doesn't work so far is knowing where I am as I'm using a simple EQ5 mount so for navigation I need some encoders or at the very least a second camera on a finder scope. I also need a good wayto predict and handle meridian flip.

The advantage of this approach is that it's all low powered and can run from batteries

. The only connection I need is one network cable. More details to follow when I.m not typing on a tablet

Is that solid or flexible cat 5?

Some people on the INDI forum have swapped to an Odroid. Best read if interested. One of them is running the Kstars end in a VM on a Mac book. It could be an Odroid or 'pi on the other end.

 I ordered an Arduino board yesterday after seeing the INDI focuser.  I have had a long break from software after all. Last time I looked at this sort of thing it needed too much kit.

John

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[jnb]

It's whatever came to hand. For most purposes I find cat5 is pretty resilient and not that critical on how good the cable is. By the same principle it's running on a raspberry pi because I happened to have one lying around.

[Ajohn]

It looks like there are other INDI like solutions. I was hoping to get some links like this when I originally posted.

http://sourceforge.net/projects/iastrohub/

http://maxchen.im/tinyastro/en/

Both of these seem to be extremely low power and very portable - no pc level power needed at all.

I was glad to see that on the first one jpg's are downloaded and raw left in the camera. While viewing in FIT's etc may be attractive it's a hell of a load from a dslr. That way download speeds get fast. Many cameras put rather a lot in jpg's these days anyway. That can be increased on some cameras via various settings.

I would say the biggest problem with these is that they use a very commercial device rather than 'pi etc. It seems to make a huge difference to plat solving times. Only problem is that I followed a link to one and out of stock. It's probably been superseded and always will be from time to time with different parts.

John

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[gkec]

 These do convert or maybe just drive ethernet cable usb style - there was a post somewhere that reckoned they had extended one by simply splicing cat 5e cable into an ordinary usb cable that had been cut in half. Other things could be going on though.

Ethernet cables do not have magical properties.  The distance a signal can travel is determined by the transceivers in the transmitting device (your laptop ot RPI USB or Ethernet port).

10/100baseT uses 2 pairs of wires and gigabit uses 4 pairs. Using pairs of cables a greater transmission distance is possible, using more than one pair increases the band width. USB is a single wire for tx/rx which can work at high speed over short distances.

[JamesF]

Ethernet cables do not have magical properties.  The distance a signal can travel is determined by the transceivers in the transmitting device (your laptop ot RPI USB or Ethernet port).

10/100baseT uses 2 pairs of wires and gigabit uses 4 pairs. Using pairs of cables a greater transmission distance is possible, using more than one pair increases the band width. USB is a single wire for tx/rx which can work at high speed over short distances.

Exactly so.  It's not the ethernet cable that's clever.  It's what its design allows the hardware at either end to do that's clever.  And then when you step up a layer, away from the physical transport mechanism, TCP/IP has all sorts of smart stuff to allow recovery from failed transmissions that allow it to work even if the connection is a bit marginal.  It's not great at that point, but it will still work.

James

[gkec]

Exactly so.  It's not the ethernet cable that's clever.  It's what its design allows the hardware at either end to do that's clever.  And then when you step up a layer, away from the physical transport mechanism, TCP/IP has all sorts of smart stuff to allow recovery from failed transmissions that allow it to work even if the connection is a bit marginal.  It's not great at that point, but it will still work.

James

Over a point to point link TCP/IP should not be necessary, or in the case of local remote control desirable, it was designed for packet loss over a wide area network and these days they tend to be reliable unless there is a physical problem.

[jnb]

There are usb to ethernet extenders about. I thought that the link I posted was to one but that turned out to be incorrect. These do convert or maybe just drive ethernet cable usb style - there was a post somewhere that reckoned they had extended one by simply splicing cat 5e cable into an ordinary usb cable that had been cut in half.

John

Exactly so.  It's not the ethernet cable that's clever.  It's what its design allows the hardware at either end to do that's clever.  And then when you step up a layer, away from the physical transport mechanism, TCP/IP has all sorts of smart stuff to allow recovery from failed transmissions that allow it to work even if the connection is a bit marginal.  It's not great at that point, but it will still work.

