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Re: USB "null modem" cables and related Linux driver questions




On Friday 01 June 2018 06:20:59 Richard Owlett wrote:

> On 05/31/2018 10:07 PM, Stefan Monnier wrote:
> >> I have two computers with USB ports.
> >> I wish them to communicate as simply as mid-20th-century computers
> >> did.
> >
> > What kind of "communicate" do you need there?
>
> Essentially any ;/
> In fact one of the thought experiments I was pursuing was how to do
> file copying/sharing over RS232 - even I would not actually attempt to
> implement.

Other than speed of the copy while the error correction verify's it 
sector by sector, why not. Both rsync and rzsz are quite capable of 
sending a file halfway around the planet with the last 17 miles on a 
barbed wire fence. And getting identical crc's in the final check.

For a circuit that was actually that dirty, I think I'd choose rzsz as 
its default packet size is 256 bytes.  If the crc of that packet fails, 
it requests a resend until it gets it right. So does rsync, but rsync's 
default packet is 64k, demanding a far cleaner path. rzsz unforch has 
several cousins 3x removed, so there is less than 100% compatibility. 
And the linux version is one of the worse compatibility violators.

> > The "way back machine" to simulate a "null modem" serial cable
> > exists, as you've seen, but it's rarely the best solution for
> > nowadays's needs,
>
> "Best" is not an invariant absolute.
>
> > since nowadays connecting two computers is something completely
> > normal, supported by a deluge of tools, but they all expect a
> > "network" connection rather than a serial cable.
> >
> > In most cases those two computers also have ethernet or wifi "ports"
> > so you can connect them via such a network (which usually offers
> > faster transmission than a serial cable, lets you seamlessly
> > multiplex several connections, and lets you use the many tools
> > working over the network to connect computers).
>
> An explicit requirement is a wired, NOT WiFi, connection.
> I that seriously. My internet access is a WiFi hotspot with its WiFi
> capability disabled.
>
> > In some cases one of the two computers's USB port is an "OTG" port,
> > meaning that it can act either as "master" or not, in which case you
> > can just use a regular USB cable (and usually you then configure the
> > OTG side to pretend it's a network card, so it ends up looking to
> > the software like you've connected the two machines via an ethernet
> > cable. That's what I use between my BananaPi "router" and my office
> > desktop).
>
> I saw it and it meets most (all?) my requirements except my reading
> suggested:
>    1. obsolete
>    2. available only for Windows/Mac
>    3. no way to determine if any of machines were equipped
>
> > If none of that are options, you can resort to using an "ethernet
> > dongle" on both sides and an ethernet cable between the two.
>
> That's a 1 versus 3 items required per connection.
>
> > All of those things will typically work "out of the box" on a
> > vanilla Linux kernel (the usbnet drivers have been incorporated
> > years ago).
> >
> > Oh, and in case those computers are somewhat old, they may also come
> > with Firewire ports, and those (contrary to USB) don't have the
> > "slave/master" distinction so you can connect your computers this
> > way with a plain normal Firewire cable (and make it appear to the
> > software, again, as some kind of ethernet-like connection).
> >
> >
> >          Stefan



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