James

IIRC the design of USB does not allow for long USB cables because the protocol deliberately has no handling of timing issues or errors. These are not so much the time for a signal to propagate along that cable but the time it takes signal reflections to die away; it's a bit like comparing two people trying to have a conversation in an anechoic chamber, the absence of reflections makes that easy (if weird) compared to trying to talk when you can hear yourself on a half second delay, which is weird and difficult. Ethernet cables behave like that anechoic chamber by being balanced so that they do not have those reflections, USB is not which makes it a simpler device to implement but with no guarantee of high speed over long connections.

[Ajohn]

The ethernet cable  is pretty clever actually. It designed in a similar fashion to the co axial cable that is used to carry radio frequencies such as analogue television signals from the aerial. There is some info on the subject on the wiki

https://en.wikipedia.org/wiki/Transmission_line

This should talk about distributed capacitance and inductance along the line because that is what makes them work. In other words some values per unit length after a fashion. If this isn't done all sort of strange things can happen. Even the actual resistance of the wire plays a part as it can help damp out the signals travelling along the line. Fourier comes into this where the square waves typically used in digital transmissions are considered. The rising edge is made up of a harmonic series of much higher frequencies. If these aren't preserved there can be overshoot, undershoot or even ringing or nothing at all coming out of the other end. Signals can even reflect back from the part that receives them.

The reason solid cored cable can carry signals for greater distances is probably down to controlling the parameters as it's made and more importantly something usually called the skin effect. DC uses all of the diameter of a piece of wire. As the frequency goes up the current levels get more concentrated towards the outer areas, this effectively means that it's resistance increases with frequency. Useful in the transmission line area as it means the wire diameter can be used to control the resistance.  Resistance offers a good way of controlling damping - how well a rising edge on a square wave is preserved along the transmission line. I'd guess they under damp. In other words a rising edge over shoots and a falling edge undershoots.  I can't find any pictorial illustrations on the web but all of these factors combine and limit maximum cable lengths for some specific design of cable.

Me - well I can believe that splicing ethernet cable into a usb cable might increase the maximum length that can be used. I wouldn't take it as fact though. The person that posted this was an amateur radio person - they generally will understand the above and be well aware of the factors involved.  On USB 3 it wouldn't surprise me if fibre optic cables become more common over time.

John

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[JamesF]

 On USB 3 it wouldn't surprise me if fibre optic cables become more common over time.

The way USB3 and Thunderbolt seem to be going to achieve greater throughput is to use multiple conductor pairs concurrently, ironically perhaps in the case of USB turning a "serial" bus into a parallel one  Thunderbolt allows for both copper and optical transmission methods, but USB may have to abandon the principle of backwards compatibility before it can do that.

James

[Gina]

Going back to the 1960s, part of my job involved designing transmission lines to carry computer digital data over 100 metres or more.  It's a whole subject in itself   Involves resistance, inductance, capacitance and the correct termination of the twisted pairs of transmission lines.  We needed parallel data lines to obtain enough bandwidth.  Apart from that I can't remember all that much about it now.  The whole thing was a very interesting research project - I really enjoyed that job

[Ajohn]

There is an American "shop" that sells cameras and such like BH ???????? can't remember the rest. There are some optical USB 3 cables on there. Getting on for $200 from memory. It's a natural way for things to go.

Actually fibre optics have their transmission line aspects as well. Single mode and multi mode. I see single mode as being like microwaves travelling along a tube of the correct size. Losses are very low and there used to be a device around where a probe could be moved along to measure voltages/current available at specific points . They varied sinusoidally so the frequency could be measured - effectively near perfect damping.

 I had to rent a line off BT once running from B'ham to Bolton. Complicated boxes at each end and running at an amazing 1200 baud - read bits / sec. One of the faster ones at the time. Some one had a VT100 on the other end running a stock control application that was in a DEC mini in B'ham.

John

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PS there is a bit on waveguides on the wiki

https://en.wikipedia.org/wiki/Waveguide

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[JamesF]

There is an American "shop" that sells cameras and such like BH ???????? can't remember the rest. There are some optical USB 3 cables on there. Getting on for $200 from memory. It's a natural way for things to go.

Hmmm.  I wonder how they work?  It seems unlikely that you'd be able to power a parasitic USB device (lots of things, then) from an optical link, but even if the device has it's own power, how does the remote end of the optical link power the electronics to convert the optical signal back into an electronic one?  Does it have to be plugged into a powered hub at the remote end?

James

[Gina]

I'm sure the remote end would need to be powered.